The Development of an Electric Tricycle and Buck-Topology

THE DEVELOPMENT OF AN ELECTRIC TRICYCLE AND BUCK-TOPOLOGY-

BASED BATTERY PACK CHARGER

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Matthew John Taschner

December, 2011 THE DEVELOPMENT OF AN ELECTRIC TRICYCLE AND BUCK-TOPOLOGY-

BASED BATTERY PACK CHARGER

Matthew John Taschner

Thesis

Approved Accepted

______Advisor Department Chair Dr. Tom T. Hartley Dr. Jose A. De Abreu-Garcia

______Committee Member Dean of the College Dr. Malik Elbuluk Dr. George K. Haritos

______Committee Member Dean of the Graduate School Dr. Yilmaz Sozer Dr. George R. Newkome

______Date

ABSTRACT

This thesis presents the design and implementation of an electric tricycle and a buck charger. The electric tricycle uses 10-14 60Ah LiFePO 4 cells for its battery pack and can accept pack voltages in the range of 28 – 48V. To achieve such a range, the electric tricycle has the capability of using two different motor controllers. These controllers are used to regulate the current supplied to a brushless DC hub motor that is placed directly in the hub of the rear wheel. A hub motor was selected since it does not require any extra form of coupling between the motor and vehicle. Speeds up to 16 mph were achieved with the electric tricycle and based on measurements and calculations, the tricycle could travel up to approximately 85 miles.

Once the tricycle has used up the energy available in the battery pack, the cells would need to be recharged. Based on the design of a switched-mode power supply, a battery pack charger was constructed. Since the cells have such a high capacity, the charger was designed to be capable of providing relatively high charge currents.

Independent of cell chemistry, the charger was capable of providing the desired charge currents for battery packs up to 150V. Also, the charger could be unified with a battery management system (BMS) so that a more accurate method for charging could be employed. Operating in the standalone mode or the unified BMS mode, the battery pack charger was able to fully charge a 10 cell LiFePO 4 battery pack with a BMS in under 6 hours and a deep cycle sealed lead-acid (SLA) battery in approximately 2 hours.

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ACKNOWLEDGMENTS

I would like to thank my advisor Dr. Tom T. Hartley for his consistent guidance over the past few years. His knowledge and supervision have helped me become the engineer I am today. I am obliged to Dr. Malik Elbuluk and Dr. Yilmaz Sozer as my committee members and thank them for their help throughout my research. Also, if it were not for Tom Vo, another graduate student in Dr. Hartley’s research group, I would not be where I am today. He has always been there for me in times of need and is one of my best friends.

I would also like to thank Erik Rinaldo for his patience with me and for providing all of the necessary tools and parts to complete my research. He also has become a good friend of mine over the years. Greg Lewis was also a huge help in providing me with the support and equipment essential to my research. I furthermore give my appreciation to

Gay Boden who has been like a mother to me over my years in graduate school.

Special thanks must be given to Allan Hoon, the University of Akron Stile Field

House Manager, for giving me permission to use the school’s indoor track for testing.

Also, Dale Ertley has been extremely helpful and tolerant by letting me use the machine shop at my leisure. I am very grateful to Professor Milton Kult for allowing me to work for him as a teaching assistant in his circuit’s laboratory and for giving me advice in life as well as in my thesis.

Lastly, I would like to thank my loving parents and brother who have been a constant driving force of inspiration and determination. I especially want to thank my dad for helping me collect data for the electric tricycle. My family has been the most supportive, loving, and understanding group of people in my life and I love them all dearly.

TABLE OF CONTENTS

Page

LIST OF TABLES ...... x

LIST OF FIGURES ...... xi

CHAPTER

I. INTRODUCTION ...... 1

1.1 Advanced Technology Vehicles...... 1

1.2 The Cell and the Battery...... 3

1.3 Power Supply ...... 4

1.4 Goals of Research...... 5

1.4.1 Electric Tricycle ...... 6

1.4.2 Battery Pack ...... 6

1.4.3 Battery Pack Charger ...... 7

1.5 Thesis Outline ...... 7

II. BACKGROUND AND RELATED WORK ...... 9

2.1 Human Energy and the Bicycle ...... 9

2.1.1 History of the Bicycle and the Velomobile ...... 11

2.1.2 The Electric Bicycle ...... 13

2.2 Switched-Mode Power Supply (SMPS) ...... 14

2.2.1 Efficiency ...... 15

2.2.2 Drivers...... 17

2.2.3 Switching Techniques ...... 18

2.2.4 Control strategies ...... 20

2.3 Charging Techniques...... 24

2.3.1 Constant Current Charging (CC) ...... 25

2.3.2 Constant Voltage Charging (CV)...... 26

2.3.3 Trickle Charging ...... 26

2.4 Conclusions ...... 26

III. HARDWARE DESIGN FOR THE ELECTRIC TRICYCLE ...... 28

3.1 Electrifying The Tricycle ...... 30

3.2 Motor Selection ...... 30

3.3 Battery Pack Design ...... 36

3.3.1 Capacity of Cells ...... 37

3.3.2 Cell Chemistry ...... 38

3.3.3 Size of Battery Pack ...... 40

3.4 Conclusions ...... 42

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IV. HARDWARE DESIGN FOR THE BATTERY PACK CHARGER ...... 44

4.1 Buck Charger Analysis...... 46

4.2 Analog Component Selection...... 52

4.2.1 AC-DC Converter ...... 53

4.2.2 Power Switching Device ...... 55

4.2.3 Inductor ...... 56

4.2.4 Free-wheeling Diode ...... 59

4.2.5 Current Sensing ...... 61

4.3 Digital Component Selection ...... 62

4.3.1 Regulated Low Power Source ...... 62

4.3.2 Microprocessor ...... 64

4.3.3 Driving the Power MOSFET ...... 65

4.3.4 Analog-to-Digital Converter (ADC) ...... 71

4.3.5 Control Panel/Active Display ...... 76

4.4 MOSFET Protection ...... 79

4.4.1 Gate Protection...... 79

4.4.2 Snubber Design ...... 80

4.5 PCB Design ...... 82

4.6 Calibration of Sensors ...... 85

4.6.1 Voltage Sensor ...... 86

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4.6.2 Current Sensor ...... 87

4.7 Other Hardware Issues ...... 88

4.8 Charging Algorithm ...... 89

4.8.1 Standalone Charger ...... 94

4.8.2 Unified with BMS ...... 94

4.9 Conclusions ...... 95

V. RESULTS ...... 97

5.1 Electric Tricycle ...... 97

5.2 PCB Design ...... 101

5.3 Standalone Charging ...... 104

5.4 Unified BMS Charging ...... 107

5.5 Conclusions ...... 110

VI. CONCLUSIONS AND FUTURE WORK ...... 112

REFERENCES ...... 117

APPENDICES ...... 121

APPENDIX A. MPLAB ® PROGRAM LISTINGS ...... 122

APPENDIX B. PRINTED CIRCUIT BOARD AND PARTS ...... 147

LIST OF TABLES

Table Page

3.1 TerraTrike Tour custom made specifications...... 29

3.2 Parameters for determining power lost due to wind drag...... 31

3.3 Different battery chemistry comparison (Battery Space, 2010)...... 38

3.4 Most important parameters and specifications for the LiFePO4 60Ah cells (Battery Space, 2010)...... 40

4.1 Charging events and associated error messages...... 92

5.1 Experimental data gathered from the electric tricycle test...... 98

5.2 Predicted tricycle data based on calculations...... 99

5.3 Estimated weight of vehicle...... 100

5.4 Charging specifications for both battery packs in standalone test...... 105

LIST OF FIGURES

Figure Page

1.1 Cumulative fuel economy for popular HEVs (Karner and Francfort, 2007)...... 2

1.2 A battery discharging through a load...... 4

2.1 Comparison of the energy cost at different speeds for various types of transportation (Exploratorium, n.d.)...... 10

2.2 Human power capability for different durations of time (Tetz, n.d.)...... 11

2.3 An image of some of the first pedalcars, or Velocars, in Paris 1925 (Van De Walle, 2004)...... 12

2.4 Synchronous buck converter with rectifier diode replaced by MOSFET (Kris, n.d.)...... 16

2.5 Typical bootstrap driver circuit for high side N-channel MOSFET drive (Balogh, 2001)...... 18

2.6 Passive resonant snubber cell in a typical buck converter (Bodur et al. , 2003)...... 20

2.7 Architecture of Differentially Enhanced Duty Ripple Control (Fan et al ., 2009). .... 22

2.8 Typical control loop for a synchronous buck converter (Kris, n.d.)...... 24

3.1 Picture of TerraTrike Tour (Utah Trikes, 2010)...... 28

3.2 The Phoenix Cruiser or X-5304R; the Brushless DC hub motor used on the electric tricycle...... 33

3.3 Crystalyte CT4840D controller. Rated at 48V with a maximum current of 40A (1,920 W)...... 34

3.4 Thumb throttle with battery voltage gauge...... 35

3.5 Rear dropout of tricycle; metal tab secures axel of motor to frame of tricycle...... 36

3.6 Example of a 60Ah LiFePO4 prismatic cell (Battery Space, 2010)...... 39

3.7 TerraTrike fully electrified with battery pack and motor completely assembled...... 42

4.1 Generic schematic for the high power buck charger...... 45

4.2 MATLAB Simulink schematic of buck charger for simulation analysis...... 46

4.3 AC-DC converter; contains rectifier, filter capacitor, and measurement ports...... 47

4.4 Waveform of current into battery with a 0.5A ripple; units in amps (y-axis) and seconds (x-axis)...... 50

4.5 Simulated voltage waveforms of the buck charger; units in volts (y-axis) and seconds (x-axis)...... 51

4.6 Zoomed in plot of the voltage across the switch; units in volts (y-axis) and seconds (x-axis)...... 52

4.7 Picture of a Mallory high voltage capacitor used for filtering the supply voltage. ... 54

4.8 Typical B-H hysteresis curve for an inductor...... 57

4.9 Inductor used in buck charger with a nickel zinc ferrite core...... 59

4.10 Path of charge current and direction of inductor voltage for different states of MOSFET...... 60

4.11 Shunt resistor placed on low side of battery pack for current measurement...... 62

4.12 Pin requirements for each of the LDO regulators (Diodes Inc., 2010)...... 63

4.13 V-I curve for the AP1117 regulators (Diodes Inc., 2010)...... 64

4.14 Pin description for the dsPIC33FJ128MC802 microcontroller (Microchip Technology Inc., 2009)...... 65

4.15 Gate charge waveform for the STB23NM60ND (STMicroelectronics, 2010)...... 67

4.16 Typical charging curve of an RC circuit with R=49 and C=2.05nF...... 68

4.17 Current flowing into the gate of the MOSFET...... 69

4.18 FOD3180 MOSFET gate driver (Fairchild Semiconductor, 2005)...... 70

4.19 MOSFET driver circuit schematic...... 71

4.20 Schematic for ADC measurements and pin connections...... 73

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4.21 Liquid Crystal Display module for the control panel of the buck charger...... 77

4.22 Control panel and active display of the battery pack charger...... 78

4.23 Placement of RC snubber across switching device...... 80

4.24 Land pattern of the STB23NM60ND MOSFET with thermal vias placed by the tab of the package body...... 83

4.25 Layer 3 of the PCB design displaying the analog and digital sections with a common ground point...... 83

4.26 Left side of buck charger PCB...... 84

4.27 Right side of buck charger PCB...... 85

4.28 Voltage calibration curve for battery pack voltage measurement...... 87

4.29 Current calibration curve for current flowing into the battery pack...... 88

4.30 Charging algorithm flowchart...... 90

4.31 Control loop for controlling the charging current (CC mode)...... 92

4.32 Control loop for controlling the charging voltage (CV mode)...... 93

5.1 Plot of speed versus power of tricycle (predicted and experimental)...... 101

5.2 High current path near ADC...... 102

5.3 Switching waveform of voltage across the MOSFETs (50V/div)...... 103

5.4 Charge current waveform with approximately 1A peak-peak ripple (5A/div)...... 104

5.5 Charge voltage of 4 cell LiFePO4 pack...... 106

5.6 Charge voltage of 4 cell LiFePO4 pack...... 106

5.7 Charge voltage of 12V SLA battery...... 107

5.8 Charge current of 12V SLA battery...... 107

5.9 Charge voltage of battery pack with BMS...... 109

5.10 Charge current of battery pack with BMS...... 109

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CHAPTER I

INTRODUCTION

Most electric vehicles available today use a battery pack as the main source of energy storage. This battery pack consists of a string of cells typically connected in series so that a higher voltage can be achieved. It is not uncommon for these strings to be connected in parallel so that the stress induced during discharge is dispersed equally among each string. Doing so increases the reliability of the battery pack. For instance, if one cell fails open in a string, the entire string is rendered useless due to the fact that each cell in that string is connected in series. In the event of a lost string, the remaining strings would endure more stress, but the battery pack would still be capable of providing energy. So, if one cell fails open in any of the strings connected in parallel, the voltage of the battery pack still remains the same.

1.1 Advanced Technology Vehicles

The Advanced Vehicle Testing Activity (AVTA), part of the U.S. Department of

Energy’s FreedomCAR and Vehicle Technologies Program, has been conducting tests of hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) since August

1995 in an attempt to provide benchmark data for technology modeling and vehicle

development programs (Karner and Francfort, 2007). Results of these tests show performance of both the vehicle and the vehicle’s battery. Fuel economy results for many of the more popular HEVs can be found below in Figure 1.1.

Figure 1.1: Cumulative fuel economy for popular HEVs (Karner and Francfort, 2007).

With so many different types of HEVs and PHEVs available on the market, transportation in the United States is primarily focused on the automobile. However, there has also been an increase of interest in recreational transportation such as the bicycle. The bicycle not only promotes good health and exercise, but can also be transformed into a vehicle capable of quick and easy transportation. Placing a motor on the bicycle along with a battery pack to supply energy to the motor can allow the rider to increase travel speed as well as overall distance. Electric bicycles may not be as convenient to most Americans, but besides recreation it is an alternative form of

transportation that can be used for delivery of goods and services as well as commuting to work. Specifically in developing countries, the electric bicycle can be used to open communication to remote villages to establish educational opportunities (Morchin and

Oman, 2006). Also, if roads are difficult to travel on with an automobile, an electric bicycle can make the journey more tolerant.

1.2 The Cell and the Battery

Batteries are electrochemical devices that store energy in a chemical form and distribute electrical energy upon demand. Mostly, batteries are portable sources of electrical power used in cell phones, MP3 players, PCs, laptops, digital cameras, power tools, and advanced technology vehicles such as HEVs and PHEVs.

The terms battery and cell are often used interchangeably. To be specific, a battery is referred to as a unified array of cells, where the cell is the basic building block of the battery. A cell consists of a positive electrode and a negative electrode which are submersed in an electrolyte (Levy and Bro, 1994). When discharging the cell, electrons flow from the negative electrode to the load, giving up most of their energy before traveling back to the positive electrode. Since a negative charge cannot accumulate on the positive electrode, this charge is neutralized by positive ions being released from the negative electrode or negative ions being released from the positive electrode. The electrical circuit is completed when positive ions move towards the positive electrode or when negative ions move towards the negative electrode through the electrolyte. This process occurs in many battery chemistries such as lithium-ion where positive ions are the charge carriers in the electrolyte. Once all of the energized material is converted to

its less-energized state, the discharge process is complete (Annavajjula, 2007). The described discharge process is shown in Figure 1.2.

Figure 1.2: A battery discharging through a load. 1.3 Power Supply

The discharge process of a battery or cell was described above, but once all of the energized material is converted to its less-energized state, the battery or cell must be recharged. A battery that is capable of reversing the discharge process by converting electrical energy into stored chemical energy is referred to as a secondary battery.

Primary batteries are only able to convert chemical energy into electrical energy, so they can only be used for one discharge process (Hartmann, 2008). Secondary batteries are

more desirable for most consumer products and applications. However, once the battery has been depleted of its energy, there must be a means of recharging it.

Most types of direct-current (DC) power supplies are suitable for charging a battery as long as its voltage and current ratings are capable of satisfying the charging specifications of the battery. Some of the more efficient power supplies available are called switched-mode power supplies (SMPS). The heart of a SMPS is a DC-DC converter which accepts a DC input and produces a controlled DC output. Switched- mode power supplies make use of electronic switching in order to process this electrical power (Perez, 2000). The fact that most electronic switches dissipate very little power is what makes a SMPS so efficient. In many applications, switched-mode power supplies typically operate at switching frequencies from as low as 50Hz all the way up to 1MHz or higher.

1.4 Goals of Research

Although there are many advantages to an alternative energy vehicle such as a

HEV or PHEV, an electric bicycle offers other very practical benefits. As will be discussed more in detail later in this thesis, the bicycle is the most efficient form of transportation and is also much more affordable than an automobile. There are also different forms of the bicycle that can be used such as the tricycle. A tricycle has one extra wheel than the bicycle making balance of the vehicle effortless. Tricycles also tend to be more comfortable and ergonomically designed. Combining the concepts of the electric bicycle and the tricycle has lead to the motivation for developing an electric tricycle for part of the research presented in this thesis. In order to electrify a tricycle, the

vehicle needs a motor and a battery pack. The battery pack should be rechargeable so that multiple trips can be made with the tricycle. In order to recharge the batteries, some form of charger is needed. This requirement is what has lead to the second portion of the research presented in this thesis on switched-mode power supplies.

1.4.1 Electric Tricycle

Designing and constructing an electric tricycle requires careful consideration of the many different aspects of the vehicle. First, it is important that the frame of the tricycle be selected so that it satisfies the rider’s intention, comfort level, and style. Once the frame is decided on, proper measurements and calculations must be made so that a suitable motor can be chosen. When selecting the motor, it is also necessary to have a desired maximum speed of travel in mind. Sizing the motor depends on multiple factors such as the frontal area of the vehicle, weight, wind drag, air density, friction, and desired velocity. The goal for this portion of the project is to select a frame for the tricycle that will allow the rider to be in a recumbent seated position and capable of traveling at speeds around 15 mph.

1.4.2 Battery Pack

Once the size and type of motor is known, it is important to design a battery pack that has the correct voltage and is capable of providing enough energy to propel the tricycle at the desired speed. Designing the battery pack involves knowledge of the voltage rating of the motor, the power rating of the motor, and the desired distance of travel. To achieve a high enough voltage, cells of a certain chemistry type should be connected in series. The chemistry of the battery pack is important since this will

determine the overall weight of the pack, the cranking current capability, and the energy density. Lithium based cells tend to have higher energy densities than most cells available on the market such as zinc-silver oxide, alkaline-zinc manganese dioxide, lead- acid, nickel cadmium (Ni-Cd), and nickel metal hydride (Ni-MH). With the goal to travel around 100 miles on a single charge as well as keeping the overall weight of the vehicle as minimal as possible, lithium based cells seem to be the best candidates.

1.4.3 Battery Pack Charger

With a properly selected battery for the electric tricycle, there must be a means to charge the battery pack. In order to quickly and effectively charge the battery pack, a battery charger should be designed and constructed with the capability of providing currents up to 10A or higher. Since the requirement for the capacity of the cells is relatively high, charging at currents in the 10A range should be sufficient. Furthermore, it is desired to have the charger be reasonably efficient, so designing a switched-mode power supply appears to be the best option. It should also be flexible and capable of charging batteries at the desired current levels within a large voltage range. Lastly, integrating the charger with a battery management system (BMS) can allow more accurate charging of individual cells in a battery pack and ensure that the pack stays balanced throughout the charge process.

1.5 Thesis Outline

The research that has been conducted is presented as a thesis that entails six chapters. Chapter I offered introductory material that is required to understand the goals for creating an electric tricycle, the need for a battery pack, and consequently the need for

a battery pack charger. The following chapter provides history on the bicycle, its predecessors, and the electric bicycle as well as background information on switched- mode power supplies and different charging strategies. Chapter II also presents related work previously published on switched-mode power supplies. Chapter III delves into detail on the design of the proposed electric tricycle including calculations for properly selecting a motor and battery pack. The hardware design and implementation of the proposed battery pack charger is discussed in great detail in Chapter IV. This includes a detailed analysis on the simulation of the battery pack charger, the motivation behind selecting both the analog and digital components, proper protection for high stress components, the PCB layout of the circuit, sensor calibration, and the software algorithms used to control the charging process. The results from testing the electric tricycle and battery pack charger are presented in Chapter V. This chapter also discusses the integration of the battery pack charger with a BMS. The final chapter, Chapter VI, concludes the work presented in this thesis and makes recommendations for future work in this area.

CHAPTER II

BACKGROUND AND RELATED WORK

This chapter discusses the history of the bicycle and other types of vehicles, switched-mode power supplies (SMPS), DC-DC converters, and different charging techniques for batteries. There is also discussion of human energy and comparisons between the different types of transportation for humans. Furthermore, this chapter presents background information on power supplies, MOSFETs, and drivers as well as previous and existing work on switched-mode power supplies.

2.1 Human Energy and the Bicycle

Human beings have the capability of generating enough power to propel many different types of vehicles. The most common type of vehicle that humans use as transportation is the bicycle. Bicycles are the most efficient form of transportation available. This can be seen from the chart in Figure 2.1, which compares many different and common types of transportation for humans. The only form of transportation that is remotely close to the efficiency of the bicycle is either walking or running. However, a bicycle can be up to five times more efficient than a human walking or running. Humans

around the world have recognized the usefulness and effectiveness of the bicycle. There are over one billion bicycles worldwide to attest to this (Exploratorium, n.d.).

Figure 2.1: Comparison of the energy cost at different speeds for various types of transportation (Exploratorium, n.d.).

The engine powering a bicycle in most cases is a human. In order to provide the energy necessary to propel the vehicle, humans must use some sort of fuel. Unlike an automobile that uses non-renewable resources like fossil fuels, humans rely on the food that they eat to generate the required amount of energy. As an example of how much more efficient a bicycle is compared to an automobile, it takes about 100 calories to power a cyclist 3 miles where the same amount of calories could only power a car for 280 feet.

It is important to maintain a healthy balance of water, protein, carbohydrates, fats, vitamins, and minerals to be able to maintain enough energy to power a bicycle far 10

distances. Some humans are capable of sustaining high levels of power for many hours.

These are primarily top class athletes that are capable of performing intensive exercise for long durations. A plot of the long term human power capability can be seen in Figure

2.2, which compares average humans to healthy and first class athletes (Tetz, n.d.).

Figure 2.2: Human power capability for different durations of time (Tetz, n.d.).

2.1.1 History of the Bicycle and the Velomobile

Bicycles have been around for centuries. Much about the history of the bicycle and its predecessors was presented by Van De Walle (2004). The first ancestor of the bicycle was built around 1817 called the Draisine. It was not until 1866 when the first bicycle was patented in the U.S. by a French mechanic named Pierre Lallemont. In the year 1885, a vehicle called the Rover Safety Bicycle was made commercially successful

by an English bicycle manufacturer called J.K. Starley. By the late 1800’s, millions of people were riding bicycles. However, in the early 1900’s when the automobile was developed, many people lost interest in cycling.

Even before the Rover Safety Bicycle in 1885, there had been designs of pedal vehicles that resembled an automobile. These vehicles, called the velomobile, consisted of three or four wheels and allowed the rider to be in a seated or semi-recumbent position. Any type of streamlined human powered vehicle was considered a velomobile.

The first well documented velomobile, or pedalcar, was called the Velocar. The concept of this vehicle came from the automobile constructor named Charles Mochet. Pictured below in Figure 2.3 are some of the first commercially available velomobiles.

Figure 2.3: An image of some of the first pedalcars, or Velocars, in Paris 1925 (Van De Walle, 2004).

During the Second World War, velomobiles were an extremely popular form of transportation. After the war there was large decline in bicycle use due to the financial 12

situation and the focus was taken off the velomobile concept and concentrated on the automobile. From the period between 1950 and 1970, there was hardly any focus on bicycle innovation, and cycling technology was temporarily put on hold. It was not until after the oil crisis in the 1970’s that the bicycle regained its prominence.

After the 1970’s, many improvements and variations of the bicycle frame and the position and orientation of the seat were made. With many different styles available, the recumbent position seemed to emerge rapidly as the most preferred rider position due to its comfort and efficiency. However, one might think that a standard was created so that all recumbent vehicles followed a particular configuration, but this is not true. Today, there are many different forms of recumbent bicycles that include three and four wheelers, different size tires, lengths, and heights.

2.1.2 The Electric Bicycle

Almost as long as the bicycle has been around, people have had the desire to propel the bicycle with other sources of power besides the human. A simple way to do this is to make an electric bicycle by placing a DC motor on one of the wheels along with a battery pack to supply energy to the motor. Electric powered bicycles have been around for almost 120 years with the first electric bicycles appearing in the late 1890s.

Many electric bicycles are built and designed specifically for travel. However, other uses of the electric bicycle include delivery of goods and services, commuting to work, and even recreation. Electric bicycles can increase the average travel speed of a pedaling cyclist by 5 to 6 mph. This not only can get the rider to a location faster, but also allows for a longer distance of travel. Electric bicycles are especially beneficial for 13

physically weak people since it can assist them on steep hills and also increase their total distance of travel (Morchin and Oman, 2006).

Since electric bicycles use batteries as the main source of energy for propulsion, the battery pack must be capable of providing enough energy for a desired distance and/or duration. To properly size the battery pack, there are certain factors such as the maximum speed of travel desired, the total distance, the type of terrain, and the wind that must be taken into consideration. With the proper battery pack, there should also be a means to recharge it. A battery pack charger is almost as important as the electric bicycle itself because without it the rider could only use the electric bicycle for as long as one charge of the battery could last. Once the battery pack has been depleted after usage, the pack should be recharged so that the electric bicycle can be operable once again.

2.2 Switched-Mode Power Supply (SMPS)

Recharging a battery pack can be very simple; set the voltage and current limit on a DC power supply with proper ratings and attach the positive and negative terminals of the battery to it. However, to charge a battery efficiently, there are different types of power supplies that can be used. One of the most efficient chargers, or power supplies, is called a switched-mode power supply (SMPS). This type of power supply is comprised of a switching regulator that maintains a constant voltage and/or current on the output.

The input voltage will typically always be higher than the output voltage for a battery charger so that high charge currents can be achieved.

Taking a DC voltage and stepping it up or down to a larger or smaller DC voltage is commonly referred to as a DC-DC converter. A converter that steps the voltage up is 14

called a boost converter, while a converter that steps the voltage down is referred to as a buck converter. Therefore, a buck converter is typically used for power supplies that are used to charge battery packs.

2.2.1 Efficiency

Battery operated systems require very high efficiency in order to make the battery life last as long as possible. DC-DC converters for this type of system must be capable of regulating the output voltage even under no load. This requires the DC-DC converter to function in discontinuous mode since there can be little to no load current. In order to maintain high efficiency, it is important to consider the types of losses associated with these converters. There are three main types of losses: load dependent conduction losses, frequency dependent switching losses, and fixed losses (Arbetter et al. , 1995).

The buck converter shown below in Figure 2.4 replaces the rectifier diode, or free-wheeling diode, with an N-channel MOSFET (Kris, n.d.). This lowers the losses in the circuit and increases efficiency since MOSFETs have a much lower on-resistance and therefore a lower voltage drop than a diode. This is called a synchronous buck converter because two complimentary PWM signals with adequate dead-time between their pulses must be generated in order to ensure that the MOSFETs do not conduct simultaneously.

This setup lowers the conduction losses due to the low on-resistance of the transistors.

Figure 2.4: Synchronous buck converter with rectifier diode replaced by MOSFET (Kris, n.d.).

During operation, a pulse is sent to the driver that controls the switching components that will causes current to flow through Q 1 and the inductor. This current must be sensed in order to properly control the converter, which is done through a shunt resistor placed in series with the inductor. When the current reaches a pre-set maximum value, the control to Q 1 is shut off and Q 2 is turned on after a small delay to ensure that

Q1 and Q 2 are not on simultaneously. Whenever the current gets to zero, the control circuitry stops driving the switches and the output voltage remains fixed.

As described above, the three main types of losses in this circuit are the conduction losses, switching losses, and fixed losses. The on-resistance of the two switches, the inductor winding resistance, the current sense resistor, and the equivalent series resistance of the capacitor are the main contributors to the conduction losses.

Reducing the resistance of any of these components will improve the efficiency of the converter. Switching losses occur from the inductor core loss, the switches’ voltage/current overlap, and the capacitor charge/discharge losses. Leakage currents of

the switches and other components as well as controller standby current are the main factors of the fixed losses. These losses are the smallest in this application, but still must be accounted for in order to accurately define the efficiency of the converter.

2.2.2 Drivers

In applications that require a MOSFET to be on the high side of the circuit, either an N-channel or P-channel MOSFET can be utilized. N-channel MOSFETs offer many benefits over P-channel MOSFETs such as their lower price, high speed, and low on- resistance. In order to properly drive the gate of this N-channel MOSFET, a driver capable of handling the violent voltage swings of the switching transitions should be used. Since this MOSFET is on the high side of the circuit, a voltage higher than the largest DC voltage in the circuit must be applied to its gate.

In many cases, a driver that employs a technology called bootstrapping is used for switching high side MOSFETs. The design process and technique of bootstrapping has been discussed and presented by Balogh (2001) and Zhou et al. (2009). Bootstrap drivers are primarily used in applications where an N-channel MOSFET is positioned on the high side of the circuit. Below in Figure 2.5 is a typical application circuit requiring a high side driver using the bootstrapping method. The driver voltage, V DRV , must be large enough to fully turn the MOSFET on, but also not exceed the gate-source junction’s maximum rating. By using a bootstrap driver, it is possible to switch a voltage higher than the largest voltage in the circuit to the gate-source junction of the MOSFET by charging a capacitor to the desired drive voltage and then applying it to the MOSFET’s gate. Since the application circuit shown in Figure 2.5 is a typical DC-DC buck

converter, the free-wheeling diode conducts during the off period of the MOSFET which references the bottom side of the bootstrap capacitor, C BST , to ground. The bootstrap capacitor then charges up to the drive voltage, V DRV , through the bootstrap diode D BST .

To turn the MOSFET on, the PWM controller sends a high signal to the driver, which internally level shifts this signal up causing the top side of C BST to switch to the output pin of the driver. Since C BST is always referenced to the source of the MOSFET, applying the top side of the bootstrap capacitor to the gate will turn the MOSFET on.

Figure 2.5: Typical bootstrap driver circuit for high side N-channel MOSFET drive (Balogh, 2001).

2.2.3 Switching Techniques

As described above in Section 2.2.1, the limiting factors on efficiency in a switched-mode power supply occur from conduction and switching losses. These losses arise from the “hard switching” of the primary MOSFET and rectifier diode or rectifier

MOSFET. Hard switching happens when there is nothing to help aid in the turn-on and/or turn-off of these switching devices leading to a maximum amount of current and

voltage across the devices. This can be reduced by assisting the devices during turn-on and/or turn-off through a technique known as soft switching.

Soft switching methods use resonant techniques to switch on at zero voltage and to switch off at zero current. The resonant circuitry employed is non-dissipative, meaning that the external circuitry added to aid in the switching does not contribute to the losses of the converter. Typical resonant circuitry, commonly referred to as a snubber cell, consists of energy storage devices like capacitors and inductors as well as some semiconductor devices such as diodes. An example of a passive resonant snubber cell is pictured in Figure 2.6. The basic concept of operation is to aid the switch device, T, during transitions so that commutation occurs under zero voltage switching (ZVS) or zero current switching (ZCS). The snubber inductor, L S, limits the current flow through the transistor, T, so that a transition can take place under near ZCS. For ZVS to occur, the buffer capacitor, CB, and snubber capacitor, Cs, are charged up to approximately the input voltage through the diode D S2 . At this point, the transistor is able to switch off with nearly zero voltage across it. Once the switch is off, C B and C S discharge through diodes

DS1 and D S3 to the load.

Using resonant snubber cells to assist the switching devices has been developed to not only protect the switching components in DC-DC converters, but also to increase the overall efficiency of the converter. There are many different techniques used for soft switching, but many lead to excessive voltage and current stresses in devices lowering power density and complicating control. There are active resonant snubber cells such as the ones presented by Hashizaka et al. (2007), Togatov et al. (2008), Adib and

Farzanehfard (2009), and Saha (2011). However, most active snubber cells are criticized due to their high cost, complexity, difficult control, and large circulating energy while passive snubber cells are cheaper and have a higher performance/cost ratio than the active ones (Bodur et al. , 2003).

Figure 2.6: Passive resonant snubber cell in a typical buck converter (Bodur et al. , 2003).

2.2.4 Control strategies

A DC-DC converter must be capable of regulating its output voltage by responding quickly to fast load current transients. To achieve such a fast response time,

Current-Mode Control (CMC) and the Hysteretic Control are used. The downside to these modes of control is the large noise sensed from the current signal during CMC and the voltage ripple on the output when using hysteretic control. Parasitic parameters of the circuit can cause degradation of performance by affecting the switching frequency during hysteretic control (Fan et al. , 2009). Two types of control strategies are presented below:

differentially enhanced duty ripple control, current-mode control, and voltage-mode control. Also, compensators used in controlling DC-DC converters are presented.

2.2.4.1 Differentially Enhanced Duty Ripple Control (DE-DRC)

In order to control the duty cycle of the MOSFET driver accurately, the voltage on the output must be known. This is sensed through a resistor divider network R S1 and

RS2 , as seen in Figure 2.7, and compared to a reference voltage. The voltage difference between these two values is amplified through a positive and negative differential difference amplifier ( DDA p and DDA n). This produces two differential error voltages, Vp and Vn. The Control Ripple Voltage, Vrd , is generated by connecting a simple RC filter,

Rr and Cr, between the switch node and the output of the DDA p. This filter acts as both a low pass and high pass filter simultaneously. The Control Ripple Voltage is comprised of two voltages, one that is a low pass filtered signal from the switch node and the other is a high pass filtered signal from Vp. Vrd is compared with the negative control voltage,

Vn, to create a trigger signal for the On-Pulse Generator. This generator, or driver, adjusts the duty cycle based upon the trigger signal, while the switching frequency remains constant. This specific design and implementation is called a differentially enhanced duty ripple control (DE-DRC) buck converter and was presented by Fan et al .

(2009).

Figure 2.7: Architecture of Differentially Enhanced Duty Ripple Control (Fan et al ., 2009).

2.2.4.2 Current and Voltage-Mode Control

The two main control strategies for switched-mode power supplies (SMPS) are voltage-mode control and current-mode control. Both of the control methods must have knowledge of the error; the difference between the desired output and the actual output.

For voltage-mode control, the time that the supply voltage is applied across the inductor is controlled by the error in voltage on the output, which indirectly controls the flow of current. Current-mode control takes the error in the output voltage and sets a threshold for an analog comparator which in turn sets the peak current flow. Voltage-mode control ends up being more stable in regards to noise, while current-mode control has a faster transient response and prevents the inductor current from increasing too quickly which can lead to saturation and MOSFET failure (Kris, n.d.).

Voltage-mode control turns to be a less efficient control strategy than the current- mode control. Using a single control loop, the output voltage is sensed for feedback and compensated for to achieve a desired set point. Although the transfer function for this mode of control is stable, the achievable bandwidth is seriously limited (Ramirez et al. ,

2003). When faced with line and load disturbances, the response of the control circuitry is quite poor and often turns out to be highly oscillatory. Due to these downsides of voltage-mode control, it is rarely used in commercial applications.

Current-mode control is a very robust control scheme that has been successfully tested and accepted as the primary mode of control. This mode of control often implements the use of proportional (P) and proportional-integral (PI) linear compensators. CMC essentially is a multiloop control scheme configured in a cascade style. There are two main loops employed when using current-mode control. The inner loop senses the inductor current for feedback purposes and uses a P-compensator for damping and protection against disturbances. An outer loop is used to sense the output voltage which implements a PI-compensator to control the output voltage to a desired set point.

2.2.4.3 DC-DC converter control compensation

Whether selecting the voltage-mode or current-mode control strategy, some type of control algorithm must be implemented. There are many types available, but the most common algorithm seen for a SMPS is the proportional, integral, and differential (PID) method. The design of control schemes and compensators for control of DC-DC converters have been presented by Ramirez and Pérez (2002), Lin et al. (2007), and

Takeshita et al. (2001). As with most control strategies and algorithms, there are delays that must be accounted for. Pictured below in Figure 2.8 is a control loop for a synchronous buck converter. The boxes enclosed by the blue dotted line can all be generated using a digital signal controller (DSC) which is a hybrid of a microcontroller and a digital signal processor (DSP). Many DSCs are capable of performing all of the functions contained within the blue dotted line. Each of these blocks has a delay associated with it. When designing a PID controller, all of these delays must be accounted for in order to achieve accurate performance. For a control loop, the time that it takes from the acquisition of the voltage and current measurements until the PWM duty cycle is adjusted us called latency. More stability is gained as latency goes down and causes the system to be more responsive.

Figure 2.8: Typical control loop for a synchronous buck converter (Kris, n.d.).

2.3 Charging Techniques

To recharge a battery that has been depleted of its energy, multiple techniques have been used. The process of charging a battery typically consists of multiple phases.

These phases are classified by the amount of energy the battery accepts during the charging process (Dörffel, n.d.).

The first phase for charging a battery is called the initial or bulk charging phase.

In this phase, most of the battery’s capacity is replenished. Based on the battery’s manufacturer, the battery accepts the highest charging current in this phase. This charge current is dependent upon the rated capacity of the battery. Next, the absorption charging phase returns the remainder of energy to the battery. In this phase, the charging current is reduced so that the battery does not become damaged. During the float charging phase, the energy is maintained by reversing the self-discharge process of the battery. Since some batteries require that all the cells in series are equalized, there is an equalization charging phase. However, this phase can take a very long time.

Many times, the charge of a battery can be restored by applying a constant current

(CC) followed by a constant voltage (CV) or some type of combination of these. Other types of charging techniques (Plett, 2004) are discussed in the following sections.

2.3.1 Constant Current Charging (CC)

During the constant current charging procedure, a power supply continually varies its voltage while applying a steady current to the battery. This process occurs in the initial or bulk charging phase. Once the battery reaches its voltage limit, the constant current charging process is terminated.

2.3.2 Constant Voltage Charging (CV)

Another charging method is called constant voltage charging. By applying a constant DC voltage greater than the manufacturer’s specification of the upper voltage limit of the battery, the current being supplied starts to taper off. This DC voltage is acquired from the AC mains using a rectifier, filter, and regulator. The float charging technique mentioned above is a variation of this constant voltage procedure. By supplying a constant DC voltage that is slightly lower than the battery’s upper limit will ensure that if the battery voltage drops a little due to self-discharge, it will remain fully charged. However, constant voltage charging is usually employed in the absorption charging phase.

2.3.3 Trickle Charging

Trickle charging is usually used for batteries that are in storage to compensate for self-discharge. This technique is exercised in the float charging phase by applying a low rate of continuous charge in order to maintain the battery at its fully charged state.

2.4 Conclusions

This chapter not only outlined the history of the bicycle and the electric bicycle, but also discussed the reason for having a power supply to charge batteries and the previous research conducted on switched-mode power supplies. Due to the ease of portability of batteries for specific applications, having batteries that are capable of recharging is essential to reliability. Different charging techniques and charging phases of these types of batteries were also covered in this chapter.

Bicycles have been around and utilized as a very efficient form of transportation for over a century. With such a long history, there have been many modifications, designs, and body types of the bicycle. Based on past documentation of bicycles, there have been two, three, and four-wheeled renditions of this vehicle. However, there could be an unlimited number of wheels used for bicycle transportation, but this does not seem practical. To ease in transportation, the electric bicycle was conceived and constructed.

This not only can help a rider get to a location quicker, but also significantly reduces the physical effort and fatigue of the rider.

Propelling the electric bicycle is most commonly done by a motor supplied by batteries. With the depletion of these batteries from usage, there is a need to recharge them. Based on previous work, a switched-mode power supply is one of the most efficient forms of supplies to charge a battery. Research has been largely concentrated on the efficiency of these supplies, or converters, with emphasis on procedures for driving the switching components, different techniques used in switching, and the control strategies developed.

CHAPTER III

HARDWARE DESIGN FOR THE ELECTRIC TRICYCLE

In order to build an electric vehicle, the first piece of equipment that needs to be designed is the frame. For this project, a tricycle is chosen as the form of transportation for its ease of use, balance, and efficiency. The specific tricycle frame that was selected is the WizWheelz TerraTrike Tour pictured below in Figure 3.1. This particular style of tricycle that TerraTrike makes allows the rider to be low to the ground and in a reclined position. The default frame with standard accessories is what is pictured below, however some additional equipment can be added on and some other equipment can be altered.

For this project the TerraTrike was built to a custom design where the specifications can be seen in Table 3.1.

Figure 3.1: Picture of TerraTrike Tour (Utah Trikes, 2010). 28

Table 3.1: TerraTrike Tour custom made specifications.

Base Model WizWheelz TerraTrike Tour Boom Length Long (45-49 X-Seam) Front Gearing Schlumpf High Speed Drive 34/85 170mm Front Derailleur None (Schlumpf) Rear Derailleur Shimano Deore LX Front Wheels Aluminum WizWheelz Rear Wheel Aluminum WizWheelz Rear Gearing 9-speed cassette Idlers TerraCycle High Performance Idler Kit Chain KMC Z9000 9-speed chain Brakes Avid BB5 Disc Brakes Fairing Mueller WindWrap XT 6mil Clear

Once the frame of the vehicle has been decided upon, the type of motor that will be used for the propulsion of the vehicle must be considered. Where the motor is going to be mounted, the power of the motor, and the type of motor are all important factors that need to be accounted for. Once the motor has been selected, there must be some form of controller to regulate and control the speed. All of these aspects have been determined and are described below in Section 3.2.

Now that a frame and motor have been designed, the next step for an electric vehicle is to decide the type of power source and where that source will be placed on the vehicle. The most widely used and convenient power source for electric vehicles is the battery pack. The nice thing about using batteries is their ability to store a large amount of energy in a relatively small amount of physical space. Another way of stating this is that batteries are capable of having high energy densities. The energy density of a cell is determined by the type of chemistry that is used in the cell. So, this is another factor that must be accounted for in the design of a battery pack. 29

The battery pack must be capable of multiple discharge and charge cycles in order for the vehicle to be reliable. There are chargers currently available on the market that can be purchased to charge the battery pack; however, many are limited to a low maximum output voltage. It would still be possible to charge the battery pack, but these chargers would not be capable of charging the entire pack at once due to the high voltage of the pack. In order to charge the entire pack, a buck charger is designed to be powered from a standard household outlet. The design of this charger is described below in

Chapter IV. Many of the components in this charger must be very carefully designed and looked at since this is a high power application.

3.1 Electrifying The Tricycle

Once the battery pack and motor have been designed and installed on the tricycle, it will be ready to ride. So, the first item to be designed is the motor in an attempt to properly propel the vehicle. There are many types of electric motors available, but the best kind of motor for this application is a hub motor that will be discussed later. Also, there are different controllers that can be used to control the motor during operation.

Depending on the type of motor that is being used, an appropriate type of controller must be selected in order to have smooth operation. Lastly, a battery pack must be designed to satisfy certain specifications of the vehicle. In the sections to follow, the motor, motor controller, and battery pack are designed and discussed in detail.

3.2 Motor Selection

For the electric tricycle to move anywhere there needs to be a motor placed on the vehicle someplace. There are different places that the motor can be mounted, but the

place that requires the least amount of mechanical alteration to the frame is to mount it directly in the hub of the back wheel. A brushless DC hub motor allows for ease of installation since there is no need for any extra coupling between the motor and the wheel. These types of motors are directly mounted between the dropouts on the rear fork of the frame.

Before the exact motor can be selected, the power of the motor must be determined. This is determined from a number of calculations based on parameters such as the weight of the vehicle plus rider, wind and rolling resistance, wind velocity, maximum speed desired for travel, etc. Shown below in Table 3.2 is a list of the important parameters needed for the determination of the power consumed due to wind drag (Morchin and Oman, 2006).

Table 3.2: Parameters for determining power lost due to wind drag.

Parameters English Units SI Units Trike Frontal Area 15 ft 2 1.394 m2 Coefficient of Drag 0.77 0.77 Air Density (Akron, Ohio) 0.0805 lb/ft 3 1.29 kg/m 3 Max Desired Ground Velocity 15 mph 6.7 m/s Average Wind Velocity (Akron, Ohio) 8 mph 3.58 m/s

First, the relative velocity of the vehicle and rider, vr, was calculated by summing the ground velocity and the wind velocity, vw. Using the maximum ground velocity and average wind velocity given in Table 3.2, the relative velocity was found to be 23 mph, or

10.28 m/s. Next, the force due to the wind drag is found by

(3.1)

where Cd is the coefficient of drag, ρ is the density of air, and A is the frontal area of the vehicle. From equation 3.1, the force of drag was found to be approximately 16.43 lbs or

73.1N. The power generated by the wind is given by

(3.2)

where vg is the ground velocity of the vehicle. The power needed to overcome the force of wind with velocity 8 mph at a vehicular ground velocity of 15 mph, was determined to be roughly 490 W. This should be about the average power needed to be overcome due to the wind. Obviously, if there are high wind velocities or gusts of wind, the required power will increase.

The rolling resistance caused by the contact of the tires with the surface being traversed is another factor that contributes to the required power of the motor. In order to determine the rolling resistance, it is important to identify certain parameters such as the width of the tire, the type of material from which they are made, and the amount of air pressure in the tube. For the standard aluminum WizWheelz tires that come with the

TerraTrike, the coefficient of rolling resistance, Cr, was determined to be 0.005. This was taken from the approximate value of rolling resistance for production bicycle tires at

120 psi (Wikipedia, 2010). Calculating the power due to the rolling resistance using

(3.3) 9.81 where m is the mass of the tricycle plus rider, yielded a result of about 34 W. This was based on a 190 lb rider on the 37 lb tricycle, which is about 103 kg. It can be seen here that the power due to the rolling resistance is almost negligible compared to the power

due to wind resistance. However, since conditions of the tires can change due to temperature and pressure, the value will be rounded up to 50 W.

It has been determined from the above calculations that the power needed to be generated to make the tricycle go 15 mph was about 540 W. This means that the motor must be at least this powerful so that the desired specifications can be met. There are many hub motors available on the market, but the best and most powerful ones are the

Crystalyte 5000 series motors. The hub motor that was selected was the X-5304R, which is more commonly referred to as the Phoenix Cruiser distributed by Electric Rider and pictured below in Figure 3.2 (Electric Rider, 2010). This motor’s power rating is over

2000W, which is more than enough to satisfy the power requirements.

Figure 3.2: The Phoenix Cruiser or X-5304R; the Brushless DC hub motor used on the electric tricycle.

Pictured above is a brushless DC motor controlled by pulsing its three coils with the use of Hall Effect sensors for feedback of the rotor’s position. This is executed by a controller that comes with the purchase of the motor. For this particular model, the X-

CT4840D controller is rated for a nominal 48V with a maximum current of 40A implying 33

that the maximum power is 1,920 W. This power limit is well above the 540 W that is needed to make the tricycle travel at a velocity of 15 mph.

The X-CT4840D controller is not the only type of controller that can be used with the Phoenix motor. There are others that can be used such as the C3635-NC controller that is distributed by Grin Technologies. It has a voltage rating of 36V with a maximum current of 35A limiting the power to 1,260 W (Grin Technologies, 2010). Again, this is well above the 540 W power specifications. The reason that these controllers were chosen is discussed later in Section 3.3. In Figure 3.3, a picture of the X-CT4840D controller is shown. The C3635-NC controller is not pictured because it takes the same form factor as the X-CT4840D.

Figure 3.3: Crystalyte CT4840D controller. Rated at 48V with a maximum current of 40A (1,920 W).

Both of the controllers selected require some form of input to adjust the speed of the motor. A thumb throttle with a 48V battery voltage gauge was included with the X-

CT4840D controller and is also compatible with the C3635-NC controller. The thumb

throttle and the battery voltage gauge are shown in Figure 3.4. This is easily mountable on one of the handlebars of the tricycle for convenient access to the rider.

Figure 3.4: Thumb throttle with battery voltage gauge. Slight modifications had to be made to the tricycle to properly mount the motor in the rear dropouts of the tricycle. The axel of the motor turned out to be slightly larger in diameter than the rear dropouts, requiring the construction of two aluminum tabs to keep the motor locked to the frame. These metal tabs were placed on both sides of the tricycle and secured to the frame to ensure the motor would not slip out of the dropouts during operation. Pictured below in Figure 3.5 is the rear dropout of the tricycle. The yellow circle highlights the area where the metal tabs are used to secure the axel of the motor to the frame of the tricycle.

Figure 3.5: Rear dropout of tricycle; metal tab secures axel of motor to frame of tricycle. 3.3 Battery Pack Design

Now that the motor has been designed and selected, it is now necessary to design the battery pack that will be used in this application. Since there are two types of controllers that can be used for the motor; the pack is designed based on the controller with the higher power rating (1,920 W). The X-CT4840D controller has a voltage rating of 48V, so the battery pack must operate approximately at this voltage in order to maintain operation. In order to achieve this voltage, a string of low voltage cells must be connected in series. Since the battery pack consists of cells connected in series to attain a higher voltage, the capacity of the pack is determined by the weakest cell in the string.

To achieve a larger capacity for the pack, multiple strings can be connected in parallel.

However, only one string of cells is used for the electric tricycle due to its ease of management. 36

3.3.1 Capacity of Cells

Since there will only be one series string of cells in the battery pack, the capacity of the pack is dependent on the individual cells’ capacity. First, the amount of current that will be drawn by the application is determined by the controller. Based on the above calculations for the amount of power needed to be overcome by the motor, the current flowing through the motor can be determined if the battery pack voltage is known. Using the X-CT4840D controller, the largest current expected is approximately 11.25A while the maximum current expected from the C3635-NC controller is about 15A. These current values are calculated assuming that the pack voltage is at its limit for the controller, 48V and 36V respectively. As the pack voltage decreases, the current must increase to accommodate the power required to move the vehicle at a velocity of 15 mph.

It must be noted that each controller has a low voltage cut-off point where the controller ceases operation. For the X-CT4840D controller, the low voltage cut-off is 40V, while the C3635-NC controller cuts off at 28V. Therefore, assuming the pack voltage will drop to these limits for the X-CT4840D and C3635-NC, the current supplied to the motor would be 13.5A and 19.3A respectively.

By determining the maximum amount of current required by the motor, the capacity of the cells can be elucidated. Since it was calculated that the maximum current seen in this application was 19.3A, the capacity of the battery pack must be large enough to handle this amount for a specified duration. Choosing 3 hours as the duration for this application to be operable, a 57.9Ah capacity would fulfill these requirements. This is based on the fact that the motor would require the maximum amount of current for the entire time, which would not practically happen. However, it is best to overdesign the 37

battery pack a little so that if the power required to move the vehicle at the desired velocity is larger than anticipated, the battery pack will be able to provide the necessary amount of energy.

3.3.2 Cell Chemistry

There are a plethora of cell chemistries available on the market today; some having a higher energy density than others. It is desired to have a relatively large energy density so that the physical size of the battery pack does not exceed the vehicle’s spatial limitations. For a comparison between some of the more popular cell chemistries out there, Table 3.3 contains information on nominal voltage, the cell’s energy density, cycle life, and its safety and environmental standings.

Table 3.3: Different battery chemistry comparison (Battery Space, 2010). Energy Working Cycle Chemistry Voltage Density Temp. Life Safety Environmental

LiFePO 4 3.2V >120 Wh/kg 0-60˚C >2000 Safe Good Lead-Acid 2.0V >35 Wh/kg -20-40˚C >200 Safe Not Good NiCd 1.2V >40 Wh/kg -20-50˚C >1000 Safe Bad NiMH 1.2V >80 Wh/kg -20-50˚C >500 Safe Good

LiMn xNi yCo zO2 3.7V >160 Wh/kg -20-40˚C >500 Better than LiCo OK

LiCoO 2 3.7V >200 Wh/kg -20-60˚C >500 Unsafe w/o PCM OK

After careful consideration, Lithium-Iron-Phosphate (LiFePO 4) cells were selected to be used for this application since they have a large energy density and are designed for high power applications such as electric vehicles. Even though LiFePO 4 has a lower energy density than other Li-Ion cells, they are chosen based upon their cycle life and safety. LiFePO 4 cells feature a nominal 3.2V, have a high discharging current, are non explosive, and have a long cycle life. Also, another nice aspect of these cells is their prismatic case seen below in Figure 3.6.

Now that LiFePO 4 has been selected as the cell chemistry of the pack, the total number of cells and their capacity must be selected. There is an excellent online source that sells these individual prismatic LiFePO 4 cells called Battery Space. This site sells many different types of cells with a wide range of capacities. Since it was determined above that the desired capacity of the pack was 57.9Ah, each cell must have this capacity rating in order for the entire pack to have the same capacity. However, batteryspace.com only sells these specific types of cells in capacities of 40, 60, 90, 100, and 160Ah. So, the cells that were selected for the electric tricycle were the LiFePO 4 prismatic module cells with a nominal voltage of 3.2V, capacity of 60Ah, and maximum discharge rate of 180A

(or 3C rate).

Figure 3.6: Example of a 60Ah LiFePO4 prismatic cell (Battery Space, 2010).

Each one of these cells has specific charging/discharging rates, physical dimensions, and specified weight. For the cells that have been chosen for this application, all of the pertinent information can be seen below in Table 3.4. It has been stated as well as seen in the specifications that the nominal, or average, voltage of these

cells is 3.2V. However, after these cells had been purchased it was seen that the actual nominal voltage for each cell was around 3.3V.

Table 3.4: Most important parameters and specifications for the LiFePO4 60Ah cells (Battery Space, 2010).

3.3.3 Size of Battery Pack

Once the proper cells have been selected, the size of the battery pack must be determined. This is quite simple since the maximum voltage for each controller is given.

The largest voltage that the pack can go up to is 48V for the X-CT4840D controller, meaning that 15 cells would suffice at a nominal 3.2V. However, since it was found that the nominal voltage was about 3.3V, 15 cells would cause the pack to be over the 48V.

So, 14 cells are used for the battery pack when the X-CT4840D controller is employed while only 10 cells are utilized for the C3635-NC controller making the battery pack a 40

nominal 33.3V. The nice thing about the C3635-NC controller is the fact that its low voltage cut-off point is 28V, so when the pack voltage dips down to this value it still is operable. As seen in Table 3.4, each cell has a low voltage cut-off of 2.5V. This means that if each cell stays relatively balanced during operation, the smallest the pack voltage will get is 25V when using the C3635-NC controller. This is good since it is very close to the cut-off of that controller. When using the larger pack (14 cells) with the X-

CT4840D controller, the smallest the pack could potential get is 35V assuming balanced discharge. However, the controller has a cut-off of 40V implying that the pack might not be using all its available capacity. Cutting off the operation earlier is actually better since there is less of a chance of over-discharging the cells leading to damage.

The electric tricycle is ready to be fully constructed now that the motor and battery pack have been designed and selected. The finished design can be seen below in

Figure 3.7, where the motor and the battery pack have been placed in the rear of the tricycle. Pictured below is the 10 cell battery pack using the C3635-NC controller.

Figure 3.7: TerraTrike fully electrified with battery pack and motor completely assembled. 3.4 Conclusions

When designing an electric vehicle, careful calculations must be made in order to determine the proper parts for the vehicle. There are specific factors that play a vital role for the power that must be provided to overcome wind and rolling resistance such as the frontal area of the frame, weight of the frame and rider, the type of tire used and its air pressure, the surface being driven on, wind velocity, etc. Once all of the specific parameters were determined, calculations were performed in order to determine the power required for the vehicle to move at a specific maximum velocity of 15 mph. The maximum power calculated to make the vehicle travel at 15 mph was found to be 350 W with no wind. Based on this power requirement, the Phoenix Cruiser hub motor was selected due to its power capabilities and ease of installation. Also, two motor controllers were chosen with different voltage and power ratings to test on the vehicle. However, depending on the controller being used, battery packs of different voltages must be used. 42

The battery pack is designed based primarily on the power the vehicle must overcome to go 15 mph, the controller being used, and the time period that the vehicle will be operable. The maximum time desired was 5 hours, implying that the battery pack have a capacity of 60Ah. There are two controllers that can be used in this application which is what determines the size of the pack. For the X-CT4840D controller, the battery pack must be between 40V to 48V while the C3635-NC controller can operate between

28V to 36V. Since each one of the LiFePO 4 prismatic module cells has a nominal voltage of 3.2V, the pack used with the X-CT4840D controller requires 14 cells while the

C3635-NC controller only needs 10 cells to satisfy the voltage requirement.

The electric tricycle was constructed after the Phoenix Cruiser hub motor and the

LiFePO 4 cells were purchased. Once the motor had been mounted between the rear dropouts some additional mechanical work was required to ensure that the motor would not slip out of the dropouts. Two aluminum tabs were constructed so that the axel of the motor remained locked to the frame of the tricycle during operation. The controller and throttle were installed in easily accessible areas of the tricycle for convenience. After the motor was installed, the battery back was placed into a metal crate that is secured to the back part of the frame. The completed electric tricycle rides very smoothly, is extremely easy to operate, and requires minimal operating instructions to the first-time driver.

CHAPTER IV