ABSTRACT SEYAM, ALI ABDELFATTAH. Weight and Size

ABSTRACT

SEYAM, ALI ABDELFATTAH. Weight and Size Reduction of a Lithium-Ion Battery Pack for an Extended Range Electric Vehicle. (Under the direction of Dr. Eric Klang).

As part of the North American EcoCAR challenge program, the North Carolina State

University team developed an extend range electric vehicle (EREV) featuring a 363-volt 22.3 kWh lithium-ion battery pack, 148 hp front-wheel drive electric motor, and 1.3L B20 diesel powered generator system. The battery consists of five A123 Systems water-cooled 72-volt prismatic lithium-ion battery modules electrically configured in series. As mandated by

EcoCAR challenge for crashworthiness, the battery structure is required to restrain the internal components at acceleration magnitudes of 8-g vertical and 20-g horizontal, with proof of achieving such structural integrity with the implementation of finite element modeling.

An initial design of the battery (year 2009 to 2010) consisted of a large aluminum and steel enclosure that resulted in excess battery pack weight and a sub-par 0.055 kWh/kg energy-to-weight ratio for vehicle lithium-ion battery standards. This research covers a revision of the battery system (year 2010 to 2011) that increases the battery energy-to-weight ratio to 0.072 kWh/kg through a unique internal structure that couples front and rear steel braces with a pair of carbon fiber sheets, with additional weight-savings obtained through the implementation of an epoxy-fiberglass non-structural exterior shell. The final product is an entirely modular unit that is hoisted into position from the vehicle rear tailgate, a different methodology from the current industry standard for large-scale electric vehicle battery packs

that are instead raised into position from the vehicle underbody. A vehicle weight savings of

250 lb over the initial version of the battery pack is achieved.

Weight and Size Reduction of a Lithium-Ion Battery Pack for an Extended Range Electric Vehicle

by Ali Abdelfattah Seyam

A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science

Mechanical Engineering

Raleigh, North Carolina

2011

APPROVED BY:

______Dr. Jeffrey W. Eischen (Member) Dr. Kara J. Peters (Member)

______Dr. Eric C. Klang Chair of Advisory Committee

BIOGRAPHY

At this stage of life, reflections of self occur at a major crossroads where one transitions from their years of growing and maturation and into the years that define who they become. The author Ali Abdelfattah Seyam has had a passion for automotives since memory, and through participation in the EcoCAR Challenge program at North Carolina State University allowed for him to pursue this passion through the latter portion of his Bachelor’s degree, through the duration of his Master’s degree, and finally into a foundation for a career as he walks through this crossroads.

Ali was born in Greensboro, North Carolina on October 13, 1987 to mother Vienna

Aly Barghash and father Abdelfattah Mohamed Seyam who were natives of Egypt and moved to the United States in 1979. He is the middle child of his immediate family with his older brother Mohamed Seyam and younger sister Afaf Seyam. The immediate family has spent most of their years raising their children in a quaint suburban lifestyle in Cary, North

Carolina, within the family home located in a neighborhood just outside of the state capital of

Raleigh and near the border of Apex. As such, Ali has gone through grade school in Apex and some of which he shared with his siblings, attending AV Baucom Elementary, Apex

Middle School, and finally Apex High School. Ali has since pursued a Bachelors and

Masters degree in Mechanical Engineering at North Carolina State University to where he is now in life as of this writing.

ACKNOWLEDGEMENTS

I would like to begin with a sports analogy that I have recently used to think of how society gives credit to great achievements:

“Often we tend to focus on the one who scores the winning goal in the basketball game while forgetting to recognize the fellow companions who passed the ball to the scorer.”

I envisaged the design associated with this work into its final form and executed the engineering effort discussed throughout this document, cultivating thoughts and ideas into a reality. But it is a matter of fact that the end result would have never been achieved without those around him who sparked some of those crucial thoughts and ideas. Moreover, the physical manifestation of those ideas into a real product would have not been possible without the handy skills and countless hours of effort around him.

It is with this gracious thought in mind that I first and foremost would like to thank those at General Motors, Argonne National Laboratory, A123 Systems, and the countless sponsors who contributed to the existence of EcoCAR Challenge and the associated graduate student funding to create the platform to exercise this engineering project.

I would like to thank Dr. Eric Klang for the idea associated with the final form of the battery pack internal structure that was subsequently executed.

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I recognize the services that were crucial to the physical creation of components associated with the battery pack, including Exact Cut Inc. in Greensboro, North Carolina for water-jet and laser cutting services for the battery mounting brackets as well as the steel and carbon fiber that compose the battery internal structure, and Jim Dean from the College of

Design who performed the CNC work necessary to create the male molds associated with the shaped fiberglass parts.

And finally, I recognize the undergraduate students who assisted with countless hours of assembly work, some of whom were self leaders and were an inspiring presence to steer this design into reality in the most critical final weeks that are the most demanding of all.

These undergraduate students include Jonathan Lohr, Brent Clay, Kyle Lunsford, Joseph

Barbour, Jay Whitaker, William Chang, Kasey (Ksenia) Sedova, and Dorian Hendricks.

TABLE OF CONTENTS

LIST OF TABLES ...... vi LIST OF FIGURES ...... vii 1.0 INTRODUCTION ...... 1 1.1 Extended Range Electric Vehicle (EREV) Architecture ...... 1 1.2 Problem Statement ...... 4 1.3 Building Blocks of the A123 Systems Battery Pack ...... 8 2.0 BATTERY REDESIGN CONCEPTS...... 12 2.1 Overview of Chapter ...... 12 2.2 Review of Composite Materials in the Automotive Industry...... 13 2.3 Review of Current Hybrid and Electric Vehicle Battery Packs ...... 20 2.4 A New Compact Packaging Scheme ...... 24 2.5 Early Concepts (Generations II to IV) ...... 27 2.6 Final Concept (Generation V) ...... 33 2.7 Generation V Mounting System ...... 38 2.8 Generation V Interface Details ...... 43 3.0 BATTERY STRUCTURAL ANALYSIS ...... 51 3.1 Review of Battery Pack Structural Elements ...... 51 3.2 Material Properties ...... 51 3.3 FEA Model of Battery Internal Structure ...... 57 3.4 Load Modes of the Battery Internal Structure ...... 65 3.5 Battery Mounting Bracket Analysis, Version 1 ...... 71 3.6 Battery Mounting Bracket Analysis, Version 2 ...... 73 3.7 Battery to Chassis Cradle Analysis ...... 76 3.8 Weight Comparison of Generation I and V Battery Packs ...... 81 3.9 Required Vibration Mount Stiffness ...... 82 4.0 INSTALLATION OF THE BATTERY PACK ...... 86 4.1 Fiberglassing ...... 86 4.2 Upper Assembly Process ...... 94 4.4 Lower Assembly Process ...... 96 4.5 Combined Assembly Process ...... 97 5.0 CONCLUSION STATEMENT ...... 104 REFERENCES...... 106

LIST OF TABLES

Table 2.1 Comparison of NCSU EcoCAR Generation I battery pack to production hybrid vehicle battery packs ...... 22 Table 3.1 Provided material properties for the Carbon Fiber Sheets (left column) versus properties input in the SolidWorks Linear Elastic Orthotropic Model (right column)...... 58 Table 3.2 Battery internal structure model-wide mimimum FOS for 8-g vertical and 20- g horizontal load modes...... 70 Table 3.3 Rear-Facing battery mounting bracket minimum FOS for 8-g vertical and 20-g horizontal load modes ...... 73 Table 3.4 Front-Facing battery mounting bracket minimum FOS for 8-g vertical and 20- g horizontal load modes...... 73 Table 3.5 Battery to chassis cradle mimimum FOS for 8-g vertical and 20-g horizontal load modes ...... 80 Table 3.6 Estimated Weight of Generation I Battery Pack and its Components ...... 84 Table 3.7 Estimated Weight of Generation V Battery Pack and its Components ...... 85

LIST OF FIGURES

Figure 1.1: Generalized schematic illustrating the B20D-EREV powertrain architecture with the A123 5 X 22S3P battery pack integrated ...... 3 Figure 1.2: CAD view of year 2010 (Generation I) A123 battery installation in NC State’s EcoCAR vehicle. The large length of the battery forced the rear seats to be located above the battery pack...... 5 Figure 1.3: Photographs of rear interior with the Generation I battery pack (a) just after battery pack installation, and (b) following the installation of the raised cargo floor cage, rear seat, and 12 volt battery...... 5 Figure 1.4: Simplified schematic of the Generation I lithium-ion battery packaging scheme ...... 6 Figure 1.5: Exploded view of the Generation I battery assembly...... 7 Figure 1.6: Photograph of a single A123 Systems 22S3P lithium-ion battery module with major features labeled. Note that the thru holes in the plastic end caps are for M8 Metric (or 5/16 English) fastener hardware...... 10 Figure 1.7: The A123 Systems battery kit as laid out when unpacked during the initial Generation I battery pack build (Li-Ion: Lithium Ion, HV: High Voltage, EDS: Electrical Distribution System, BMS: Battery Management System). .. 11 Figure 2.1: The revised packaging scheme to replace the Generation I battery pack (a) from the top view, and (b) from the bottom view...... 25 Figure 2.2: Early Generation II battery pack concept ...... 29 Figure 2.3: Generation III battery pack concept...... 31 Figure 2.4: Refined method of attaching the lower assembly to the upper invented during Generation IV...... 32 Figure 2.5: Finite element analysis loading condition for the Generation IV concept upper assembly steel plate, 8-g downward load mode...... 34 Figure 2.6: Simplified view of the mechanics of the Generation V battery internal structure during 8-g downward (negative z direction) loading...... 36 Figure 2.7: Procedure for Generation V concept upper assembly...... 37 Figure 2.8: Welding procedure for construction of the rear steel brace...... 38 Figure 2.9: Procedure for Generation V concept lower assembly...... 39 Figure 2.10: Procedure for joining the upper and lower assemblies in the Generation V concept...... 40 Figure 2.11: Final installation procedures for Generation V battery pack...... 41 Figure 2.12: Generation V battery pack mounting system...... 42 Figure 2.13: Methods of anchoring the battery to chassis cradle to the vehicle chassis itself...... 43 Figure 2.14: Planned internal wiring connections for the Generation V battery pack concept...... 46 Figure 2.15: Views of the four exterior interfaces on the Generation V battery pack ...... 47

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Figure 2.16: Detailed design of fuse access panel passage through the fiberglass shell shown in Figure 2.14a ...... 47 Figure 2.17: Detailed design of liquid cooling pipe passages through the fiberglass shell shown in Figure 2.14b...... 48 Figure 2.18: Detailed design of high voltage wire passages through the fiberglass shell shown in Figures 2.24c and 2.24d...... 48 Figure 2.19: CAD view of the cool and warm water manifolds of the battery pack...... 50 Figure 3.1: Orthographic and isometric views of all structural elements in the Generation V battery pack concept...... 52 Figure 3.2: Defined contact patches on the battery internal structure finite element model...... 60 Figure 3.3: Defined part-to-part relations in the finite element model of the battery internal structure...... 64 Figure 3.4: A view of the element mesh for the battery internal structure model. This type of mesh is termed a mixed mesh, using a combination of solid and shell elements...... 65 Figure 3.5: 8-g vertical (z-axis) and 20-g transversal (y-axis) load modes of the battery internal structure finite element model...... 66 Figure 3.6: 20-g fore (-x) load mode force and boundary conditions for the battery internal structure finite element model...... 69 Figure 3.7: The six load modes of the rear-facing (version 1) battery mounting brackets 74 Figure 3.8: The six load modes of the front-facing (version 2) battery mounting brackets ...... 75 Figure 3.9: The 8-g down load mode of the battery to chassis cradle...... 78 Figure 3.10: The 8-g upward load mode of the battery to chassis cradle...... 79 Figure 3.11: The 20-g horizontal load modes of the battery to chassis cradle. In addition to constraining the anchor bolt holes of the model as noted here, the weld bead constraint as shown in view (b) of Figure 3.10 is also applied...... 80 Figure 3.12: Major weight inducing components of the Generation I battery pack ...... 83 Figure 3.13: Major weight inducing components of the Generation V battery pack ...... 84 Figure 4.1: Process for producing the fiberglass components of the battery pack ...... 89 Figure 4.2: Finished appearance of the four fiberglass components of the battery pack: .. 91 Figure 4.3: Cross-section mechanics of the fiberglass lower module cradle when airborne...... 92 Figure 4.4: Photography of upper assembly sequence prior to installation of the front and rear steel braces. Assembly progresses from views (a) to (e)...... 95 Figure 4.5: Photography of final upper assembly details...... 98 Figure 4.6: Illustration for installing vertical M8 threaded rod fasteners through front and rear steel braces...... 99 Figure 4.7: Photography of lower assembly process...... 100 Figure 4.8: Illustration of joining the lower assembly to the upper using the four vertical M8 rods associated with the lower assembly...... 101

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Figure 4.9: Photography showing the major steps associated with the combined assembly process of the battery pack...... 102 Figure 4.10: Photograph of rear vehicle interior following the completed installation of the battery pack...... 103

1.0 INTRODUCTION

1.1 Extended Range Electric Vehicle (EREV) Architecture

The year 2009 to 2011 North American EcoCAR challenge program (mainly sponsored by the United States Department of Energy and General Motors) challenged selected universities to convert a donated stock vehicle into a vehicle for the future, namely that the vehicle is more environmentally friendly (smaller greenhouse gas and petroleum energy usage footprint, and a reduced onboard energy consumption). Simultaneously, the challenge requires the converted vehicle retains consumer acceptability through maximal retention (or addition) of amenities, cargo and passenger capacity. The stock vehicles for the EcoCAR program were provided by General Motors, in the form of a five passenger compact- crossover SUV (Sport Utility Vehicle). The stock powertrain was front-wheel drive consisting of a transversely mounted V6 internal combustion engine coupled to an automatic transaxle.

For the EcoCAR challenge program, the North Carolina State University team implemented a B20D-EREV (B20 Diesel Extended Range Electric Vehicle). In the recent movement of automotive powertrain hybridization (whereby two or more onboard energy sources propel the vehicle), the EREV (extended range electric vehicle) is most electrified as it is nearly 100% plug-in battery electric vehicle, with the exception of a small internal combustion engine-generator system activated when the battery SOC (state of charge) declines to low levels. The electrical power of the generator supplements the range capabilities of the vehicle such that longer trips are possible. And unlike a 100% plug-in battery electric vehicle, an EREV allows you to immediately fuel and go on long trips rather than the waiting period for a full charge from the plug. This is because the instantaneous power from the engine-generator system is sufficient for continued driving. If the battery has high energy content, the plug-in ability of an EREV allows the consumer to have a large AER (all electric range) before the engine-generator activates. This allows for avoiding the

1 consumption of fuel altogether during daily commutes, given the round-trip is less than the AER. An EREV presents itself as a solution for today until battery technology evolves to higher energy densities that allow 100% battery electric vehicles to have ranges near today’s internal combustion engine vehicles.

The NC State EcoCAR team has integrated an A123 Systems 5 X 22S3P lithium-ion battery system into its B20D-EREV architecture. The overall battery pack consists of five prismatic 22S3P modules (22 units electrically in series, each unit containing 3 cells electrically in parallel). The five 72 volts modules are oriented electrically in series, providing an overall nominal open circuit voltage of 363. At 100% state of charge (SOC), the battery pack contains 22.3 kWh of energy. With proper thermal regulation, the battery can perform at extremely high charge and discharge rates. Provided is a simplified schematic of the overall design to understand the battery’s interaction with the major components of the vehicle, Figure 1.1.

The illustration in Figure 1.1 shows that the battery pack interacts with four major vehicle components through a high voltage DC (direct current) electrical interface. During plug-in charging, NLG513-Sc unit manufactured by BRUSA provides the battery with up to 3.3 kW of power when plugged into a 240 VAC outlet.

Because the engine in an EREV is demoted to merely driving a generator when the battery SOC declines, the engine tends to be off during a large portion of the driving experience. Therefore accessories that are engine belt-driven in conventional vehicles must be powered electrically in EREV or pure electric vehicles. The vehicle’s DC-DC converter shown in Figure 1.1 which converts the high voltage from the lithium-ion battery to 14 volts typically produced by a belt-driven alternator. Such a function is always necessary as the standard 12 volt lead-acid battery cannot solely support a modern vehicle’s 12-volt system.

Figure 1.1: Generalized schematic illustrating the B20D-EREV powertrain architecture with the A123 5 X 22S3P battery pack integrated

During all-electric operation of the vehicle, the electric motor (also called an ETS, or electrical traction system) takes DC electricity from the battery pack to propel the vehicle. The model 101X ETS comes from General Motor’s Chevrolet Fuel Cell Equinox, and includes a motor controller that converts the DC electricity into AC waveforms for the motor. The motor controller also reverses the motor into a generator for regenerative braking (hence the bi-directionality shown between the battery and electric motor blocks in Figure 1.1).

The 1.3L diesel engine was also provided by General Motors, and a custom-built belt drive unit provides a 2.4:1 ratio between the engine and generator. The permanent generator manufactured by Perm GMBH in Germany is a water-cooled (model PGS 150) unit capable of 20 kW at 6000 rpm. The three AC (Alternating Current) lines coming from the generator feed a three-phase diode bridge rectifier responsible for conversion to DC, with sufficient capacitance provided by the other components on the DC bus ensuring the rectifier DC output is smooth. The 2.4:1 drive ratio between the engine and generator is such that the engine runs at a constant fuel-efficient RPM associated with a 20 kW output. During engine operation, the battery acts as a buffer between power produced by the generator and load consumed by the DC-DC converter and electric motor. For example if the total DC load is 6 kW while the generator is producing 20 kW, the battery absorbs the remaining 14 kW, hence becoming charged during that instant.

1.2 Problem Statement The scope of work conducted for this thesis was a complete revision of the vehicle’s year 2010 battery installation with an improved and finalized design for year 2011, mainly to: 1. To improve the aesthetic appeal and functionality of the vehicle’s rear interior volume 2. To reduce the vehicle mass, which exceeded its GVWR (gross vehicle weight rating)

The problematic year 2010 installation (called first generation, or Generation I) packaged the batteries in a steel/aluminum rectangular enclosure in the vehicle rear atop of the chassis rear cargo floor. The resulting large enclosure forced the rear seats to be located above battery, eliminating practical head room for rear passengers (see Figures 1.2 and 1.3).

Through the known mass of the A123 modules and calculating the mass of the rectangular enclosure (using SolidWorks 3D CAD volume solver and known densities of the materials), the year 2010 Generation I battery system weighs nearly 900 lbs (breakdown of the weight detailed later in Chapter 3). Coupled with rear seats, vehicle curb weight rose to 5163 lbs, just above its acceptable GVWR (gross vehicle weight rating). During the year

2010 EcoCAR competition at the General Motors Proving Grounds in Yuma, AZ, the rear seats (127 lbs total) were removed to be deemed safe (below GVWR) for dynamic events. The Generation I curb weight at which the vehicle participated in dynamic events was 5036 lbs. To transform this vehicle into a practical five passenger vehicle, it was paramount to reduce the battery mass such that the vehicle can stay below GVWR when including the rear seats.

A closer look at the Generation I battery packaging scheme is necessary to understand its excess volume and weight. For cooling, the lithium-ion modules have aluminum surfaces to allow conduction of heat to cold plates that have internal flow of automotive antifreeze. In the Generation I scheme, all five modules were sandwiched between two cold plates (as shown in Figures 1.4 and 1.5), applying cooling to both sides of each module. This forced a spread-out layout of the system.

Figure 1.2: CAD view of year 2010 (Generation I) A123 battery installation in NC State’s EcoCAR vehicle. The large length of the battery forced the rear seats to be located above the battery pack.

Figure 1.3: Photographs of rear interior with the Generation I battery pack (a) just after battery pack installation, and (b) following the installation of the raised cargo floor cage, rear seat, and 12 volt battery.

As outlined in the technical rules of EcoCAR challenge, it is required that the designed battery enclosure restrains the battery modules and their cold plate(s) at 20-gs horizontally and at 8-gs vertically. Such a strict requirement is to ensure that battery packs designed by the student teams do not have modules escaping the enclosure during accidents. Figure 1.5 shows that the interior of Generation I was organized into two interior assemblies, one that clustered three modules, the other that clustered two. As shown in Figure 1.4, each lithium-ion module and cold plate pair is clamped to its interior assembly steel frame. Moreover, the interior assembly steel frames are bolted to rubber cup mounts (vibration isolation requirements covered in Section 1.3), the mounts in turn bolted to the aluminum base plate. The aluminum base plate and its re-enforcing underbody steel frame are then anchored to the pair of fore-aft structural rails of the chassis. The 20-g horizontal and 8-g vertical load path from the modules to the chassis rails goes through the interior steel frames, vibration cup mounts, and then finally to the aluminum base plate/underbody steel frame. To meet the 20-g/8-g requirements, twenty-six small cup mounts were necessary. In all, the massive weight was due to ten cold plates, twenty-six mounts, a large aluminum base plate/re-enforcing underbody steel frame to cover the spread-out layout, and equally large aluminum walls and lid.

Figure 1.4: Simplified schematic of the Generation I lithium-ion battery packaging scheme

A final concern regarding the Generation I design is the difficulty of assembly, which occurs within the tight vehicle interior. This caused challenges in using hand tools in portions

6 of the battery and discomfort while performing high voltage wiring safely. To increase safety and ease of servicing, the revised battery pack must be completely modular from the vehicle; that is be entirely assembled in a controlled environment outside the vehicle, subsequently attaching it to the vehicle with a few simple steps. Such modularity has been commonplace in smaller hybrid vehicle battery packs where the electric propulsion works in parallel with the internal combustion engine. However in highly-electrified EREVs or fully electric vehicles where the scale of batteries are larger and heavier to support 100% electric propulsion and larger AER (all electric range), the concept of modularity is not as obvious to achieve.

Figure 1.5: Exploded view of the Generation I battery assembly.

In summary, a successful replacement to Generation I would feature: 1. A compact packaging scheme to reduce the battery occupational volume. 2. Reduced weight (from smaller size, light materials, and downsized cooling system) while still meeting strength requirements for 20-g horizontal and 8-g vertical acceleration.

3. The concept of complete modularity despite a potentially large size and mass relative to conventionally smaller hybrid vehicle battery packs The timeframe to design and build a replacement to the Generation I battery spanned from June 2010 to June 2011 in time for the final EcoCAR challenge competition. Such a timeframe limited the solution to using state-of-the-art, already available materials and design tools while maximizing creativity.

1.3 Building Blocks of the A123 Systems Battery Pack With their prismatic lithium-ion module series, A123 Systems develops a kit of modular components that give auto manufacturers flexibility with packaging schemes. The modularity of the kit components allows A123 Systems to mass produce the same components that can be used in a plethora of battery pack models. Rather than having to redesign each battery pack by rearranging hundreds of cells, the auto manufacturer need only to reconfigure a few modules.

The A123 Systems kit includes the prismatic lithium ion battery modules as well as the necessary ECUs (electronic control units), associated communications wire harness, and sensors. This vehicle used a 5 X 22S3P kit consisting of five 22S3P lithium-ion battery modules.

Each 22S3P module has a total weight of 89 lbs, stores about 4.5 kWh of total energy, and is 72 volts nominal overall. A single photographed module is shown in Figure 1.6. As mentioned earlier in Section 1.1, each module contains twenty-two units electrically in series, with each unit containing three prismatic cells electrically in parallel. Each module is a continuous stack of thin, rectangular shaped cells. Each cell’s top has an electrical positive and negative tab, and the stack of cell tabs in each module are laser welded together to provide the 22S3P electrical configuration. A123 Systems can offer other xSyP electrical configurations for their modules, x being the number of units in series, and y being the

8 number of cells in parallel per unit. A module is complete with top plastic covers to prevent accidental contact with the cell connections and folded aluminum fins that come in contact with the cells to conduct heat away. The aluminum surfaces come in contact with metallic cold plates that circulate coolant. To enhance thermal conductivity, the aluminum surfaces have thermal paste applied.

Finally each module has two plastic end caps, one on either end of the module. Each end cap houses an MBB (measurement and balance board), which are ECUs that monitor the temperature and voltage of each cell, and are also responsible for managing cell voltage balance during charging and discharging. The plastic end caps feature horizontal and vertical thru holes to allow the passage of mounting bolts or other hardware (size M8 Metric or 5/16 English), giving versatility for the battery pack designer to restrain the modules in a variety of ways. It should be noted that the centerlines of the horizontal and vertical thru holes intersect such that either the horizontal or vertical pair of thru holes are useable on each module, but not both pairs simultaneously. The compression straps are designed to pull the end caps together, providing pre-compression that holds the prismatic module assembly together. The plastic end caps each feature an embedded harness plug for their MBB ECU, and high voltage electrical terminals with threaded holes that give access to the module’s overall positive and negative connection points. Each of the five modules receives a pre- determined number, M1 (module 1) to M5 (module 5). The electrical positive and negative are on opposite ends of the module such that each end cap is designated as the module’s N (negative) or P (positive) end. The MBBs embedded in the end caps identify themselves as M1P (module 1 positive) through M5P (module 5 positive), or M1N (module 1 negative) through M5N (module 5 negative). This yields a network of ten total MBBs in a 5 X 223SP configuration.

Figure 1.6: Photograph of a single A123 Systems 22S3P lithium-ion battery module with major features labeled. Note that the thru holes in the plastic end caps are for M8 Metric (or 5/16 English) fastener hardware.

Technical requirements specified for the health of the A123 Systems modules include thermal management that ensures the maximum cell temperatures remain below 60 deg C, and the modules are to be isolated from vertical vibrations of the chassis through the use of rubber mounts. To prevent resonance from vehicle vibrations, it was stated that the natural frequency of the overall battery system must meet or exceed 50 Hz.

The entire A123 Systems 5 X 22S3P kit is shown in Figure 1.7. In addition to the battery modules, the remainder of the battery pack kit components include: 1. BMS Module: The BMS (Battery Management System) is an ECU provided by A123 Systems that serves as the master controller for the battery. It interfaces to the vehicle CAN (car area network) network for receiving commands to activate/shut down the battery, and also interfaces to the sensor and communications harness inside the battery pack to receive information from the MBBs.

2. EDS Module: The EDS (Electrical Distribution System) contains current and voltage measuring devices that report to the BMS in addition to isolation relays. The isolation relays are controlled by the BMS, and isolate (engage) the high voltage from the battery pack from the vehicle DC bus when the vehicle is off (on). 3. LLS: The LLS (liquid level sensor) provides information to the BMS in regards to cooling system leak detection. It is critical to position the LLS at the bottom of the battery pack interior to detect the leak as soon as possible. When a collected pool of fluid is detected, the BMS immediately opens the isolation relays in the EDS. 4. MIS: Included with the high voltage wiring cables is a connector to support an MIS (manual isolation switch). As a required safety feature, the MIS is located electrically after module 3 positive (M3P) and module 4 negative (M4N). When the MIS is opened and locked out during vehicle high voltage servicing, the battery pack is disabled from being able to induce 363 volts if the vehicle was attempted to be started.

Figure 1.7: The A123 Systems battery kit as laid out when unpacked during the initial Generation I battery pack build (Li-Ion: Lithium Ion, HV: High Voltage, EDS: Electrical Distribution System, BMS: Battery Management System).

2.0 BATTERY REDESIGN CONCEPTS

2.1 Overview of Chapter In the previous introduction chapter, namely at the end of Section 1.2, the problem statement of redesigning the Generation I battery pack was laid out, specifically the goals to achieve in the redesign. The largest issue declared in the problem statement was the reduction of battery pack weight such that the vehicle can fall below its GVWR (gross vehicle weight rating). The use of composite materials (both for structural and non-structural elements) was explored in the conceptual phases of the battery pack redesign as a partial solution for enhancing weight reduction. A literature review in this chapter explores the use of composites in automotive applications to scope previous and current trends in this area.

For the sake of easing high voltage wiring and overall serviceability, the problem statement laid out at the end of Section 1.2 also targeted complete modularity of the redesigned battery pack such that its entire assembly-disassembly can occur outside the vehicle. The redesigned battery pack also requires a downsized cooling system to aid in a more compact packaging scheme. A literature review in this chapter of current hybrid and electric vehicle battery packs targets their packaging schemes and level of modularity, as well as methods of thermal management and any conventional or lightweight materials used to create their structural enclosure. This is to compare approaches taken in the redesigned battery pack to current battery pack solutions. A look at the energy to weight ratio (kWh/kg) of each reviewed battery pack is included to rate the designs in this thesis. This metric gives an overall sense of how well-designed the battery pack is, with low ratios indicating too much weight for the energy stored.

Following the literature review is a discussion of redesign concepts. There were a total of four complete concepts rendered fully in SolidWorks 3D CAD, designated as Generations II through V. Generation V became the implemented solution.

2.2 Review of Composite Materials in the Automotive Industry During the recent push for more environmentally friendly vehicles in the automotive industry, there is a discussion of using composites that feature natural fibers over traditional fiberglass and carbon fiber [1-5]. For example Ashori discusses the concept of wood-plastic composites, which feature wood or plant fibers such as jute or flax as the reinforcing fiber embedded within a thermoplastic or thermoset resin [1]. Marsh proposes a strategy of recyclable thermoplastic resins in conjunction with natural plant fibers as opposed to non- recyclable thermoset resins and synthetic fibers (such as aramid or fiberglass) that cost energy to produce [2].

Literature suggests that these environmentally friendly composites fill a niche of “secondary and tertiary structures…(that) do not require the high mechanical properties that advanced composites possess” [3]. Examples of secondary automotive structures using plant fibers include elements of the cargo floor, door panels, seats, interior trays and shelves, interior roof headliners, and underbody filler panels such as in the Mercedes A-class [2,4]. Due to their hollow tubular structure, plant fibers offer superior noise and heat insulation for panels separating the engine and interior compartments, as well as insulation from outside noise when applied in door panels [3]. These plant fiber based composites are displacing their over-engineered fiberglass predecessors in these secondary structural niches while forty percent less dense than traditional fiberglass fibers for vehicle weight savings [2,4].

For its hay baler doors, John Deere uses a polyurethane resin (called SoyOyl) that is derived from soybean oil as opposed to petroleum. The resin features preservatives to prevent biodegradation during the lifetime of the product, and has equivalent mechanical strength to the petroleum based equivalent. To reduce weight, John Deere replaces the steel doors on their balers with SoyOyl resin-fiberglass reinforced doors. Although this is a more structural

role than the aforementioned automotive roles in bio-composites, note that this is not achieved with the use of natural plant fibers [2,4,5].

A sense of the global environmental impact, from manufacturing the material, to in- product use, to end of life recyclability, is being increasingly emphasized. For example a study by Joshi et. al. discusses reduced emissions and increased fuel economy for the in- vehicle use of plant fiber composites that are lighter than their fiberglass equivalent, and also notes that the production of synthetic fiberglass requires significant energy consumption over naturally grown plant fibers [5]. In addition, synthetic fiber production releases dust, carbon dioxide, nitrous oxide, and sulfur oxides [2]. There is also a tendency to increase the volume fraction of fibers in the plant-based composites over their fiberglass equivalent due to relatively lower strengths in plant fibers. Despite this, the low density of plant fibers still allows for a lighter product relative to the fiberglass version, and the higher volume fraction of plant fibers displaces the use of plastic resins which also require a high amount of nonrenewable energy consumption when manufactured [5]. Concerning end of life environmental impact, while the burning of natural plant fibers may release carbon dioxide into the atmosphere, the absorption of carbon dioxide when nature creates the plant fibers offsets this effect [5]. On the resin side of the equation, thermoplastics (such as polypropylene, polyethylenes, nylons, polyvinylchlorides, polystyrene) offer recyclability as they can be melted down, while thermosets (polyester, vinylester, epoxy and polyurethane) cannot and are harder to dispose of [2,3,4]. The previously mentioned SoyOyl resin- fiberglass used by John Deere, and the previously mentioned natural fiber-synthetic resin composites are all one part natural and one part synthetic. This dichotomy creates a challenge at the end of its life. It is not completely natural to be recycled agriculturally, nor is it completely synthetic to be recycled industrially. The composite is either downcycled, meaning that when recycled it is worth less, or it is incinerated to capture some of its energy in the form of heat [3]. Examples of such downcycling including grinding down thermoset composites into powder that is reused as filler for another composite, or the re-heating and

14 re-shaping a thermoplastic composite into a new part. Cycling a thermoplastic part has limitations as properties degrade with each cycle [6].

To make composites that can be truly recycled through biodegradation, it will be important to use resins and fibers that do breakdown from enzymatic reactions that take place during composting. In addition to natural resins, there are also a select few synthetic resins that are biodegradable. Netravali and Chabba mention a few examples of fully biodegradable composites, including Biopol resin manufactured by Monsanto coupled with pineapple and henequen fibers that achieve the strength of wood in the grain direction with a mere 28% of fiber content. For primary structural functions, the authors cite research where such bio-composites achieve strengths equivalent to fiberglass-epoxy or steel. For example the combination of ramie fibers and soy protein concentrate (SPC) resin offer a 275 MPa strength, and hemp fiber with starch-based resin achieves a strength of 200 MPa. Research into spider silk fibers showing tensile strengths very superior to steel is underway. For the near term, these fully biodegradable highly structural composites are still in the research phase, and the current lack of mass production makes material costs for the bio-resins quiet higher than today’s thermoplastic and thermoset synthetic resins [3]. Marsh also discusses a push to bring natural fiber composites from their present non-structural roles to more structural roles in the future [2].

Prior to the recent environmentally-friendly bio-composites movement, the automotive industry has been involved with synthetic composites for the last several years to some extent, with the limitations dictated by economics of low-cost mass production required in the industry. In a 1986 journal article, Beardmore and Johnson note that only low-volume exotic or experimental vehicles showed extensive use of fiber-reinforced polymers (FPRs) in the chassis, such as Lotus vehicles that mix FRP pieces with a metal backbone taking the largest loads, or an entirely FRP chassis prototype made by Ford using graphite fibers and hand lay-up. The Ford prototype saved 1246 lbs over the steel equivalent while achieving the same performance, showing that FRPs can take dominant structural roles, the only problem

15 being that the concept car used a very slow hand lay-up process not suitable for mass production [7].

Beardmore and Johnson frequently refer to the SMC (sheet molded compound) method for mass produced, secondary structure parts such as grille opening panels near the radiator, hood panels, trunk panels, door panels, or in some cases secondary structural passenger cabs that are dropped onto a structural frame in heavy-duty trucks. SMC composites typically consist of chopped fiberglass pre-impregnated with a high viscosity thermoset resin rolled into sheets. The sheets are unrolled at the final production, cut into segments called charges, and the charges are compressed under 1000 psi in heated steel presses that cure and shape it into the final product. Only 80% of the charge covers the mold initially, and it expands under the heat and pressure during the process (three to four minutes). In order for SMCs to take more structural roles in automotives, the fiberglass must be continuous rather than chopped, and uncertainties need to be addressed in regards to how to control the fiber positions and orientation as the charge expands in the mold. Without precise control of continuous fibers, the finished part would feature structural variability and uncertainty. Over-engineering to compensate for this and the fact that a large-area structural part would need to also be designed to withstand the 1000 psi in the press, SMCs may not provide the lightest solution possible in automotive structures [7].

Other ideas discussed by Beardmore and Johnson include high speed resin transfer molding (HSRTM), illustrated by the authors as first taking a large cut sheet of fiberglass and pressing it initially into a dry glass preform (achievable with an epoxy binder press). For structural stability, the preform provides more control of fiber orientation over SMCs in that the fibers will not move during the final press process. To finish the product, the preform is then placed into a low-pressure heated steel press, where resin is injected under pressurized feed throughout the preform while it cures. Because the press clamps with less pressure than in the SMC method, this reduces having to engineer the part for the press. The HSRTM concept is a faster and more automated version of today’s resin transfer molding (RTM), and

16 can be sped further with multiple resin injection sites and the development of lower viscosity resins. A variation of the HSRTM method is the RIM (reaction injection molding) method, where the glass preform is inserted to an RIM press that injects reactants that cure into a resin within thirty seconds. This provides a more attractive option for mass producing composite automotive structures, but significant monetary investments are needed in HSRTM or RIM presses to prove the concept [7].

The aforementioned SMC, HSRTM, and RIM methods all use thermoset resins, while the method of thermoplastic stamping uses fibers that are pre-impregnated with a thermoplastic resin. The pre-impregnated material is heated to the molten point of the thermoplastic, and placed into a stamping press where it cools into its final shape. This method has been used successfully to produce secondary structural automotive parts such as secondary beams within bumpers, cargo floors, and seat components. Typically, these parts are made with polypropylene or other thermoplastic resin featuring chopped or continuous fiberglass, or wood fillings. Because the overall product does not exhibit high strength for primary structural applications, there have been attempts to add pre-impregnated unidirectional tapes to critical areas of the component. Having to add the tape and the fact that thermoplastic composites are harder to complexly shape make it challenging to develop large-scale primary automotive structures using thermoplastic stamping [7].

What has changed in the world of automotive composites since the above-discussed 1986 journal article by Beardmore and Johnson [7]? 1. Thermoplastic stamping (with polypropylene the most popular choice) incorporating natural plant fibers has been adopted for secondary structural automotive parts in the recent environmentally-friendly bio-composites movement, although some combinations of plant fiber and thermoplastic resin are not compatible due to degradation of the plant fibers during the heating process [1,2,3,4]. There has been research in ensuring quality control of continuous fiber thermoplastic stamped products. Challenges include thermal gradients that induce viscosity gradients,

leading to non-uniform displacement (also called draw-in) of the underlying continuous fiber and hence inconsistent structures [8,9]. Development of thermoplastic sandwich structures that can be pressed in one step (in as little as 40 seconds) can lead to efficiently-produced car body structures [10,11]. Thermoplastic structures also show potential in energy absorbing crash zone structures [12]. The first primary structural application using a thermoplastic composite was done by Volvo, applying a thermoplastic rear differential support in the chassis to reduce vehicle weight. Lighter and vibration absorbing engine air intake manifolds; front end modules incorporating accessories such as headlight openings and fan supports; in- door secondary structural carriers; decorative exterior body panels (fenders and tailgates for example); secondary bumper beams that are not damaged in minor incidents; primary crush zone bumper systems such as in the BMW M3 or Chevy Venture; and under-engine shields such as those on the Citroen C5 and Chrysler PT Cruiser are all recent advents of thermoplastic composites [6]. Dow Automotive developed an adhesive to bond polypropylene to metal, leading to hybrid metal- thermoplastic composite structures that do not need welds or fasteners. Such technology was employed for a thermoplastic-metal hybrid Volkswagen front end module (holding radiator, headlights, hood latch). The module fills the gap between the upper and lower transverse front steel chassis structural members, a gap that allows for more access to the engine compartment but forces the module to compensate for front-end structural stiffness lost from the gap. The contiguous adhesive bond between the thermoplastic and reinforcing steel elements of the module does not induce stress concentrations traditional fasteners create, and therefore module is overall stronger, easier to design, and lighter [6, 13, 14]. 2. Recent work in RTM (resin transfer molding) and RIM (reaction injection molding) includes research into speeding the mold closed-open cycle time, having so-called Class-A surface finishes for exterior automotive panels that are manufactured by RTM, and modeling of mold forces and resin flow with CAE (computer aided engineering) to optimize mold tooling design and ensure quality parts [15-20].

General Motors developed a viable prototype vehicle front end for crashworthiness consisting of upper and lower transverse chassis rails that are constructed with tri- axially woven fibers over foam core, coupled with an RTM panel joining the upper and lower members [21]. The Mercedes Benz McLaren SLR uses a variety of RTM processes, including one to create the upper body structure and roof from fifteen preforms that are assembled in a mold. The structure uses a sandwiched foam core, with foam injection taking 15 seconds, and the epoxy resin injection and curing taking 30 minutes. The upper body structure in the McLaren SLR is responsible for the torsional and bending stiffness of the chassis [22]. Exterior panels and large cargo boxes for Chevrolet pickup trucks have been employed using a chopped fiberglass preform that is subsequently placed in an RIM press where resin injection lasts a mere five seconds, a cure time is only two minutes [6]. 3. Recent applications of SMC (sheet molding compound) are several panels of the new generation Ford Thunderbird, the Renualt Espace and Avantime exterior panels are largely SMC, and the new generation Chevrolet Corvette uses a variety of SMC exterior panels as it has for several years. Because SMCs have potential to release gas under high temperatures when in the paint booth, coatings have been employed for preparation for painting, or embedded pigment technology that has SMC panels pre- painted before vehicle manufacturing. Work has progressed to provide increased shelf life for raw SMC, improved so-called Class A finishes for vehicle exterior panels through lower density formulations, and the introduction of S-glass and woven rovings to strengthen SMCs. This makes SMCs more durable for applications of spoilers, sedan rear boot lids, and truck tailgates [6].

In summary, there are previous examples of entire composite vehicle chassis, recalling the hand lay-up 1980s Ford graphite concept for instance [7]. This means that composites can take primary structural roles in automotives, the problem being that composite automotive structures are not available for high volume by means of a cost- effective mass production technique. Precise control of fibers for primary structural

19 applications through the development of preforms that are subsequently placed in RTM and RIM presses may realize mass production of primary automotive structures in composite form such as a full chassis [7]. In fact, elements of such RTM and RIM techniques have been adopted for the primary structure of the Mercedes McLaren SLR roof [22], the mass production of the cargo boxes in recent Chevrolet pickup trucks [6], and concepts such as the front crash zone of a vehicle developed by General Motors [21]. In addition to RIM/RTM, the prior mentioned Volvo rear differential support is a primary structural role filled by a thermoplastic composite [6], and research continues to improve the quality of thermosplastic composites to upgrade them to structural roles [8-12]. But today, a full-body composite chassis still belongs to low-volume, exotic or specialty cars such as the Tango two-passenger electric car manufactured by slow techniques such as hand lay-up [6]. Also note that all aforementioned primary structural automotive composites in this paragraph use synthetic composites, with natural plant fibers reserved for sub-structural roles [2,3,4]. Fully biodegradable composites using bio-resins and natural fibers strong enough for primary structural roles are still in the research phase [2,3].

2.3 Review of Current Hybrid and Electric Vehicle Battery Packs Zolot et. al. [23] directed their study of a Toyota Prius battery pack in terms of the packaging of the battery pack cells, and evaluate the resulting thermal performance of the pack to determine the quality of the battery pack packaging. Similar to the A123 System battery modules discussed in Chapter 1, the Prius battery pack uses stacked prismatic cells. Unlike the A123 5 X 22S3P system which has five stacks of prismatic cells (where each stack is termed a module), the Prius battery has a single long stack that comprises the battery. The cell chemistry is NiMH (nickel metal hydride), and is sized smaller for a parallel hybrid application rather than a large-sized pack for EREV/fully-electric vehicles. The cells are air cooled by a blower, with intake air drawn from the passenger cabin through an intake plenum and exhausted to the vehicle trunk and outside. This forced convection system is sufficient for cooling the Prius pack for its parallel hybrid demands. The NiMH cells are spaced from each other by protruding dimples on the cell sidewalls, allowing for slots of air to flow

20 between cells in the forced convection system. The Prius pack features 38 cells in series for a pack voltage of 273.6 V, and an amp-hour capacity of 6.5 Ah (or 1.78 kWh if the two numbers are multiplied). From photography shown in the study, the stack of prismatic cells is encased in a thin-walled metal enclosure transversely bolted to the chassis behind the rear seats and in front of the spare tire pan. The unit appears to be removable as a single modular unit. The complete battery pack mass is 53.3 kg [23], yielding a 0.03 kWh/kg energy density figure for the Prius pack.

The first generation Honda Insight vehicles also featured a NiMH battery located in the cargo area, and houses its Panasonic cells carrying a total of 0.9 kWh of stored energy in a package that weighs 20 kg [24]. The Honda Insight battery also uses air cooling [25].

In the mid-1990s, General Motors filed a patent for a novel electric vehicle battery pack restraining system [26]. An array of battery modules are lined to run fore-aft in the vehicle, with a second set of modules lined perpendicular to the fore-aft line to create an overall T-shaped configuration from an aerial view of the battery. The mechanical anatomy of the packaging system is complex but unique, beginning with the large T-shaped foundation tray at the battery pack bottom. The T-shaped foundation tray features a multitude of rectangular recesses that battery modules sit within, and sculpting in such recesses structurally stiffens the tray. The foundation tray’s recesses provide some level of restraint at the bottom of the modules they hug, with restraint further up on the modules provided by what are termed interlock plugs. At either end of the battery modules are mechanical sockets that plug into corresponding sockets in the interlock plugs. For neat packaging, the interlock plugs feature passages that allow module-to-module wiring and the passage of cooling lines if the batteries require water cooling. Finally the sides of the interlock plugs feature vertical slots that accept corresponding vertical ribs located in the interior sidewalls of the battery pack’s top lid. As the top lid is installed, a spacer of spongy polypropylene foam (Eperan-pp made by Kaneka Texas Corporation) becomes compressed between the top interior surface of the lid and the top of the modules, with the preloaded foam pushing down on the modules to

21 further secure them. The patent emphasizes that the battery pack foundation tray and lid together serve a structural role in the chassis, and more importantly that the battery enclosure does not weigh excessively such that it compromises electric vehicle range and performance. Therefore, the patent specifies the top lid and foundation tray be made of polypropylene reinforced with fiberglass (top lid featuring 10% by weight fiberglass, foundation tray featuring 30% by weight fiberglass) [26].

Table 2.1 Comparison of NCSU EcoCAR Generation I battery pack to production hybrid vehicle battery packs Vehicle Pack Energy Energy to Chemistry Cooling mass (kg) (kWh)1 Weight Ratio Method (kWh/kg) NCSU 408 22.3 0.055 Lithium-Ion (Li-ion) Water EcoCAR Gen. I Battery 2002 Toyota 53.3 1.78 0.033 Nickel Metal Hydride Air Prius2 (NiMH) First 20 0.9 0.045 Nickel Metal Hydride Air4 Generation (NiMH) Honda Insight3 Tesla 450 53 0.118 Lithium-Ion (Li-ion) Water Roadster5 GM EV I, 1175 16.5 0.014 Lead Acid (PbA) Air First Gen.6 GM EV I, 481 29.1 0.060 Nickel Metal Hydride Air Second (NiMH) Gen.7 Chevrolet 197 16 0.081 Lithium-Ion (Li-ion) Water Volt8 1 Wh capacity of packs calculated by product of pack voltage and amp-hour capacity. 1 kWh = 1000 Wh 2 Toyota Prius pack figures from reference [23] 3 Honda Insight pack figures from reference [24] 4 Note on Honda Insight battery cooling from reference [25] 5 Tesla Roadster battery pack figures from reference [32] 6 Information on GM EV I battery statistics for first generation from reference [27] 7 Information on GM EV I battery statistics for second generation from reference [28] 8 Chevrolet Volt battery pack figures from articles, see references [29] and [30]

More or less, the GM EV1 electric car featured a T-shaped battery pack in the likeness of the aforementioned GM patent. The first generation EV1 battery pack featured twenty-six Delphi lead acid batteries producing 312 volts with a total 53 Ah capacity (16.5 kWh if the two numbers are multiplied), with the battery pack mass at 1175 kg, awarding a 0.014 kWh/kg energy density [27]. The second generation EV1 battery pack featured twenty- six Ovonic brand Nickel Metal Hydride (NiMH) batteries producing 343 volts with a total 85 Ah capacity (29.1 kWh if the two numbers are multiplied), with the battery pack mass at 481 kg, awarding a much improved 0.060 kWh/kg energy density [28]. Air cooling was the method of thermal management in the EV1 battery system [25]. A look at the new EREV Chevrolet Volt T-shaped battery pack also reveals a likeness to the patented T-shape design discussed in the previous paragraph, this version using 288 lithium-ion prismatic cells from LG-Chem packaged in 9 modules total delivering 16 kWh of stored energy in a 435 lb (197 kg) package [29,30]. This yields an energy density of 0.081 kWh/kg. The Volt battery pack uses a water cooling system, with thin aluminum plates integrated within the cell stacks that feature small water channels. During cold ambient temperatures, an electric heater warms the water. A chiller within the air conditioning circuit assists during hot ambient temperatures; otherwise the coolant circulates through a radiator at the front in normal conditions [29]. The T-style battery pack is a single modular unit that is installed into the chassis from under; the vehicle is raised on a lift and the battery is pushed up on a jack into its designated tunnel space underneath the vehicle chassis [31].

The Tesla roadster battery pack takes a different approach to electric vehicle battery pack design. Rather than using a prismatic cells stacked into battery modules, the Tesla roadster pack uses several small (about AA sized) cylindrical lithium-ion cells, 6800 total quantity for one pack. The individual cells are steel-shelled, which the authors claim can increase the overall pack safety if the cells are subject to significant mechanical loads. Tesla chose aluminum housing for the battery pack. Although specific details are not mentioned, Berdichevsky et. al. claim to house the battery “with layers of protection” and “high levels of redundancy” [32]. The Tesla pack is water cooled with a 50/50 mix of water/ethylene glycol.

Each cell’s outer steel shells provide good thermal conductivity to the cooling; and by using several cylindrical cells, cooling surface area is increased [32]. The Tesla battery unit is also completely modular, with installation occurring by first lifting the vehicle, afterwards lowering the vehicle onto the battery pack and subsequently fastening it to the chassis [33].

With the recent growth spurt of hybrid electric and fully-electric vehicles, these are just some of the many battery packs in the industry. The Toyota Prius and Honda Insight batteries represent the increasingly popular parallel hybrid vehicle market where a small electric assist motor propels the vehicle in parallel with an internal combustion engine. The GM EV I represents a fully electric vehicle that showed potential in the 1990s and early 2000s, while the Tesla Roadster and Chevrolet Volt demonstrate new electric vehicles and highly-electrified hybrids (EREVs). Table 2.1 is a summary of major statistics regarding each reviewed battery. A trend can be noted with the chemistry versus the energy density (kWh/kg), with heavy lead-acid batteries taking the bottom, NiMH taking the middle, and lithium ion being superior. The listed mass for the Generation I battery in the NCSU EcoCAR vehicle is broken down later in Chapter 5. Note in Table 2.1 that the heavyweight Generation I battery is behind the lower end of the lithium ion class in terms of energy density. With each A123 22S3P module weighing 89 lbs (40 kg) and having 4.5 kWh of energy, the maximum potential achievable (that is if there was no enclosure) is 0.11 kWh/kg. Simply put, the heavyweight Generation I solution sub-optimally takes advantage of the high energy content in the 22S3P modules.

2.4 A New Compact Packaging Scheme A much more compact packaging scheme than in the previous Generation I battery pack was necessary to reduce intrusion into the rear interior volume such that the rear seating space could be reclaimed, and also to reduce the size of the surrounding enclosure to begin weight reduction potential. The brainstorming process occurred in the 3D CAD environment provided by Unigraphics (NX) 6.0 software, where a donated full model of the vehicle was

available courtesy of General Motors. CAD models of the A123 battery modules were inserted and positioned in a variety of configurations.

Figure 2.1: The revised packaging scheme to replace the Generation I battery pack (a) from the top view, and (b) from the bottom view.

The chassis of the subject compact-crossover SUV features an underlying skeleton of steel structural tubes/rails, with sheet metal steel filling the gaps between the rails. To avoid extensive waiver processes in the limited one year timeframe of this project, it was decided not to modify any structural members of the chassis to create space for the redesigned battery pack. Figure 2.1 shows the most compact solution derived with minimal impact to the vehicle structure. The sheet metal pan for the spare tire is removed behind the rear seat region, creating a sizeable vertical pass through between the fore-aft structural rails of the vehicle such that some of the battery could protrude underneath the vehicle. This strategy was not taken in Generation I, the entire battery was placed above the fore-aft structural rails which began the problem of high intrusion in rear interior volume.

As seen in Figure 2.1b, the vertical pass through size was fitting for only a single battery module, which is located offset to the passenger side to avoid interference with the

diesel fuel pump and filter. The remaining four battery modules are placed side by side in the rear cargo volume as shown in Figure 2.1a. To reduce costs and expedite the build phase, the cold plates from Generation I are recycled into this design. Figure 2.1 shows that the insertion of cold plates between the modules. Unlike Generation I which used ten cold plates (two per battery module), the downsized cooling system only requires that a single side of each module be exposed to a cold plate, reducing the total number of plates to four. As shown in Figure 2.1a, three of the four cold plates are placed in alternation with the battery modules. The fourth cold plate comes into contact with a single side of the lone module underneath the vehicle.

The overall shape of the battery pack in Figure 2.1 shows a top-heavy system. In other words, the top of the battery is larger than the bottom and the vertical pass through provided by removal of the spare tire pan. This meant that as a single modular unit, the battery pack would need to be hoisted in from the rear tailgate, and dropped into position. The overall vertical dimension of the scheme in Figure 2.1 was well within the vertical dimension of the chassis’s rear tailgate opening, realizing the potential for such an approach. The literature review in Section 2.3 suggests that this approach is different than the normal protocol taken for large-scale batteries in EREV and fully-electric vehicles where the chassis is lowered onto the battery, and not vice versa as proposed here. Batteries that are lowered into the chassis rear interior volume are normally reserved for smaller parallel hybrid batteries such as in the Insight or Prius that are light enough to be lifted in and out by hand. In the case of this battery, its sheer weight would require the use of a hoist when lowering into the rear interior volume.

Note the establishment of the vehicle coordinate system shown in Figure 2.1, where x is the fore-aft direction, y is the lateral direction, and z is the vertical direction. Positive x points toward the rear, positive y points toward the passenger side, and positive z points upwards. This Cartesian coordinate system is used forthwith. There were a total of four

concepts utilizing the scheme in Figure 2.1, designated as Generations II through V. Generation V became the design that was implemented.

2.5 Early Concepts (Generations II to IV) Early in the redesign phases, the battery pack was divided into two main sub-assemblies, the upper assembly consisting of the four module-three cold plate stack above, and lower assembly consisting of the bottom module, its associated cold plate, and the BMS and EDS units (recall the discussion of the BMS and EDS from Chapter 1, Section 1.3). The two subassemblies are buildable on bench tops, afterwards hoisting the upper assembly over the lower and mechanically fastening them together. This would mean that the upper assembly would require integrated lift points. The combined upper and lower assemblies are then re- hoisted, a sealing outer shell is bolted on, and the complete battery pack is lowered into the vehicle rear interior. A battery to chassis cradle is installed prior, which carries a plurality of vertical vibration-isolating mounts for the battery pack to rest on and be fastened to.

The early Generation II concept was the most composite-rich. For primary structural components, it was chosen to propose synthetic fibers as natural fibers are not yet primarily structural in the automotive industry (recalling Section 2.2). Concerning the resin side, currently it is much easier to access widely available synthetic resins over bio-resins. For this battery pack production, the most affordable and simplest method for building shaped composite components is a thermoset wet hand lay-up of the molded composite pieces over a mold rather than outsourcing autoclave facilities that can form a composite with thermoplastics. Another inclination to use thermosets over thermosplastics for this design’s primary structural components is the reluctance of thermoplastics to take significant structural roles in automotives (although there are limited examples such as the Volvo rear differential support [6] mentioned in Section 2.2 or the structural polypropylene exterior of the GM T-pack concept [26] in Section 2.3). Although choosing thermoset resins and synthetic fibers for structural parts is the least recyclable option; to cover the argument of future sustainability, it was concluded at the end of Seciton 2.2 that highly structural fully

biodegradable composites for primary structures are still in the arena of research and a possibility in the future.

For non-structural composites in this design, challenges in accessing thermoplastic composite forming facilities again limited this design to a wet hand lay-up with thermoset resins. Natural plant fibers have been used in non-structural automotive roles as mentioned in Section 2.2, and their incorporation into thermoset resins have been attempted [3]. For the compressed timeframe of this project, it was preferable to use internal knowledge and experience with synthetic fiberglass. Undoubtedly it is possible to investigate thermoset resin-natural fiber for similar projects with a longer design cycle time. And for the argument of future sustainability, based on Section 2.2 it is not difficult to imagine that the automotive industry can currently mass produce recyclable thermoplastic resin-natural fiber non- structural composite shells mass produced in the shapes and sizes conjured for this project as such parts are on the road today.

Figure 2.2 provides a look of the early Generation II concept and materials selected. The chosen thermoset resin-synthetic fiber composites were wet hand lay-up epoxy- fiberglass for shaped parts, and epoxy-fiberglass G10-FR4 sheets that are widely available for purchase as raw material for flat parts. It is intended that the main structure for providing the 8-g vertical and 20-g horizontal support is provided by the: 1. Ring subassembly that attaches to the upper assembly module-cold plate stack (Figure 2.2a) 2. The two-piece fiberglass-epoxy bottom lid (Figures 2.2c and 2.2d) 3. The fiberglass-epoxy cradle (Figure 2.2b) that connects the lower assembly to the two-piece bottom lid. With this in mind, one structural flaw includes the 8-g vertical support for the upper assembly module cold-plate stack, merely provided by the four M8 horizontal rods (Figure 2.2a) spanning the distance of the upper assembly. 8-g vertical and 20-g horizontal support is also questionable at the mere adjacency between the vertical walls of the bottom lid and

28 extended horizontal mounting flanges pictured in Figure 2.2e. A major fabrication challenge is also presented by the bent aluminum anchor brackets (Figure 2.2b) that attach the lower assembly to the upper. These issues led to abandonment of the early Generation II concept.

Figure 2.2: Early Generation II battery pack concept: (a) upper assembly, (b) attachment of lower assembly to the upper, (c) and (d) the attachment of fiberglass bottom lids, (e) idea to attach battery pack to vehicle. Aluminum bars with threaded holes (not shown) bolt onto the lower assembly fiberglass cradle such that the blue-colored bolts shown in views (c) and (d) allow the structural bottom lids to be connected to the fiberglass cradle.

In the Generation III concept, the horizontal M8 rod of the upper assembly are alleviated from carrying the 8-g vertical loads via carbon fiber-foam core panels placed above and below as shown in Figure 2.3a. The panels are exposed to significant bending in the 8-g vertical as they are pressed mid-section by the upper assembly stack and constrained

29 on either end by the vertical M10 fasteners. Loads captured by the M10 rods are then directly transferred to the ring subassembly via steel load transfer links in Figure 2.3c. The panels are chosen due to their superior bending strength-to-weight ratio. The upper assembly horizontal M8 rods are also alleviated from carrying 20-g fore-aft (x-axis) loading via the fore-aft restraining system in Figure 2.2b, the associated plates being made of steel as battery pack interior space limits here prevent the use of thicker carbon fiber-foam core panels. 20-g lateral (y-axis) loading from the upper assembly is directly caught by the ring subassembly (Figures 2.3a and 2.3c). The switch from Generation II to III also saw the downgrade of the bottom lids from structural to non-structural members. Part of this maneuver was to be able to slightly re-shape the exterior lids of the battery pack (due to changing cargo floor designs) without consequence to the structural finite element model. The structural connection from the ring subassembly to the vehicle chassis bypasses the bottom lids and goes directly to steel battery mounting brackets (Figure 2.3e) that bolt through the lids and into threaded holes found in the ring subassembly itself. Figure 2.3d shows that the rear of the two lower carbon fiber-foam core panels carries the weight of the lower assembly. Because threaded holes are not possible in composites, the addition of the aluminum spacer plate above the panel carries threaded holes to screw in the vertical bolts of the lower assembly.

The vertically thin dimension of the Generation III aluminum spacer plate in Figure 2.3d means limited thread engagement with the lower assembly vertical bolts, translating into potential structural issues with only a few aluminum threads supporting the weight of the lower assembly (especially at 8-gs). Therefore Generation III was slightly modified into a fourth generation concept, with the rear of the two lower carbon fiber-foam core panels replaced with a solid steel plate as shown in Figure 2.4. This increases the thread engagement with the lower assembly bolts and also allows for a novel tooth-interlocking mechanism for easing the alignment of the upper and lower assemblies as detailed in Figure 2.4.

Figure 2.3: Generation III battery pack concept.

Figure 2.4: Refined method of attaching the lower assembly to the upper invented during Generation IV. One of the carbon fiber-foam core panels beneath the upper assembly from Generation III is replaced with a steel plate with equivalent thickness. The steel plate features four threaded holes to accept the vertical M8 bolts of the lower assembly. Furthermore, a steel guide plate attached to the cold plate attached to the cold plate of the lower assembly features a tooth pattern that matches a corresponding tooth pattern on the steel plate. When lowering the upper assembly onto the lower, locking the tooth patterns together indicates the four vertical M8 bolts are aligned with the four threaded holes in the steel plate such that the M8 bolts are ready to be pressed up and spun into the threaded holes.

Some initial finite element analysis was conducted to determine the viability of the Generation IV structure meeting the required 8-g vertical and 20-g horizontal load requirements. The most critical loading is that of the 8-g downward case on the upper assembly steel plate. If this test failed, it was likely that the 20-g fore-aft loading cases against the fore-aft restraining plates shown in Figure 2.3b would fail as well.

The total mass of the upper assembly module-cold plate stack is 402.5 lbs (four 89 lb modules, three 15.5 lb cold plates). The upper assembly steel plate is one of two plates supporting the bottom of the upper assembly, and therefore it sees half of that weight at 1-g (201.25 lbs). Amplified for 8-gs, this yields a pressure load of 1610 lbs from applied at the contact patch between the module-cold plate stack and the steel plate. Meanwhile, the lower

32 assembly has a module-cold plate mass of 104.5 lbs (one module, one cold plate). If it is assumed this load is distributed evenly across the four vertical M8 bolts of the lower assembly, each bolt is pulled in tension at 26.125 lbs. Magnified for the 8-g down loading case, each bolt is pulled in tension at 209 lbs. Therefore each M8 threaded hole in the upper assembly steel plate sees a downward 209 lb force. The 8-g downward FEA (finite element analysis) loading on the upper assembly steel plate is shown in Figure 2.4, with six constraints applied at the contact patch between the steel plate and M10 washers the steel plate is pressed downward against.

During finite element simulations of the loading case shown in Figure 2.5, the steel plate failed with stress concentrations occurring against the washer bearing surfaces. Even if the steel plate had the cut-outs removed (i.e. was a solid piece), the downward deflection was not sufficiently reduced to minimize the stress concentration against the washers. If it became challenging to solve the 8-g downward restraining problem, it would be even more challenging to design a viable 20-g fore-aft restraining system (shown in Figure 2.3b). Furthermore, an overall view of the Generation IV upper assembly showed a plurality of required fasteners (for example the vertical M10 rods, hardware for the fore-aft restraining system, and load transfer links in Figure 2.3c). This increases the complexity of the assembly process, and therefore a rather different approach was necessary for the battery internal structure.

2.6 Final Concept (Generation V) A simplified abstraction of the Generation V concept battery internal structure is shown in Figure 2.6. The studies on the Generation IV system components such as the upper assembly steel plate indicate that the vertical 8-g loading cases are tough challenges associated with mechanical bending. This is because the battery pack mass is suspended between supporting points on either end by battery mounting brackets as shown in simplified form in Figure 2.6a. The Generation V battery internal structure consists of a rear and front steel brace that slip over the upper assembly module-cold plate stack. Carbon fiber sheets are clamped between

33 aluminum shims and the steel braces. The thin carbon fiber sheets have very little resistance to out-of-plane loading, but are far superior in handling in-plane loads. Because the carbon fiber sheets span open air between the steel braces, it is undesirable to have the battery modules of the upper assembly press down on them in open air as they will sag due to their little out-of-plane resistance. The purpose of the aluminum shims is to ensure that the only point of contact between the upper assembly modules and the carbon fiber sheets is where the sheets have steel brace material directly below for out-of-plane support when pressed on vertically by the modules.

Figure 2.5: Finite element analysis loading condition for the Generation IV concept upper assembly steel plate, 8-g downward load mode. The steel plate is at 3/8 inch thickness, and approximately 28 inches in length and 9 inches in width, and made of ASTM-A36 steel.

During the 8-g downward load cases, the steel braces will tend to bend downward as shown in Figure 2.6b. Because the top portions of the steel braces move closer together, the top carbon fiber sheet will trend toward compression. The steel braces move further apart on the bottom, and thus the lower carbon trends toward tension. Because the carbon fiber sheets are rather stiff in-plane (resistant to compression/tension), the sheets ideally reduce the downward deflection of the steel braces, hence reducing stresses experienced within the braces. When bolting the lower assembly mass to the rear steel brace as shown in Figure

2.6c, the rather large moment about the rear steel brace’s battery mounting bracket support causes it to rotate significantly, pushing the upper carbon fiber sheet in the –x direction, and pulling the lower carbon fiber sheet in the +x direction. Because the carbon fiber sheets translate, this causes the front steel brace to rotate in the same direction as the rear steel brace, creating an overall “parallelogram effect” as shown. This is undesirable as the carbon fiber sheets mainly translate rather than undergoing stress absorption. A diagonal link (Figure 2.5d) corrects the situation.

A finite element model of the structure shown in Figure 2.6 is detailed in Chapter 3. There were several iterations conducted with the model. The behavior illustrated in Figure 2.6 was indeed what occurred in finite element simulations. Figure 2.7 shows the upper assembly process for Generation V rendered in 3D CAD, while the same is shown for the lower assembly process in Figure 2.9. Figure 2.8 shows the construction of the rear steel brace, which consists of quarter inch thick ASTM-A36 steel sheets that are water-jet cut for precision. Note carefully that the shapes generated for each piece are designed to interlock with each other during the welding process, saving time and adding convenience for the welder in that no measurements are necessary when tacking on each piece. The AISI 1045 steel pieces on the sides shown in Figure 2.8e stiffen the steel braces further during 8-g vertical loading. AISI 1045 is chosen over A36 as the A36 equivalent would yield at stress concentrators during finite element simulations.

Note from Figures 2.7f and 2.7g the module-cold plate stack of the upper assembly uses the vertical M8 fastener thru holes of the modules rather than the horizontal ones as the previous design concepts (Generation II through IV) used. To eliminate the manufacture of as many parts as possible, horizontal compression plates for the upper assembly-module cold plate stack were dropped from Generations II through IV. In Generation V, what provides the horizontal compression to keep the upper assembly cold plates from slipping is the precise positioning of the vertical M8 fasteners in views (f) and (g) in Figure 2.7. Precise positioning of these vertical M8 fasteners is achieved by water-jet cutting the steel brace components

35 from machines that import the CAD model geometry. The horizontal center-to-center dimensions of the M8 holes in the steel braces are designed to provide a rather tight fit of the cold plates inserted between the modules. This tight fitting provides the horizontal compression of the upper assembly. This concept proved successful during the actual assembly shown in Chapter 4.

Figure 2.6: Simplified view of the mechanics of the Generation V battery internal structure during 8-g downward (negative z direction) loading. The battery internal structure (a) consists of a front and rear steel brace, a pair of carbon fiber sheets, and aluminum shims that define the contact patch between the upper assembly battery modules and carbon fiber sheets. Ideally if (b) the upper assembly modules pressed downward with an 8-g force, this would induce bending where the top carbon fiber sheet is in compression and the bottom is in compression. Because carbon fiber is rather stiff, this would reduce the deflection in the steel braces such that stresses in the steel braces are lessened. When (c) adding the lower assembly module in the 8- g vertical cases, a “parallelogram effect” occurs where the carbon fiber sheets translate rather than extend/compress, leading to high stresses in the steel braces. Adding a diagonal link (d)

corrects this, allowing the carbon fiber sheets to extend/compress and stiffen the structure properly.

Figure 2.7: Procedure for Generation V concept upper assembly. Assembly progresses from views (a) to (m). As shown in (k) through (m), two of the M8 vertical rod fasteners from view (f) are borrowed for installation of the battery pack fuse.

Recall from Section 2.5 that the Generation IV concept had a novel tooth-interlock concept for aligning the upper and lower assemblies during the joining process. This concept was recycled into Generation V as shown in Figure 2.10. Figure 2.11 shows the final installation procedures after joining the upper and lower assemblies, including how to attach the top lid. View (d) in Figure 2.11 shows that a lip made of aluminum is attached with thru fasteners (nut and bolt combination) to a perimeter flange formed by the joined two-piece bottom lid. Steel threading is ideal over aluminum due to robustness against thread stripping,

37 and therefore steel nuts are bonded to the aluminum lip by JB weld adhesive (bonding is ensured by scuffing the smooth aluminum surface with sandpaper). The zoomed-in inset for Figure 2.10d shows that the aluminum lip is able to sit flat on the bottom lid perimeter flange as the bonded nuts underneath the lip pass through larger recess holes drilled along the perimeter flange. Note in Figure 2.11 that the top lid is attached after the battery pack is hoisted into the rear interior. The bonded nuts attached to the aluminum top lid lip provide threaded holes for which to screw in the securing bolts of the top lid.

Figure 2.8: Welding procedure for construction of the rear steel brace. Assembly progresses sequentially from views (a) through (h). The front steel brace undergoes a similar procedure. Unless otherwise stated, all flat steel pieces are quarter-inch thick ASTM-A36.

2.7 Generation V Mounting System Figure 2.12 provides a more detailed view of the installation of the Generation V battery pack to its battery to chassis cradle. When the battery pack is dropped onto the cradle, the battery mounting brackets at each corner of the pack are aligned with the M12 rods of the

38 powertrain mounts in addition to vertical M16 bolts welded to the cradle. Vertical support is provided when the battery pack sits on the four vibration-isolating mounts of the cradle. Because the horizontal support is at a rather strict 20-g requirement, finite element studies on the battery mounting brackets revealed that horizontal restraining solely by the powertrain mount M12 rods could not be achieved successfully (bearing stresses on the M12 holes exceeded yield stress). This is where the addition of the vertical M16 bolts are needed, which provide additional horizontal support. This also reduces the horizontal shear on the mounts in the 20-g cases.

Figure 2.9: Procedure for Generation V concept lower assembly. The assembly occurs on top of a pair of wooden 2” x 4” beams. Assembly progresses from views (a) to (e).

Figure 2.10: Procedure for joining the upper and lower assemblies in the Generation V concept. The interlocking teeth concept for aligning the upper and lower assemblies is maintained from Generation IV (Figure 2.12).

Figure 2.13 details the attachment of the cradle to the vehicle chassis. This is achieved through the utilization of anchor bolt connections (M16 size). The anchor bolt connections come in two varieties designated as types I and II. A type I connection is a classic nut-bolt combination that clamps the cradle to the chassis’s fore-aft box structural rails. Type II connections are designed for attaching the cradle to transverse structural rails which the cradle floats above. The type II anchor bolt connection is detailed in the right view of Figure 2.13. The 8g-up loading case represents the battery pack pulling up on the cradle with high force such as during a vehicle rollover accident. In such a scenario, the powertrain mounts pull up at their respective locations on the cradle while the cradle itself is pinned down by the washers beneath the anchor bolt heads. Note that the rear two powertrain mounts are behind all anchor bolt heads, the closest being the type I connections nearby. Therefore one can imagine a moment arm (about the y-axis) from the two rear powertrain mounts to the nearest type I anchor bolt heads. 8g-up loading cases in finite element show that such a moment would snap off the rear portion of the cradle. When fixing rear portions of the cradle to the chassis fore-aft structural rails (such as by welding), this eliminated the excess moment arm and in the 8g-up loading cases. This explains specifying welding the rear portion of the cradle to the chassis fore-aft structural rails as shown in Figure 2.13.

Figure 2.11: Final installation procedures for Generation V battery pack. Procedure is sequential from views (a) through (g). Procedures (a) through (d) are performed with the battery pack airborne on a hoist, afterwards the battery pack is dropped onto its four mounts and fastened down in the rear interior of the vehicle, view (e). The four MIS (Manual Isolation Switch) fasteners in view (g) are vertical bolts that pass through the MIS and holes in the top lid, and screw into threaded holes found in the MIS foundation shown in Figure 2.6i.

Figure 2.12: Generation V battery pack mounting system. The battery to chassis cradle (a) utilizes four total M12 threaded studs (from each mount) in addition to eight vertical M16 threaded studs to restrain the battery pack. A view of the entire system after dropping the battery pack onto the battery to chassis cradle is shown in view (b). The battery pack is secured with four nuts that screw down on the vertical M12 studs and clamp the battery mounting brackets to the powertrain mounts.

Figure 2.13: Methods of anchoring the battery to chassis cradle to the vehicle chassis itself. This is achieved through a system of anchor bolts, designated as types I and II as shown above. Type I anchor bolt configurations are classic nut-bolt combinations that clamp the cradle to the fore-aft box structural rails of the chassis. Type II connections attach the cradle to transverse structural rails the cradle floats above as shown in the detail to the right.

2.8 Generation V Interface Details Figure 2.14 shows a detailed view of all high voltage wiring connections necessary for the Generation V battery pack. These connections wire the five battery modules in series for a total of 363 volts. This voltage is readily available at the EDS, which (as discussed in Chapter 1, Section 1.3) features a pair of controlled isolation relays that engage/disengage the 363 volt supply to the remainder of the vehicle high voltage components upon vehicle startup/shutdown. As shown in view (a) of Figure 2.14, the EDS-to-vehicle connection is achieved with an external plug, which is essential for complete battery pack modularity from the vehicle in that the battery pack is essentially plugged (unplugged) from the vehicle when installed (removed). Two passages through the exterior fiberglass shell are required for the

43 battery pack wiring, one for the plug associated with the EDS-to-vehicle connection, a second associated with the plug for the manual isolation switch (MIS). A third passage through the exterior fiberglass shell is required for the inlet and exit pipes of the liquid cooling system. Lastly, a fourth passage on the top lid is designed for the fuse access panel. The location of these passages is shown in Figure 2.15. The fuse access panel, cooling pipe, and wire passage designs are detailed in Figures 2.16, 2.17, and 2.18 respectively. All passages shared common challenges in ensuring a seal from the outside environment (to prevent the entry of dust, water, etc.) coupled with the ability to be reversible (i.e. undone) in case the battery pack required future disassembly.

The central hub for the fuse access panel interface is what is referred to as the panel ring, made of 1/8-inch thick steel and located on the interior side of the top fiberglass lid as shown in view (a) of Figure 2.15 and 2.16. Four flanges on the extremities of the panel ring feature thru holes that allow for thru fasteners (nut and bolt combination) for attaching the panel ring to the top fiberglass lid. The location of the four sets of thru fasteners is shown in Figure 2.15a. Because the fiberglass holes are drilled slightly larger than the bolt shanks of the thru fasteners, a leak is possible in the gap between the bolt shanks and holes. This gap is sealed via RTV sealant applied underneath the thru fastener bolt heads as shown in Figure 2.16. Once secured to the top fiberglass lid, the panel ring is permanently a part of the lid such that the semi-permanent RTV sealant does not need to be undone. The panel ring provides a set of threaded holes to accept the ends of the bolts associated with the aluminum fuse access panel. An underlying gasket provides a seal as the aluminum fuse access panel is tightened against the exterior face of the fiberglass top lid.

Details of the internal cooling system configuration are shown in Figure 2.19. Located interior-side on bottom lid part 2, a 1/8-inch thick steel pipe entry/exit plate is the central hub for the pair of liquid cooling pipe passages through the fiberglass shell. The shape of the plate is shown in Figure 2.15b, while the method of attaching or separating the fiberglass bottom lid piece from the plate is shown in Figure 2.17. Figure 2.15b shows that

44 the pipe entry/exit plate features two large holes that allow for the passage of the pipes. The plate is water jet cut from a CAD model for precision, with the hole diameters on the plate specified to be equivalent to the outer diameters of the pipes such that the plate fits snug on the pipes while still allowing for the plate to be slipped up or down on the pair of pipes during view (a) of Figure 2.17. As seen in that same view, a gasket is tacked onto one side of the plate with RTV sealant such that the gasket stays in position throughout installation. While progressing from view (a) to (b) in Figure 2.17, the plate is first slid down the pipes such that it later becomes pushed up by the bottom fiberglass lid, ensuring the plate is pressed against the fiberglass lid during installation. The plate also features three threaded holes to accept the ends of the bolts during view (b) that secure the fiberglass lid piece to the plate. The plate becomes increasingly clamped against the fiberglass lid as the bolts are tightened, compressing the gasket in between to create a seal between the plate and fiberglass lid. Additional RTV sealant is applied during view (b), at the circumference of the pipes to ensure a seal between the outer diameter of the pipes and the holes of the plate. Ideally during future assemblies and disassemblies, the RTV sealant permanently attaches the gasket to the plate and the plate to the pipes as shown in view (c) such that these elements stay in. However, re-application of RTV sealant as shown in (a) and (b) is possible if the RTV bond is lost.

Views (c) and (d) of Figure 2.15 show a pair of 1/8-inch thick steel wire grip plates, one located on the interior side of the top lid for the MIS (manual isolation switch) wire interface, the second on the interior side of bottom lid part 2 for the EDS-to-vehicle wire interface. Because the plugs associated with these interfaces each feature two high voltage wires, each wire grip plate features two standard, purchasable wire grips (also called cord grips in the industry) to support each wire’s passage through the exterior of the battery pack. View (a) of Figure 2.18 shows all preparation for the installation of the wire grip plate. Each wire grip plate is water jet cut from a CAD model for precision, with each plate featuring a pair of holes cut slightly larger than the outer diameter of the wire grip bodies inserted through them. The back side of the wire grip body outer diameters feature threads such that a

securing nut also sold with the wire grips can be screwed on. As shown in view (a) of Figure 2.18, the securing nuts in this case are screwed onto the wire grip bodies to clamp them to their wire grip plate. RTV sealant is applied underneath the securing nuts such that the sealant is compressed when wet to fill the gap between the outer diameter of the wire grip body and its associated hole in the plate. The tackiness of the sealant also ensures that the wire grip is permanently attached to the plate.

Figure 2.14: Planned internal wiring connections for the Generation V battery pack concept. M1N to M5N indicate modules 1 to 5 negative, while M1P to M5P indicate modules 1 to 5 positive.

Figure 2.15: Views of the four exterior interfaces on the Generation V battery pack. Each interface requires a scheme for a passage through the exterior fiberglass shell while maintaining a seal. For ease of disassembly, each interface also requires reversibility (i.e. the ability to be undone). The interface assembly schemes are shown in Figures 2.25 through 2.27.

Figure 2.16: Detailed design of fuse access panel passage through the fiberglass shell shown in Figure 2.14a

Figure 2.17: Detailed design of liquid cooling pipe passages through the fiberglass shell shown in Figure 2.14b. View (a) is during the first time installation, view (b) is after complete installation, and view (c) is after the fiberglass shell is removed during any future battery pack disassembly.

Figure 2.18: Detailed design of high voltage wire passages through the fiberglass shell shown in Figures 2.24c and 2.24d. View (a) is during installation, view (b) is after complete installation.

After attaching the wire grips to their corresponding wire grip plates, further preparations are necessary in view (a) of Figure 2.18. A gasket is tacked onto one side of the plate with RTV sealant such that the gasket stays in position throughout installation. Prior to the plug being installed on the wires, each wire is routed through its corresponding wire grip. The free ends of the wires are then stripped and crimped with the plug’s conductive crimp

48 lugs, afterwards having the plastic plug body slip and lock onto the crimp lugs. The front sides of each wire grip feature an additional nut that is tightened and loosened. As with any standard wire grip, this nut when tightened actuates fingers that squeeze an internal circumferential rubber element, which in turn squeezes the outer diameter of the wire such that the wire is (1) locked into position, and (2) a seal is created between the outer diameter of the wire and inner diameter of the wire grip. During view (a) of Figure 2.18, it is not yet known where exactly the wire grip plate should reside along the wires, so the wire grip nuts are kept loose such that the plate can be slid along the wires. When joining the wire grip plate to its associated fiberglass lid piece from views (a) to (b), the wire grip plate can be pulled/pushed along the wires and steered into position by grabbing from the wire grips themselves, afterwards securing the plate to the fiberglass lid piece by application of bolts that screw into threaded holes in the plate as shown in view (b). The gasket creates a seal between the plate and fiberglass lid piece as it becomes compressed in between during the tightening of the bolts. Once the wire grip plate is secured, the wire grip nuts are finally tightened as shown in view (b) to prevent the wires from sliding inside the grips while also creating the seal between the outer diameter of the wires and inner diameters of the grips.

Figure 2.19: CAD view of the cool and warm water manifolds of the battery pack. The manifolds are constructed of flexible PEX plastic piping that permits the curvatures shown in view (a). The segments of PEX piping are joined by quick connect “sharkbite” fittings shown in view (b).

3.0 BATTERY STRUCTURAL ANALYSIS

3.1 Review of Battery Pack Structural Elements Figure 3.1 provides orthographic and isometric views of the Generation V battery pack system with all non-structural elements (battery modules, cold plates, and exterior fiberglass shell) suppressed. The three main structural systems are: 1. The battery internal structure, a system consisting of the front and rear steel braces, diagonal links, and pair of carbon fiber sheets. The mechanics of this system was reviewed in Chapter 2, Section 2.6. 2. The four battery mounting brackets at each corner of the battery pack 3. The battery to chassis cradle that hosts the vibration mounts the battery pack sits upon. The purpose of this chapter is the documentation of the finite element analyses of these structures (executed in SolidWorks Simulation 2009) showing the capability to restrain the battery module-cold plate mass at the required, statically-induced 20-gs horizontal and 8-gs vertical. This over-static compensation ensures the restraining of the battery module-cold plate mass under dynamic conditions. Each module has a mass of 89 lbs, and each cold plate has a mass of 13.5 lbs.

Recall the vehicle coordinate system established in Chapter 2, with positive x pointing toward the vehicle rear, positive y pointing toward the vehicle passenger side, and positive z pointing upwards. Therefore x is the fore-aft coordinate, y is the transversal (or side-to-side) coordinate, and z is the vertical coordinate. For the purpose of orientation, views throughout this chapter will use this coordinate system as they did in Chapter 2.

3.2 Material Properties The materials used in all structural components of the battery pack include ASTM A36 hot- rolled steel, AISI 1045 cold-rolled steel, and the carbon fiber sheets. The material properties for A36 and 1045 steel are from the SolidWorks database.

Figure 3.1: Orthographic and isometric views of all structural elements in the Generation V battery pack concept.

Note from Figure 3.1 that the battery internal structure’s carbon fiber sheets lie in the vehicle coordinate’s xy-plane. When defining material properties, the carbon fiber sheet material coordinates will also follow the vehicle coordinates, meaning the x, y, and z of the carbon fiber sheets are aligned with the vehicle x, y, and z coordinates. The carbon fiber sheets are manufactured by Protech Composites based in Vancouver, WA [34]. The chosen product is the 1.7mm thick (no gloss finish) carbon fiber sheets, the properties of which were provided by consultation with the company. Given properties include the in-plane tensile strength S+, in-plane compressive strength S-, in-plane tensile modulus E+, in-plane compressive modulus E-, flexural strength, flexural modulus, short beam shear strength Sxy, and interlaminar shear strength (Syz/Sxz). To represent a single laminate of the carbon fiber sheet, a custom-defined linear elastic orthotropic model is defined in SolidWorks Simulation

2009, which requests elastic moduli Ex, Ey, and Ez; shear moduli Gxy, Gxz, and Gyz; poisson’s ratio Qxy,Qyz, and Qxz; shear strength Sxy; tensile strengths Sx+ and Sy+; and finally compressive strengths Sx- and Sy-.

Because the in-plane weave appearance does not show bias in the x or y, the carbon fiber sheet is assumed to be quasi-isotropic in the xy-plane. To deduce an elastic modulus E for the xy-plane, an average is taken between the reported tensile and compressive elastic moduli. This elastic modulus E is set equal to Ex and Ey: E E E E E (Eq. 3.1) x y 2

Because Poisson’s ratio is not given, it is assumed that Poisson’s ratio Qxy,Qyz, and Qxz are 0.23. With isotropy assumed in the xy-plane, the assumed Poisson’s ratio and elastic modulus E from Equation 3.1 can be used to estimate the shear modulus in the xy-plane: as is done in classic isotropic materials: E Gxy (Eq. 3.2) 2(1Q xy )

Now the challenge remains in defining the out-of-plane mechanical properties Ez, Gxz, and Gyz, which requires a visit to Hooke’s Law in three-dimensions:

H ij SijklV kl (Eq. 3.3) where Hij and Vkl are the strain and stress tensors respectively, and Sijkl is the compliance tensor. Classic solid mechanics instructs that in order for an infinitesimal element to have a moment balance, the off-diagonal shear terms in the stress and strain tensors need to be equal

(i.e. Wkl = Wlk for the stress tensor). Because the stress and strain tensors are symmetric, the same must hold true for the compliance tensor Sijkl. In Voigt notation, Equation 3.3 can be expanded, and for an orthotropic material is:

ª 1 Q yx Q zx º « 0 0 0 » E E E « x y z » «Q xy 1 Q zy » H 0 0 0 V x ½ « E E E » x ½ ° ° « x y z »° ° H y V y ° ° «Q xz Q yz 1 »° ° ° ° « 0 0 0 »° ° °H z ° E E E °V z ° ® ¾ « x y z »® ¾ (Eq. 3.4) H yz 1 W yz ° ° « 0 0 0 0 0 »° ° °H ° « 2G »°W ° ° zx ° « yz »° zx ° H 1 W ¯° xy ¿° « 0 0 0 0 0 »¯° xy ¿° « 2Gzx » « 1 » « 0 0 0 0 0 » ¬« 2Gxy ¼» Note that in Voigt notation, the strain and stress tensors have been reduced from second order to first order (i.e. are column vectors), and the compliance tensor has been reduced from fourth to second order (i.e. is a matrix). However, the symmetry law still applies to the compliance matrix, and therefore the off-diagonal terms must be equal, providing: Q Q yx xy E E y x (Eq. 3.5a) Q Q xz zx E E x z (Eq. 3.5b) Q Q yz zy E E y z (Eq. 3.5c)

Note that for an orthotropic material that there are a variety of Qij and Qji, but it is not

necessarily true that Qij equals Qji. The difference between the two notations is ij indicates the amount of shrinkage (negative strain) in the j direction when the material is pulled in the i, and vice versa for notation ji. Recall from the quasi-isotropic xy-plane assumption in

Equation 3.1 that Ex and Ey are equal, and therefore Equation 3.5a will mandate Qxy equalsQyx.

This result is sensible in that the weave in the xy does not show bias in the x or y, which led to the quasi-isotropic assumption to begin with. Equations 3.5b and 3.5c each have two

unknows at this point, for instance 3.5b has unknowns Ez and Qzx. Recalling earlier that Qxz

was assumed to be 0.25, it could be assumed that Qzx is also 0.25 such that Ez could be determined. However the moment it is assumed the two Poisson’s ratios in Equation 3.5b (or

3.5c) are equal, this would imply that Ez =Ex=Ey=E, creating a fully-isotropic material where all E and Q are equal at this point. In fact, SolidWorks states that if all elastic moduli E are

input at the same value, it will default to an isotropic solution (ignore all G input by the user,

computing all G from isotropic relations). It is known that a layered, laminated carbon fiber sheet cannot be fully-isotropic, so the Poisson’s ratiosQzx and Qzy are assumed to be 0.10, which is less than the counterpart values Qxz and Qyz which were assumed to be 0.25 earlier.

This ensures that when Ez is calculated by either Equation 3.5b or 3.5c, it will be less than

Ex=Ey=E, which is sensible in that the z direction is orthoganol to the stiffer and stronger woven xy plane. Finally, the Huber relation [43] for orthotropic materials can be used to compute a projected value for Gxz and Gyz: E E G y z yz 2(1 Q Q ) yz zy (Eq. 3.6a) E E G x z xz 2(1 Q Q ) xz zx (Eq. 3.6b)

In summary, Table 3.1 shows the values used in the SolidWorks Simulation 2009 linear elastic orthotropic model representing the carbon fiber sheet material.

The strength properties S of the linear elastic orthotropic model in Table 3.1 are used in a Tsai-Hill failure criteria calculated by SolidWorks. The carbon fiber sheets are modeled as shell elements using the composite shells application in SolidWorks, with the number, thickness, and orientation of plies specified by the user. The shell elements are in plane stress

assumption, meaning that the out-of-plane stresses Vz, Wxz, and Wyz are zero, resulting in the following compaction of Equation 3.4:

ª 1 Q yx º « 0 » Ex E y H x ½ « »V x ½ ° ° «Q xy 1 »° ° ®H y ¾ 0 ®V y ¾ (Eq. 3.7) « E E » °H ° « x y »°W ° ¯ xy ¿ 1 ¯ xy ¿ « 0 0 » « » ¬ 2Gxy ¼ Note that all material properties with superscript z have dropped out in the compacted compliance matrix, meaning that those material properties are not used (and therefore do not affect the results) in the composite shells application despite their entry into the linear elastic orthotropic model. Those material properties are reserved in case the material is applied to solid elements rather than shell elements. At each shell element node, multiple plane stress states (Vx,Vy,Wxy) are computed, one per ply. Therefore, the Tsai-Hill failure index FI has multiple values at each node, one per ply as well:

V 2 V V V 2 W 2 FI x x y y xy S 2 S 2 S 2 S 2 x x y xy (Eq. 3.8)

It should be noted that if the axial stresses V become negative (i.e. compression), the compressive strength value is used in place of the tensile strength value (i.e. Sx- is used in place of Sx+ if Vx becomes negative). The failure index FI varies from 0 to 1 (a value exceeding 1 is considered a failure), and therefore the Tsai-Hill factor of safety is the inverse of the failure index

The steel components are modelled three-dimensionally in the FEA models, and

SolidWorks applies 3D tetrahedral elements to such components. At each node, a stress state

(Vx, Vy, Vz, Wxy, Wyz, and Wxz) is resolved. Each stress state is subsequently transformed into the

56 three principal stresses (V1, V2, and V3). The Von Mises stress is calculated at each node as such:

(V V )2 (V V )2 (V V )2 V 1 2 2 3 1 3 Von_ Mises 2 (Eq. 3.9)

The Von Mises factor of safety n is: V n yield V Von _ Mises (Eq. 3.10) where the yield stress is referenced from the material’s information in the SolidWorks database.

For each 20-g horizontal or 8-g vertical load mode in the finite element models, success or failure in each case is judged by observing the overall global minimum factor of safety (FOS) across the model, whether that minimum develops from the Von Mises criteria in a steel component, or develops from the Tsai-Hill criteria within the carbon fiber sheets.

The over-static compensation provided by setting the requirements at such high 20-g horizontal and 8-g vertical values does not require a factor of safety above unity. In other words, success of each load mode occurs if the global minimum factor of safety is at or above 1.0.

3.3 FEA Model of Battery Internal Structure The finite element model of the battery internal structure includes the front and rear steel braces, the pair of carbon fiber sheets, the diagonal link, and the aluminum shims that create the vertical contact between the upper assembly module-cold plate stack and the carbon fiber sheets. The definition of so-called contact patches with respect to components that border the battery internal structure are necessary for defining loads and boundary conditions in various

57 load mode studies. Figure 3.2 shows all defined contact patches for the battery internal structure model highlighted in blue.

Table 3.1 Provided material properties for the Carbon Fiber Sheets (left column) versus properties input in the SolidWorks Linear Elastic Orthotropic Model (right column). Provided Material Properties Linear Elastic Orthotropic Model Values * ASTM D-638 Tensile 123 ksi Elastic Modulus Ex 8 Msi Strength S+ * ASTM D-695 Compression 87 ksi Elastic Modulus Ey 8 Msi Strength S- ** ASTM D-695 Tensile 8.1 Msi Elastic Modulus Ez 3.2 Msi Modulus E+ ASTM D-695 Compression 7.9 Msi Poisson’s Ratio Qxy, Qyz , 0.25 *** Modulus E- Qxz † ASTM D-790 Flexural 98 ksi Shear Modulus Gxy 3.2 Msi Strength ‡ ASTM D-790 Flexural 7.2 ksi Shear Modulus Gyz 2.18 Msi Modulus ‡ ASTM D-5379 Short Beam 7.5 ksi Shear Modulus Gxz 2.18 Msi Shear Strength Sxy †† ASTM D-2344 Interlaminar 8 ksi Tensile strength Sx+ 123 ksi Shear Strength Syz = Sxz †† - - Tensile Strength Sy+ 123 ksi ‡‡ - - Compressive Strength Sx- 87 ksi ‡‡ - - Compressive Strength Sy- 87 ksi ††† - - Shear Strength Sxy 7.5 ksi * Result Derived from Equation 3.1 **Result Derived from Equation 3.5b or 3.5c, with assumption Qzx = Qzy = 0.10 ***These Poisson’s Ratios are assumed to be 0.25, located in the mid-span of typical material Poisson’s Ratios. †Result from Equation 3.2 ‡ Results from Equations 3.6a and 3.6b, with assumption Qzx = Qzy = 0.10 †† Based on the provided ASTM D-638 tensile strength ‡‡ Based on the provided ASTM D-695 compressive strength ††† Based on the provided ASTM D-5379 short beam shear test result

Often the system being studied by the finite element model comes into contact with a neighboring component excluded from the model, which is where: 1. Boundary conditions are applied if the modeled system pushes into, rolls against, or is bonded/welded to the neighboring component. 2. Loads are applied if the neighboring component presses into a region of the system modeled. The contact region between the finite element model and a neighboring component does not necessarily constitute an entire surface of the finite element model, but often merely constitutes a region of that surface. In such cases, a contact patch on the surface of the finite element model is necessary. A method was developed in the SolidWorks environment to custom-define contact patches, beginning with a standard SolidWorks 2-D sketch defined on the surface in question. This 2-D sketch represents the size and shape of the contact patch region. When complete, the 2-D sketch is extruded by the smallest amount permissible by SolidWorks, creating what is termed here an infinitesimal extrusion. Care was taken during post-mesh to zoom-in and observe that additional elements were not added along the depth of the extrusion, but only in the pattern of the original 2-D sketch. When defining the boundary conditions, each infinitesimally extruded contact patch can be selected exclusively, permitting the application of desired boundary conditions or loads to all element nodes that fall in the contact patch. In Figure 3.2, there are a total of five contact patches for the battery internal structure finite element model. Four are associated with the contact region against the flat side of battery mounting brackets and at each corner of the battery pack, and the fifth located underneath the rear steel brace is the contact region between the lower assembly module-cold plate mass and underside of the rear steel brace.

Figure 3.2: Defined contact patches on the battery internal structure finite element model.

Another complexity associated with the battery internal structure model is the definition of relationships between the components that comprise the model. The entire battery internal structure model is an assembly file in SolidWorks. The front and rear steel braces are assembly files of their own that are imported into the model as subassemblies. The diagonal links are ASTM-A36 steel pieces modeled as a part file, and mated appropriately to the steel braces (concentric mates at the diagonal link bolt holes on the steel braces, flush mates against the surface of the steel brace the links rest against). Prior to importing the carbon fiber sheet and aluminum shim parts into the assembly file, they are first downgraded from three-dimensional parts to two-dimensional. Otherwise, SolidWorks would attempt to mesh these components with three-dimensional tetrahedral elements rather than two- dimensional shell elements (which of important note for the carbon fiber sheets would prevent the use of the composite shell application). A two-dimensional surface is first created from a flat surface of the initially three-dimensional sheet part, this surface positioned mid- plane within the part. Afterwards, the three-dimensional geometry in the part files are suppressed with the addition of a SolidWorks body delete, leaving behind the two- dimensional mid-plane surface as the part geometry. The now 2-D carbon fiber sheet and

aluminum shim part files are subsequently imported into the assembly file, positioned horizontally in the xy by concentric mates between the bolt holes of the sheets and corresponding bolt holes of the steel braces. The z positioning of the 2-D parts represent their mid-plane location in space, achieved with appropriate distance mates from 3-D parts.

This assembly file sequence leads to some automated assumptions about part-to-part interactions when the assembly is in the static FEA application of SolidWorks Simulation 2009. A default “global contact set bonded” assumes that for every three-dimensional part file that is mated to another flush, the parts will be bonded together such that the element mesh is contiguous as if the multiplicity of parts is one entity. Because the front and rear steel brace subassemblies consist of mated parts that come into contact with each other flush, the “global contact set bonded” will apply across these subassemblies. The diagonal links are also mated flush to the front and rear steel braces such that the braces in conjunction with both links are ultimately treated as one large piece in the “global contact set” default. This assumption is reasonable for the front and rear steel brace subassemblies in which the pieces in reality are welded together. However for the diagonal links, this assumption is not realistic in that the links are not welded to the steel braces, but rather bolted on. To override the “global contact set bonded” in this case, the link surfaces that come into contact with the steel brace surfaces are set into a free-free interaction with respect to each other, with a bolted connection type III as denoted in Figures 3.3a and 3.3e used.

SolidWorks Simulation 2009 does not automatically define relationships between 2-D (carbon fiber sheets and aluminum shims in this model) and 3-D parts. Therefore as shown in Figure 3.3b, a “contact set bonded” relationship is applied between the aluminum shim and carbon fiber sheet, and another “contact set bonded” relationship is applied between the carbon fiber sheet and the nearby interior surface of the steel brace. In reality, the shims, carbon fiber sheets, and steel braces are not bonded to each other. However, the designed dimensions of the steel braces are such that the vertical gap between the battery modules and interior horizontal surfaces of the steel braces are for a rather tight fit of the aluminum shims

61 and carbon fiber sheets in between. With these sheet components tightly pinched vertically, it is assumed that they will move together as if they were bonded, although it is possible in reality there could be some slight horizontal slippage between the modules, aluminum shims, carbon fiber sheets, and the interior surfaces of the steel braces. However, such slippage cannot be quantified, is likely to be limited further (or even eliminated) by the fact the vertical M8 bolts of the upper assembly go through all these components.

Across the battery internal structure FEA model, there are three types of bolted connections shown in Figure 3.3. The vertical M10 bolts of the rear steel brace use a bolted connection type I, where the bottom ends of the bolt screw into threaded holes found in the bottom of the rear steel brace. This is because Module 1 and its associated cold plate are mounted flush against the bottom of the rear steel brace such that there is no space for nuts underneath the rear steel brace. Unlike the rear steel brace, the front brace has clearance underneath to accept nuts. The vertical M10 bolts of the front steel brace use a bolted connection type II, where the bolt passes entirely through the front steel brace and is secured with nuts on the underside of the brace. The type III bolted connection is associated with the bolts that secure the diagonal links to the steel braces. Threaded holes are located in the steel brace assemblies to accept the ends of the bolts that secure the diagonal links. For bolted connections that use threaded holes such as types I and III in Figure 3.3, SolidWorks requires the selection of the circular hole edge and an interior cylindrical bearing surface of another hole. The cylindrical bearing surface represents the location of the threaded holes that the bolt screws into, while the selected circular hole edge represents the location of the bolt head. For bolt-nut combinations such as the type II connection in Figure 3.3, SolidWorks requires the selection of two circular hole edges that demarcate where the bolt head and nut are located. All bolted connections are assumed to be made of plain carbon steel in this model. The SolidWorks bolted connections remain invisible to the user after they are defined; however, they act as cylindrical entities that behave in the material they are defined as, transmitting nodal displacements within one circular entity (hole edge or bearing cylindrical surface) of one part to another set of element nodes located on a circular entity of another

part. In this way when the finite element model is loaded, two parts that are bolted to each other displace together as they would in reality due to the presence of the bolt.

When complete, the battery internal structure model is a mixed mesh as seen in Figure 3.4, using a combination of solid and shell elements. Solid tetrahedral elements represent the three-dimensional steel parts of the model, while shell elements are used for the aluminum shims (using SolidWorks database material properties of 6061-T6 aluminum) and the carbon fiber sheets. The carbon fiber sheets use the SolidWorks composite shell elements, which requires defining the number, thickness, and orientation of plies. Protech composites specified a 0.015 inch thick per ply thickness, which creates a total of five plies for the 1.7 mm thick sheets used in this structure. In SolidWorks, each ply was defined with the linear elastic orthotropic model properties listed in Table 3.1, all with a 0 degree orientation such that the xy coordinates of each ply aligns with the xy of the vehicle coordinate system. Recall from Section 3.2 that the in-plane xy directions of the carbon fiber sheets are indeed aligned with the xy of the global vehicle coordinates. Moreover, each ply uses a gridded weave superimposed upon each other in identical directions throughout the depth of the carbon fiber sheets such that all receive a 0 degree orientation. Of secondary note, because the ply weave does not have bias in the x or y, a 90 degree orientation would be equivalent to the 0 degree orientation.

The individual shell element meshes are not contiguous with the solid elements, which requires node-to-node mapping from the shell portions of the model to the solid. The most time-consumptive portion of the model runs are associated with node-to-node mapping between the aluminum shim shell elements to the carbon fiber sheet shell elements, and then from the carbon fiber sheet shell elements to the interior surface elements of the steel braces. During each load mode study, the appropriate shell element sections are suppressed to increase the speed of the model runs in each load mode.

Figure 3.3: Defined part-to-part relations in the finite element model of the battery internal structure. Views (a) and (b) highlight the overall relationships in the model, while views (c) through (e) highlight the three types of SolidWorks bolted connections used in the model.

Figure 3.4: A view of the element mesh for the battery internal structure model. This type of mesh is termed a mixed mesh, using a combination of solid and shell elements.

3.4 Load Modes of the Battery Internal Structure Each finite element model requires six statically-applied load modes associated with restraining the battery module-cold plate mass: 8-g downward, 8-g upward, 20-g fore, 20-g aft, 20-g pointed toward the passenger side of the vehicle, and finally 20-g pointed toward the driver side of the vehicle.

The 8-g vertical (z-axis) and 20-g side-to-side (y-axis) load modes of the battery internal structure are shown in Figure 3.5. The upper assembly module-cold plate stack consists of four modules at 89 lbs each and three cold plates at 13.5 lbs each, yielding a total of 402.5 lbs for the entire stack. Amplified for 8-gs, this is a total force of 3220 lbs. A 20-g amplification results in a total force of 8050 lbs.

Figure 3.5: 8-g vertical (z-axis) and 20-g transversal (y-axis) load modes of the battery internal structure finite element model.

In the 8-g vertical cases, the total 3220 lb force of the upper assembly stack is dived in half to 1610 lbs per appropriate aluminum shim as seen in views (a) and (b) of Figure 3.5. When the upper assembly module-cold plate stack shifts horizontally, it will transmit the load through the total of sixteen vertical M8 bolts running through the stack. Each vertical M8 bolt twice passes through its corresponding steel brace, therefore providing a total of thirty-

two points on the steel braces at which the horizontal load is applied. Dividing the 20-g 8050 lb force from the upper assembly stack into thirty-two equal parts yield a 251 lb force per steel brace M8 hole as shown in views (c) and (d) of Figure 3.5.

The application of load from the lower assembly battery module (89 lbs) and its cold plate (13.5 lbs) is simpler. The total lump weight of the module and cold plate is 104.5 lbs, which is amplified to 836 lbs for 8-g cases and 2090 lbs for the 20-g cases. In view (a) of Figure 3.5 (for the 8-g downward case), the 836 lb downward loading is split into four equal parts, applied at the cylindrical surfaces of the four threaded holes that are associated with the M8 bolts that fasten the lower assembly lump mass to the bottom of the rear steel brace. The 2090 lb 20-g horizontal load from the lower assembly lump mass is applied equally among the four same holes, except the forces are applied horizontally in the correct direction of the load mode as seen in views (c) and (d) of Figure 3.5. In view (b) for the 8-g upward case, the 836 lb force associated with the lower assembly lump mass is applied as a pressure load across the lower assembly contact patch shown earlier in Figure 3.2.

The boundary conditions for the 8-g vertical (z-axis) and 20-g side-to-side (y-axis) load modes are all the same in Figure 3.5. All battery mounting bracket bolts that screw into the front and rear steel braces run along the x-direction, orthogonal to both the z and y directions of these load modes. Therefore what restrains the battery internal structure from motion in the y and z directions are the bolt shanks of the battery mounting bracket bolts, and so the cylindrical surfaces of the battery mounting bracket bolt holes in the steel braces are fully constrained as noted in Figure 3.5.

The 20-g fore and aft load modes for the battery internal structure are somewhat more complex. The peculiarities of the 20-g fore load mode are shown in Figure 3.6. Similar to the 20-g side-to-side load cases discussed previously, each M8 vertical bolt hole in the front and rear steel braces receives a 251 lb force to represent the upper assembly module-cold plate stack, all pointed in the –x for the 20-g fore case, and pointed in the +x in the 20-g aft case.

Also similar to the 20-g side-to-side load cases, the 2090 lb force from the lump mass of the lower assembly is applied equally amongst the four associated bolt holes on the underside of the rear steel brace, except pointed in the –x for the 20-g fore case, and pointed in the +x in the 20-g aft case. In the case of the fore load mode, what restrains the battery internal structure are the back faces of the front battery mounting brackets and the thread engagement with the bolts associated with the rear mounting brackets. This is why the aft-side battery mounting bracket bolt holes and fore-side battery mounting bracket contact patches are constrained as shown in Figure 3.6.

Interesting deformation is observed with the front and rear steel braces in the fore and aft load modes that need to be controlled with additional boundary conditions. For instance when focusing on the 20-g fore load mode case in Figure 3.6, note that all forces are applied above or below the constraints associated with the battery mounting brackets. Therefore, each force has a vertical moment arm starting from their location and ending in the region of the battery mounting brackets. In the rear steel brace, all forward acting forces are located in front of the battery mounting bracket constraints such that their moment arms cause the brace to cave into the battery modules. Because the modules are incompressible, a roller constraint is necessary on both aft-side aluminum shims as shown in Figure 3.6 such that the top and bottom portions of the rear steel brace slide over the battery modules rather than cave into them. In the front steel brace, the forward acting forces are located behind the battery mounting bracket constraints such that the brace is pried open by the moment arms. This is permissible since it is possible for the brace to separate from the battery modules rather than cave into them, and so the fore-side aluminum shims do not have roller constraints applied in the 20-g fore load mode.

Figure 3.6: 20-g fore (-x) load mode force and boundary conditions for the battery internal structure finite element model. To represent the upper assembly module-cold plate stack, 251 lb per M10 vertical bolt hole on the rear and front steel braces are applied in the –x direction as shown. Not shown is the application of the 2090 lb load applied from Module 1 and its cold plate, which works similar to views (c) and (d) in Figure 3.5 except that the loads applied at the four bolt holes on the underside of the rear steel brace are pointed in the –x direction as well. Finally, the constraints are applied on the fore and aft side of the battery internal structure as shown.

The 20-g aft load mode works in the same manner as the fore load mode, except all forces are reversed to the +x direction, what is applied boundary condition-wise on the fore

69 side is applied to the aft side, and what is applied boundary condition-wise on the aft side is done to the fore side.

Table 3.2 Battery internal structure model-wide mimimum FOS for 8-g vertical and 20- g horizontal load modes - Min. FOS (With Min. FOS (No Carbon Fiber Sheets) Carbon Fiber Sheets) 8g Down 3.10 1.70 8g Up 3.40 2.00 20g Fore 1.42 - 20g Aft 1.70 - 20g Passenger 2.80 - Side 20g Driver Side 2.80 -

Table 3.2 is a summary of the model-wide minimum factor of safety (FOS) for each load mode. The determination of model-wide FOS was discussed previously in Section 3.2. Across all load modes, the minimum FOS occurs in a highly-stressed region of the steel braces as determined by the Von Mises criteria. For the carbon fiber sheets, SolidWorks reports the minimum Tsai-Hill FOS on a per-ply basis. A scroll through all ply results on both sheets show Tsai-Hill minimum FOS values on the order of tens in all load mode cases. SolidWorks also determines the interlaminar shear stress distribution between plies. A scroll through all interlaminar shear stress values shows that the carbon fiber sheets were below the 8 ksi maximum allowable listed in Table 3.1. The employment of the carbon fiber sheets in this structure was to resolve the challenging 8-g vertical load modes as mentioned in Chapter 2, Section 2.6. When comparing the 8-g no carbon fiber versus the carbon fiber results in Table 3.1, the minimum FOS is nearly double when employing the carbon fiber sheets. Table 3.1 also shows that the structure passes (minimum FOS > 1) even without the carbon fiber sheets. As is common with FEA studies regarding composites, uncertainty with the material properties exists without knowledge of the Poisson’s ratio Q or shear moduli G. Assumed ratios were used instead as discussed in Section 3.2 to derive the shear moduli. To allow the structure to pass the required 8-g vertical and 20-g horizontal requirements with as little

70 doubt as possible during the short one-year design and build time, the structure was designed to pass without the carbon fiber sheets. Theoretically, it is possible to design the structure to be even lighter than what is shown here to bring the carbon fiber case 8-g minimum FOS values closer to 1.0, as long as the modifications would not compromise the 20-g horizontal minimum FOS.

3.5 Battery Mounting Bracket Analysis, Version 1 Note in Figure 3.1 that the rear facing battery mounting brackets are slightly different than the front facing brackets. This section pertains to the rear facing brackets, made of ASTM A36 steel pieces welded together. The assembly file of the bracket used in the static study setting of SolidWorks Simulation 2009 is a composition of modeled part files that represent the individual steel pieces. As the individual parts are mated together flush in the assembly file, SolidWorks Simulation 2009 by default applies the previously discussed “global contact set bonded” relationship across the FEA model of the bracket (reasonable in that this is a welded assembly).

As there are four mounting brackets (one at each corner of the battery pack), note that each bracket is assumed to support approximately one-quarter of the 507 lb total module- cold plate mass (five 89 lb modules, four 13.5 lb cold plates). Amplifying this quarter load for 8-gs yields a total of 1014 lb per bracket, and for 20-gs is amplified to 2535 lb per bracket. Figure 3.7 shows the boundary conditions and force definitions for the bracket in all load modes.

In the 8-g vertical load modes, the 1014 lb force is divided equally into eight parts (126.75 lb per hole) applied at the battery mounting bracket bolt holes. In the downward case, the bracket is pressed against the top surface of the mount located underneath, and so an appropriately sized and shaped contact patch is defined underneath the bracket to represent the contact region, constrained in the 8-g downward case. In the 8-g upward case, the bracket is pressed up against the lone washer and nut that fastens the bracket to the threaded stud of

its associated mount. Likewise, a contact patch shaped in the form of a circular washer is defined on the top surface of the bracket and is fully constrained during the 8-g upward case.

What restrains the battery mounting brackets in all 20-g horizontal load mode cases are the shanks of the vibration mount M12 threaded stud and M16 bolts that project from the battery to chassis cradle. The holes associated with these bolt shanks are fully constrained as shown in views (c) through (f) of Figure 3.7. In the 20-g aft case for the rear facing battery mounting brackets, the battery internal structure pushes into the flat face of the bracket. For this load mode, a contact patch sized for the amount of the face that becomes loaded by the battery internal structure is defined and loaded as a uniform pressure with a 2535 lb magnitude as seen in view (c) of Figure 3.7. This contact patch is the same size and shape as the mounting bracket contact patches on the battery internal structure FEA model seen earlier in Figure 3.2. In the 20-g fore cases for the rear battery mounting brackets, the battery internal structure now moves away from the bracket. The bolts that secure the brackets to the internal structure are also pulled forward with the internal structure, causing the bolt heads and underlying washers to press into the bracket in the fore (-x) direction. Contact patches in the shape of the washers are defined on the appropriate face of the bracket, and the 2535 lb load is divide equally into eight parts (316 lb) for application to each washer bearing surface as seen in Figure 3.7d. The battery mounting bracket bolts are pushed sideways (along the y direction) by the battery internal structure when an inertial shift occurs toward the passenger or driver side, and so the 316 lb loads are applied at the associated eight bolt holes of the bracket in the appropriate directions for the 20-g driver and passenger side cases.

The factor of safety (FOS) distribution across the bracket is determined using the Von Mises criteria as the bracket is made entirely of isotropic steel. Table 3.3 shows the minimum FOS of the bracket for each load mode. As seen in Table 3.3, the bracket performs exceedingly better in the 20-g passenger and driver side cases when compared to the other cases. However if the weight-reducing cut-outs in the thick portion of the bracket were expanded to lower the passenger and driver side FOS results, the FOS in the other load

72 modes became compromised. This became the optimal design while keeping the minimum FOS values in all load modes acceptable.

Table 3.3 Rear-Facing battery mounting bracket minimum FOS for 8-g vertical and 20-g horizontal load modes - Min. FOS 8g Down 1.47 8g Up 1.70 20g Fore 1.54 20g Aft 1.47 20g Passenger Side 4.28 20g Driver Side 4.28

3.6 Battery Mounting Bracket Analysis, Version 2 Note in Figure 3.1 that the rear facing battery mounting brackets are slightly different than the front facing brackets. This section covers the FEA for the front facing brackets, which works very similar to the FEA of the rear facing brackets covered previously in Section 3.3. Note that that each bracket is assumed to support approximately one-quarter of the 507 lb total module-cold plate mass. Figure 3.8 shows the load modes of the FEA associated front facing brackets.

Table 3.4 Front-Facing battery mounting bracket minimum FOS for 8-g vertical and 20- g horizontal load modes - Min. FOS 8g Down 1.66 8g Up 1.32 20g Fore 1.36 20g Aft 1.35 20g Passenger Side 2.84 20g Driver Side 2.80

Figure 3.7: The six load modes of the rear-facing (version 1) battery mounting brackets

Figure 3.8: The six load modes of the front-facing (version 2) battery mounting brackets

3.7 Battery to Chassis Cradle Analysis The battery to chassis cradle is constructed of ASTM A36 steel. The large xy-plane footprint of the cradle is created by a single 3/16” thick piece. To stiffen the cradle in order to meet the out-of-plane vertical loading requirements, 1/2" thick reinforcement sections are welded underneath the footprint in the vicinity of the vibration mounts. There are also vertical 3/16” thick ribs welded on the top side of the footprint for this same purpose. Figure 3.9 shows the 8-g downward load mode study to which these reinforcements were tailored to. Note the front vertical rib makes an appearance in the top side view (a) and underside view (b) as this particular rib is inserted through a horizontal slot cut in the footprint, afterwards welded to the footprint. The rear vertical rib in view (a) is simply measured and welded into location. The front and rear 1/2" thick sections were designed as light as possible through the use of weight-reducing cut-outs that were tailored simultaneously to not compromise the 8-g downward load mode minimum FOS. While the addition of the 1/2" thick sections and vertical ribs reduced deflection and stresses in the 8-g downward study, another issue developed toward the rear portion of the cradle. When the rear pair of vibration mounts press downward, the rear transversal section would bend downward between the fore-aft chassis rails. Meanwhile, the portion of the rear cradle receiving support from the fore-aft rails beneath did not deflect down due to the fixed constraints beneath to simulate the existence of the rails. The deflection gradient from downward to zero deflection created stress concentrations computed by the FEA model. The stress concentrations were alleviated by the addition of small 1/2" thick block reinforcements on the top side of the cradle footprint seen in view (a) of Figure 3.9.

The 8-g downward load mode was created by first defining the contact patches on the top surface of the cradle that represent the shape and size of the vibration mounts bases that press downward on the cradle. The base of the mounts are triangular shaped with three bolt holes, and the vibration mount contact patches in Figure 3.9a are in the likeness of the mount bases. Recall from Section 3.5 that in the 8-g vertical cases, each bracket is assumed to

transmit a 1014 lb load. In the 8-g downward case for the cradle, 1014 lb per contact patch are applied as seen in Figure 3.9a. View (b) shows contact patches on the cradle underside associated with the region of contact between the cradle and fore-aft structural rails of the chassis, and are constrained to simulate their support from underneath when the cradle is pressed downward. In Figure 2.12 of Chapter 2, a type II anchor bolt connection is specified for fastening the front portion of the cradle to transversal (y-axis running) structural rails of the chassis which the cradle floats above. Each type II anchor bolt connection features a nut underneath the cradle to support it from the underside in downward loading cases. The six constraints in view (c) of Figure 3.9 are associated with the supporting nuts, defined in contact patches in the shape and size of contact between the cradle and the supporting nuts.

The specifics of the cradle 8-g upward load mode are shown in Figure 3.10. The battery pack pulls up on the four vibration mounts, which are secured to the cradle each with three bolt-nut fastener combinations in a triangular pattern, and so these fasteners are pulled upward with the mounts as well. The nuts and washers on the bottom of these fasteners subsequently push upward into the bottom surface of the cradle. The 1014 lb 8-g vertical load per battery mounting bracket is divided equally into three 338 lb loads, each applied on a contact patch in the shape and size of the nuts as shown in view (a) of Figure 3.10. What prevents the cradle from moving upward are the top washers and bolt heads of all anchor bolts, and so contact patches on the top surface of the cradle are defined in the shape and size of the washers and are constrained as seen in view (c) of Figure 3.10. With the loading in view (a) and only the constraints in view (c), a failure mode develops in the rear portion of the cradle. The rearmost 338 lb forces in view (a) are all located behind the nearest constraints in view (c), causing each force to have a moment arm starting from their locations and ending at the nearest constraints, bending the rear portion of the cradle excessively upwards. The size and shape of any stiffening elements in the rear region is limited due to the space occupied by the vibration mounts on the cradle top surface. To simulate a weld bead that could bolster the support for the 8-g upward case, constraints at the edges of the interface between the fore-aft chassis rails and cradle as shown in view (b) of Figure 3.10 were

77 applied, and indeed this remedied the problem. This implied that the cradle, especially towards the rear portion, would need to be welded to the structural rails in order to meet the required 8-g upward loading requirement. At this point in the cradle design, it was decided that the cradle would be first fastened in place by the anchor bolt connections. Afterwards it is welded to the fore-aft structural rails, meaning the cradle is permanent to the vehicle body.

Figure 3.9: The 8-g down load mode of the battery to chassis cradle. The stiffening elements highlighted in red were designed around this load mode.

In the 20-g horizontal load modes, forces are applied at a multitude of bolt holes on the cradle. Recall from Section 3.5 that each battery mounting bracket is assumed to transmit a total of 2535 lb in the 20-g horizontal cases. Each bracket in the horizontal is constrained at three points, two vertical M16 bolts projecting from the cradle, and the vertical M12 rod with its associated vibration mount. The 2535 lb load transmission is thus divided into three equal parts, providing 845 lb per rod. As noted in Figure 3.11, each M16 bolt hole receives an 845

lb horizontal load. The M12 rod of each vibration mount is assumed to receive the third 845 lb load applied by its corresponding battery mounting bracket. Because each mount is bolted by three M10 fastener sets to the cradle, this 845 lb load is split into 292 lb per M10 bolt hole as noted in Figure 3.11. All 20-g horizontal load modes experience the same magnitude of forcing, but the direction of the forces is applied in the appropriate direction (+x for aft, -x for fore, +y for passenger side, -y for driver side). The constraints of the 20-g horizontal load modes are all identical. The anchor bolt shanks prevent the horizontal motion of the cradle, and therefore all anchor bolt holes of the cradle are constrained as noted in each view of Figure 3.11. Furthermore as noted in the caption of that figure, the weld bead constraints applied in Figure 3.10b are also applied in all 20-g horizontal load modes.

Figure 3.10: The 8-g upward load mode of the battery to chassis cradle.

Figure 3.11: The 20-g horizontal load modes of the battery to chassis cradle. In addition to constraining the anchor bolt holes of the model as noted here, the weld bead constraint as shown in view (b) of Figure 3.10 is also applied.

Table 3.5 summarizes the minimum FOS (factor of safety) results for each load mode of the battery to chassis cradle.

Table 3.5 Battery to chassis cradle mimimum FOS for 8-g vertical and 20-g horizontal load modes - Min. FOS 8g Down 1.20 8g Up 1.22 20g Fore 1.41 20g Aft 1.41 20g Passenger Side 1.68 20g Driver Side 1.68

3.8 Weight Comparison of Generation I and V Battery Packs Using the detailed SolidWorks CAD models of both the Generation I and V designs, the weight reduction between both designs can be estimated. A123 Systems provides the weight for each of its 22S3P battery modules, the EDS module, and the BMS controller. Each created part file in SolidWorks is registered with its material, providing part properties including mass density. The software is capable of calculating the volume of each part file, and coupled with the assigned density, the software calculates the weight of the part (menu option Tools -> Mass Properties). Figures 3.12 and 3.13 list the major weight inducing components of both the Generation I and V battery pack designs, and Tables 3.6 and 3.7 are the estimate of the overall battery pack system weights using the CAD model estimations.

The end of Section 2.3 (in Chapter 2) concluded that if no structural enclosure was required, the maximum potential energy-to-weight ratio with the five 22S3P battery modules is 0.11 kWh/kg. Using the estimates provided in Tables 3.6 and 3.7, the Generation I battery only achieved 50% of maximum potential, with an increase to 65% of maximum potential in the Generation V battery design. The Generation V battery (0.072 kWh/kg) is more toward the lower end of the lithium ion class than its predecessor (Generation I), for instance just behind the technology in the Chevrolet Volt (0.081 kWh/kg) and the Tesla Roadster (0.118 kWh/kg).

Comparing the overall estimated weights between Tables 3.6 and 3.7 shows a weight loss of about 100 kg (220 lbs) between the Generation I and V battery pack systems. The vehicle competed in year 2010 with a weight of 5036 lbs with the Generation I battery, and the vehicle weigh-in with the Generation V during the June 2011 competition is 4790 lbs. On the surface, the difference in these two weights (246 lbs) suggests that the CAD weight loss estimation of 220 lbs is quite accurate. However, the 5036 lb weigh-in is without rear seats, while the 4790 lb weigh-in includes rear seats. The Generation I vehicle curb weight with rear seats climbs to 5163 lbs. A direct comparison of both vehicle curb weights with rear

seating suggests a vehicle weight loss of 373 lbs, more than suggested by comparing the CAD models of the Generation I and V batteries. This suggests that something in addition to the battery pack change caused vehicle weight loss. For instance the cargo floor cage over the Generation I battery was made of steel square tubing and spanned a larger area than the Generation V battery. Because the Generation V battery had a much more compact form factor, the revised cargo floor cage could be smaller, and its weight was further reduced by using aluminum angle over steel square tubing.

3.9 Required Vibration Mount Stiffness As discussed previously in Chapter 1, it is a technical requirement for the A123 System 22S3P battery modules to not be directly connected to the chassis, but rather have vertical vibration isolation from the chassis. To reduce the risk of resonance, it was required that the natural frequency of the battery pack system to be above 50 Hz. The Generation V battery pack receives its vibration isolation through the four battery to chassis cradle powertrain mounts its rests upon at its mounting brackets. Welded to the cradle are also M16 bolts that run through the mounting brackets to essentially help in catching the battery pack during the 20-g horizontal load modes. From a vibration analysis perspective, these bolts also prevent the battery pack from moving in the x or y, leaving only a single degree of freedom in the z direction. The natural frequency (in Hz) fn of the battery pack system is thus estimated with the classical single degree of freedom equation: 1 k f cr n,cr 2S m (Eq. 3.11)

The notation fn,cr denotes critical natural frequency, which is set to 50 Hz for this particular problem. The total system stiffness k should not be below the crtical vaue kcr in order to achieve a natural frequency greater than or equal to fn,cr. m is the sprung mass of the battery pack system. The relationship in Equation 3.11 implies that if the battery pack is heavier (larger m), then a larger kcr is required to meet or exceed the critical natural frequency fn,cr. This means the mounts must be stiffer the heavier the battery pack. For a

battery pack that has a significant mass such as designed here, the required powertrain mounts tend to be rather stiff. To determine the sprung mass of the battery pack, the total 308 kg mass in Table 3.7 has the mass of the battery to chassis cradle subtracted, leaving 290.7

kg or 641 lbs (or about 20 slugs). In order to calculate kcr in lbf/ft, the mass m must be in slugs and critical frequency fn,cr must be in Hz. Since m and fn,cr are known, the critical 6 stiffness kcr is solved for by Equation 3.11 and found to be 1.97 x 10 lbf/ft, or 164327 lbf/in. The selected four vibration mounts for the cradle are stiff mounts normally used to support transmissions, and the sum total stiffness of the four mounts combined exceeds the calculated kcr.

Figure 3.12: Major weight inducing components of the Generation I battery pack

Table 3.6 Estimated Weight of Generation I Battery Pack and its Components Component Quantity Mass (kg) A123 22S3P Modules 5 40.3 Aluminum Cold Plate Assemblies 10 7.0 2-module cluster interior assembly steel frame 1 6.7 3-module cluster interior assembly steel frame 1 9.3 Aluminum Base Plate & Underbody Steel Frame 1 56.1 Long Aluminum Sidewalls (Passenger & Driver Side) 2 6.5 Short Aluminum Sidewalls (Front & Rear Side) 2 4.9 Vibration Mounts 26 0.5 Aluminum Lid Braces 14 0.071 Aluminum Corner Braces 4 1.1 Aluminum Sidewall Braces 12 0.17 Aluminum Top Lid 1 21 Total Pack Mass (kg) 408 Total Pack Energy (kWh) 22.3 Pack Energy to Weight Ratio (kWh/kg) 0.055

Figure 3.13: Major weight inducing components of the Generation V battery pack

Table 3.7 Estimated Weight of Generation V Battery Pack and its Components Component Quantity Mass (kg) A123 22S3P Modules 5 40.3 Aluminum Cold Plate Assemblies 4 7.0 Rear Steel Brace 1 22.0 Front Steel Brace 1 18.7 Steel Guide Plate (for Lower Assembly) 1 1.14 Rear Battery Mounting Bracket 2 4.7 Front Battery Mounting Bracket 2 4.6 Battery to Chassis Cradle 1 17.3 Aluminum Fuse Access Panel 1 0.197 Steel Fuse Access Panel Ring 1 0.16 Steel Wire Grip Plate for MIS Plug 1 0.26 Steel Wire Grip Plate for HV DC Interface Plug 1 0.20 Total Pack Mass (kg) 308 Total Pack Energy (kWh) 22.3 Pack Energy to Weight Ratio (kWh/kg) 0.072

4.0 INSTALLATION OF THE BATTERY PACK

4.1 Fiberglassing The most complex procedure of the battery pack build process is the creation of the four fiberglass components, which includes the one-piece top lid, two-piece bottom lid system, and the lower assembly cradle. The form factor (size and shape) of each fiberglass piece was designed in the SolidWorks environment at first as a solid piece, afterwards applying the shell feature command to the solid to create the likeness of a part created on a mold. The interior surfaces of the shell are used to generate a corresponding three-dimensional male mold geometry. The male molds of each fiberglass part were created from foam pieces carved into shape by a three-axis CNC machine at the North Carolina State University College of Design facility. The vertical axis of the used machine could only handle foam slabs of three-inch vertical increments. As a result, the male mold geometry in SolidWorks was divided into three-inch vertical increments, each increment becoming its own part file. Each part file is then exported in IGS file format that is programmable into the CNC machine. The three-inch thick high-density foam slabs are purchasable from most construction supply companies, the particular foam used in this project being blue-colored scoreboard.

Figure 4.1 is a summary of the entire fiberglassing process through actual photography taken during various stages. The three-inch thick slabs in view (a) are first carved by the 3- axis CNC machine into shapes representing vertical sections of the male molds. The creation of the mold is complete as shown in view (c) once the slabs are stacked together and bonded with liquid nails adhesive. To prevent gaping between the stacked slabs, the curing molds are weighted as shown. Care is also taken not to apply adhesive toward the edges of slabs such that seepage of adhesive does not occur onto the mold surfaces when the molds are weighted. View (b) is a photograph of some of the required materials, including woven and non-woven fiberglass mat in various ounces, two-part epoxy (West Systems 105 resin and 206 hardener), and turtle wax. View (d) is the surface preparation of the male mold. The glossy appearance

in the photograph is due to the packaging tape first applied to the surfaces to which fiberglass layup occurs. The tape prevents the epoxy from seeping into the foam mold pores (accidentally bonding the finished part to the mold), and so it is important for the individual rows or patches of tape to slightly overlap. To ensure the cured fiberglass part separates from the mold, several applications of turtle wax are applied with a sponge as seen in view (d). The most time consumptive process is the fiberglass-epoxy layup itself, which requires the following simultaneous processes: 1. The mixing of epoxy resin and hardener in proper portions. The West Systems 105 and 206 tanks are each purchased with special pumps. One pump of each tank provides the proper ratios, and are mixed together in a disposable plate or tray. 2. Cutting woven or non-woven fiberglass mat into patches for layup. 3. Application of epoxy-wetted fiberglass patches onto the mold. The dry fiberglass patch is applied to the mold and brushed with epoxy either with paint brushes or plastic cards.

A photograph of a curing fiberglass part post-lay-up is shown in view (e) of Figure 4.1. When finished with the lay-up of an individual part, all tools that come into contact with epoxy are immediately bathed in acetone to ensure the epoxy separates from the tools. Due to the caustic materials used (epoxy and acetone), the entire process occurs in a well-ventilated facility. Following the lay-up is a twenty-four hour waiting period to ensure all epoxy is cured. The curing part remains in a well-ventilated area to ensure fumes during that time are evacuated. Prior to separating the cured fiberglass part from the mold, the rough and excess edges are trimmed down with a dremel rotary cutting tool, the mold edges themselves defining where exactly the final clean edges of the fiberglass part should be. A mask in necessary due to the fine glass powders produced when the part is trimmed down. The fiberglass part is then pried and separated from the mold. Each part is then inspected for imperfections, those that are immediately visible to the eye (such as gaps or small holes) circled with marker as targets for repair. The repair is rather simple as shown in view (f) with the application of additional epoxy, or epoxy and a piece of fiberglass mat on the exterior and

87 interior side if the imperfection is large enough. To catch imperfections not visible by eye, the fiberglass part is then filled with water, and any seepage that is observed identifies where additional sealing epoxy need be applied. Finally in view (g), the fiberglass part receives an exterior and interior coating of automotive spray-on rubberized coating to give the part a more finished appearance.

All passageway and bolt holes are first marked with templates generated from CAD models (templates either being print-outs or water-jet cut parts themselves that are to be bolted on the fiberglass part), then drilled or cut to size/shape with a dremmel tool. When drilling holes in the fiberglass parts, it is important to employ slow drilling speeds and gradually step up the drill bit size. Drilling the hole in one step using the final diameter drill bit and going at rapid speeds may cause the bit to walk into the fiberglass part and create a deformed-shape hole rather than a circular one. Finally bolt heads are to never come into direct contact with the fiberglass, applying flat washers where necessary. The pressure induced by the relatively smaller bolt head when the bolt is tightened can cause the fiberglass to become crushed underneath the head. By step (g) in Figure 4.1, the top lid has the fuse access panel interface (Figure 2.16) installed permanently, while the wire grip plate accessories on the top lid and bottom lid part 2, as well as the coolant pipe entry/exit plate on bottom lid part 2 are temporarily bolted on for an initial test fit. The wire grip plates and pipe entry/exit plate systems are then removed, later installed in the combined assembly phase shown later in Section 4.2 of this chapter.

Figure 4.1: Process for producing the fiberglass components of the battery pack: (a) CNC-carving foam slabs into pieces for the male molds, (b) raw materials for fiberglassing, (c) formation of male molds by bonding the CNC-carved slabs, (d) surface preparation of male molds, (e) hand lay up of fiberglass and epoxy resin, (f) post-cure correction of imperfections, (g) final surface finish and installation of accessories

The first created fiberglass part was bottom lid part 1, the rear-facing of the two-piece bottom lid system. The chosen lay-up schedule was a few layers of 6-oz woven fiberglass, followed by a single middle layer of 24-oz woven fiberglass, then finished with an additional stacking of 6-oz woven fiberglass. As shown in view (a) of Figure 4.2, this created a less- desirable gridded (or waffle-looking) appearance due to the large weave of the 24-oz layer underneath. For bottom lid part 2 and the top lid, a similar sequence was employed as done for bottom lid part 1, except the middle layer was thick non-woven fiberglass as opposed to 24-oz woven. This provides the smoother surface finishes seen in views (c) and (d) of Figure 4.2. Around the perimeter flanges of the fiberglass lid pieces, the thick middle layer (whether 24-oz woven or thick non-woven) had difficulty in contouring to the mold, and so the flanges were generated with several lay-ups of 6-oz woven fiberglass cut in narrow strips. The lower module cradle male mold also had many curvatures associated with stiffening ribs, so the entire cradle was laid-up with several layers of 6-oz woven fiberglass, generating the smoothest of all finishes as seen in view (b) of Figure 4.2.

Figure 4.2: Finished appearance of the four fiberglass components of the battery pack: (a) bottom lid part 1, (b) lower module cradle, (c) bottom lid part 2, and (d) top lid.

Figure 4.3: Cross-section mechanics of the fiberglass lower module cradle when airborne. The fiberglass surrounding the foam inserts increases the overall alpha stiffness of the cradle (D1 > D2). The vertical ribs preserve the beta stiffness (E1 = E2). If the vertical ribs were not present, the local beta stiffness would lessen, compromising the overall alpha stiffness. The combination of the fiberglass surrounding the foam inserts and vertical ribs function to increase the overall alpha stiffness when the cradle is airborne.

The lower module cradle functions to create attachment points for the BMS controller and EDS module in the lower assembly of the battery pack. The cradle attaches to the lower battery module (Module 1) through the four vertical M8 bolts that run through the module.

The cradle itself cannot be shaped as a tub that would otherwise block the ends of the module where high voltage wiring is attached. Instead, the cradle is shaped as a uniform extrusion with two vertical sidewalls for attaching the BMS and EDS. Because it is not shaped as a tub, the overall stiffness and strength of the cradle is questioned due to the fact the weight of the EDS and BMS can pull down the vertical side walls whereas a full tub would allow for adjacent vertical walls that prevent each other from becoming pulled down. The addition of vertical ribs along the sidewalls in the photograph of Figure 4.3 were designed in CAD to prevent the sidewalls from deflecting excessively downward when the BMS and EDS are attached. Note from Section A-A’ in Figure 4.3 that the cross-section of the cradle’s extruded shape is that of the letter ‘W,’ created by the fact that there are bottom table top surfaces designed to be lower in height from the M8 bolt head surface. These lowered table top surfaces allow the entire lower assembly to sit flat and hence be assembled on a table top despite the presence of the M8 bolt heads beneath. Once the lower assembly becomes airborne when installed to the upper assembly, a problem is created by the fact that no stiffening element exists between the table top surfaces and M8 bolt head surface, causing deflection angle D1 to be large as shown in the left view of Section A-A’. Angle D provides a sense of the overall stiffness and strength of the cradle under the weight of the BMS and EDS, a larger D implying a weaker and less stiff cradle, a smaller D implying a stiffer cradle. Realizing this issue after the cradle was produced, three foam inserts were added to the M8 bolt head surface as shown in the photograph of Figure 4.3, at locations that do not block the M8 bolt holes. The foam inserts were enclosed with additional fiberglass layup. The fiberglass added around the foam inserts act as horizontal beams that prevent the table top surfaces from moving relative to the M8 bolt head surface, reducing angle D(D D) as shown in the right view of Section A-A’. The original vertical ribs are still crucial to the structure as they preserve the local beta stiffness of the cradle (E E); that is the vertical ribs prevent the vertical sidewalls from bending downward relative to the horizontal table top surfaces.

4.2 Upper Assembly Process The upper assembly process begins with the placement of the lower carbon fiber sheet and pair of aluminum shims as shown in view (a) of Figure 4.4. In order to provide clearance for nuts underneath associated with the vertical M8 fasteners of the assembly, the upper assembly is built on a pair of wooden beams that keep the bottom side elevated. The alternating module-cold plate stacking sequence then progresses as shown in views (b) and (c), placing all vertical M8 rods through the modules along the way to secure them in position. Views (d) and (e) are activities associated with the front facing M8 vertical rods. The rear rods are removed between views (c) and (d) to allow the rear steel brace to be installed, otherwise the rods would interfere with the installation of the brace. Meanwhile, the front rods in view (e) are pushed up by a jacking stud to lock both carbon fiber sheets and front aluminum shims from shifting horizontally while the rear steel brace is installed. As far as the rear aluminum shims, it was hoped that the rear steel brace would pinch them stationary while installed as the vertical gaping between the brace and the module-cold plate stack is narrow, designed for a tight fit of the carbon fiber sheets and aluminum shims in between. In reality, the rear aluminum shims often shifted horizontally while the rear steel brace was installed, requiring a few trials until the shims had not shifted to obstruct the final installation of the rear M8 vertical rods. In hindsight, the assembly process should require the aluminum shims to be bonded to the carbon fiber sheets to avoid this issue.

Figure 4.4: Photography of upper assembly sequence prior to installation of the front and rear steel braces. Assembly progresses from views (a) to (e).

The horizontal center-to-center dimensions of all bolt holes in the carbon fiber sheets, aluminum shims, and steel braces associated with the vertical M8 rods are such that the cold plates fit tightly between the battery modules. This ensures solid contact between the cold plates and modules for thermal conductivity and serves to prevent the cold plates from sliding between the modules. Recall from the previous paragraph that the rear M8 rods are removed prior to sliding on the rear steel brace. Because the positioning of the M8 rods forces the battery modules tightly against the cold plates, the overall side-to-side (transversal) dimension of the module-cold plate stack relaxes and expands slightly when the rear rods are removed. The expansion of this dimension is to the degree that a compression strap is necessary as shown in view (a) of Figure 4.5 before the rear steel brace can fit over the stack. The fit of the rear steel brace is sufficiently tight that it is pushed into position with gentle assisting taps from a rubber mallet. Once the brace is positioned, the rear vertical M8 rods are re-installed through the brace, carbon fiber sheets, aluminum shims, and battery modules permanently. The front steel brace is installed in a similar manner. Then as shown in view

(c), the passenger and driver sides of the upper assembly are ready for the installation of the diagonal links. On the top side of the upper assembly, the MIS (Manual Isolation Switch) foundation and fuse bracket accessories are installed as shown in view (d). With the mechanical portion of the upper assembly complete, the extra length copper pipes shown in view (e) can be trimmed back to proper length. Excessive length causes the shark bite fittings introduced in Chapter 2 (Figure 2.19) to not fit within the fiberglass shell of the battery pack. The shark bite fittings require a one-inch insertion depth to create a seal against the pipe, and too short of a copper pipe length will cause the fitting to butt against the steel brace before consuming one inch of pipe. Therefore, the pipe shortening is an iterative process during the first-time assembly of the battery pack.

Because standard M8 bolts are not sufficiently tall to pass through the entire vertical length of the steel braces, custom fasteners were created using M8 threaded rod cut to length. The final installation of the M8 threaded rod fasteners through the front and rear steel braces is shown in Figure 4.6.

4.4 Lower Assembly Process The hub of the lower assembly process is the fiberglass lower module cradle, the manufacture of which was discussed in Section 4.1. Recall from the discussion of the cradle’s production that its underside features low-height table top surfaces that allow the lower assembly to sit flat and hence be built upon a table top despite the presence of vertical M8 bolt heads beneath. As shown most prevalently in Figure 4.7e, the cradle’s table top surfaces are rested on a pair of 2x4 beams rather than the table top itself to increase the accessibility to the M8 bolt heads underneath.

Once positioned on the beams, lower assembly accessories are attached to the cradle such as the BMS controller (Figure 4.7a), LLS bracket (Figure 4.7b), and EDS module (Figure 4.7c). For each set of fasteners associated with these accessories, washers are placed

96 where necessary to prevent bolt heads from contacting the fiberglass cradle. The flat washers distribute the pressure of the bolt head over a larger area to prevent the crushing of the fiberglass. Battery module 1 is subsequently placed on the cradle (Figure 4.7d), followed by the placement of its associated cold plate (Figure 4.7e). In preparation for its mating to the upper assembly, the lower assembly is temporarily secured with four vertical M8 rods as shown in Figure 4.7f, with temporary nuts on the top sides of the rods to prevent them from falling through. Figure 4.11 is an illustration of how the vertical M8 rods of the lower assembly are secured to the upper assembly during the mating process, starting from Figure 4.7f and moving forward. It is crucial during the phase in Figure 4.7f that the M8 rods are checked for vertical looseness (i.e. that the cold plate and battery module have not shifted horizontally as to bind the rods from moving vertically). Vertical looseness eases the process of pushing the rods into the upper assembly as seen in Figures 4.11b.

4.5 Combined Assembly Process Following the joining of the lower assembly to the upper is the final combined assembly process. Highlights of the major steps associated with the combined assembly process are shown with photography in Figure 4.9. View (a) shows that the cooling system manifolds for the upper and lower assemblies are built to completion prior to the joining of the two assemblies. Installation of remaining cooling system plumbing and additional electrical accessories (such as high voltage wiring) onto the combined assembly is not an instantaneous process, and therefore the hoisted combined assembly is lowered onto a flat table with the rear portion supported by the lower assembly itself and front portion supported by a stack of wooden beams as shown in view (b). View (c) shows the combined assembly re-hoisted following the complete installation of all accessories, followed by view (d) which shows the beginning of sealing the battery pack with the fiberglass exterior shell beginning with bottom lid part 1. The second part of the bottom lid is associated with the passages of wiring and cooling system components through the exterior of the battery pack, the details associated with the design and installation procedure of such interfaces covered earlier in Section 2.10 in Chapter 2.

Figure 4.5: Photography of final upper assembly details. Assembly progresses from views (a) to (d), with (e) displaying an overall view of the completed upper assembly.

Following the installation of both fiberglass bottom lid pieces is hoisting and subsequently lowering the battery pack onto its mounts associated with the battery to chassis cradle. The process is best illustrated by the sequence of three-dimensional CAD views shown earlier in Chapter 2, Figure 2.10. As a final step, the fiberglass top lid is mounted and manual isolation switch (MIS) is secured. Underneath the vehicle, the battery pack’s high

98 voltage DC interface plug is connected to the vehicle high voltage DC bus, and the pair of exposed entry/exit PEX pipes is connected to a pair of sharkbite fittings similar in type used to connect the internal plumbing network. The sharkbite fittings underneath the vehicle are adapted to a pair of rubber hoses leading to the associated cooling system radiator. Figure 4.10 shows the final product installed as viewed from the rear interior of the vehicle.

Figure 4.6: Illustration for installing vertical M8 threaded rod fasteners through front and rear steel braces. View (a) shows initial steps, view (b) shows final steps.

Figure 4.7: Photography of lower assembly process. The assembly progresses from views (a) to (f)

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Figure 4.8: Illustration of joining the lower assembly to the upper using the four vertical M8 rods associated with the lower assembly. View (a) represents the state of the assembly process shown by the photograph in Figure 4.7f. Procedure progresses sequentially from views (a) to (c).

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Figure 4.9: Photography showing the major steps associated with the combined assembly process of the battery pack. Assembly progresses chronologically from views (a) through (d).

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Figure 4.10: Photograph of rear vehicle interior following the completed installation of the battery pack. A cargo floor cage constructed of aluminum angle is subsequently placed to take loads associated with cargo and provide a finished aesthetic appeal to the rear interior. The cargo floor cage features a carpeted floor that can be hinged upward to provide access to the fuse access panel and manual isolation switch (MIS) of the battery pack.

The installation exercise successfully conducted in this chapter demonstrates the feasibility of the final Generation V design conceived towards the end of Chapter 2. Moreover, the build and installation achieved in this chapter results in a successful integration of a functioning battery pack that reduces the overall vehicle weight and has a much more compact form factor as best demonstrated by comparison of Figure 4.10 with Figure 1.3b shown in the problem statement of the thesis in Chapter 1.

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5.0 CONCLUSION STATEMENT

For the 2009 to 2011 North American EcoCAR Challenge Program, North Carolina State University developed an Extended Range Electric Vehicle (EREV), at its heart a 363 Volt, 22.3 kWh lithium-ion battery consisting of five modular lithium-ion units from A123 Systems electrically oriented in series. The first conceived battery design (Generation I) implemented in year 2010 housed the lithium ion modules in an oversized aluminum and steel rectangular enclosure. The Generation I battery design consumed excessive vehicle interior space and was equally excessive in weight, effectively forcing the total vehicle weight above its GVWR (Gross Vehicle Weight Rating).

The challenge tackled by the efforts associated with this thesis was to re-package the battery pack in the most compact fashion while simultaneously seeking the most lightweight possible structure that can theoretically restrain the battery modules at a required 20-gs horitzontally and 8-gs vertically (for vehicle crashworthiness). Both the design and build phases of the re-conceived battery pack were to be executed within a single year (2010 to 2011). Given the limited timeframe of this challenge, the redesigned battery pack was to not require modifications to structural members of the vehicle chassis. The resulting revised and far more efficient packaging scheme of the battery modules and associated water cooling system hardware resulted in a "top-heavy" shape requiring the battery pack to be hoisted into position from the rear tailgate of the vehicle. Literature review of current large-scale EREV and electric vehicle battery packs suggests this is a unique approach to large-scale battery pack installation, where it is more conventional to raise the system into position from underneath the vehicle.

The revamped packaging scheme also positioned all battery internals through a vertical pass-through in the rear of the vehicle chassis. This presented a challenging mechanical bending problem in the vertical 8-g load mode cases whereby the inertia of the heavy internal battery modules push downward/upward while the associated structure is

104 merely constrained (connected) to the vehicle chassis from its sides rather than bottom. The final design concept (Generation V) utilizes a unique steel and carbon fiber battery internal structure developed with finite element analysis where the carbon fiber sheets theoretically assist in the 8-g vertical load cases by nearly doubling the global structure minimum factor of safety (FOS). Additional weight conservation was attained with a durable, thin-walled non- structural epoxy-fiberglass shell exterior constructed over wet hand lay-up molds.

In addition to significant aesthetic improvement due to the reclamation of vehicle interior space, the successful design and integration of the Generation V battery concept into the North Carolina State University EREV vehicle resulted in an overall weight reduction of nearly 250 lb over Generation I (see Section 3.8 of Chapter 3). The final Generation V battery pack concept yields an estimated pack energy density of 0.072 kWh/kg, on the lower end of the lithium-ion class of automotive battery systems and superior to the Nickel-Metal Hydride spectrum of automotive hybrid vehicle batteries. This is a significant improvement in the vehicle battery pack energy density which was previously (with Generation I) 0.055 kWh/kg, on the order of the Nickel-Metal Hydride class.

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