Comparison of Tab-To-Busbar Ultrasonic Joints for Electric Vehicle Li-Ion Battery Applications

Article Comparison of Tab-To-Busbar Ultrasonic Joints for Electric Vehicle Li-Ion Battery Applications

Abhishek Das * , Anup Barai, Iain Masters and David Williams WMG, The University of Warwick, Coventry CV4 7AL, UK; [email protected] (A.B.); [email protected] (I.M.); [email protected] (D.W.) * Correspondence: [email protected]; Tel.: +44-247-657-3742

Received: 26 June 2019; Accepted: 12 September 2019; Published: 14 September 2019

Abstract: Recent uptake in the use of lithium-ion battery packs within electric vehicles has drawn signiﬁcant attention to the selection of busbar material and corresponding thickness, which are usually based on mechanical, electrical and thermal characteristics of the welded joints, material availability and cost. To determine joint behaviour corresponding to critical-to-quality criteria, this study uses one of the widely used joining technologies, ultrasonic metal welding (UMW), to produce tab-to-busbar joints using copper and aluminium busbars of varying thicknesses. Joints for electrical and thermal characterisation were selected based on the satisfactory mechanical strength determined from the T-peel tests. Electrical contact resistance and corresponding temperature rise at the joints were compared for diﬀerent tab-to-busbar joints by passing current through the joints. The average resistance or temperature increase from the 0.3 mm Al tab was 0.6 times higher than the 0.3 mm Cu[Ni] tab, irrespective of busbar selection.

Keywords: electric vehicle; thin metal ﬁlm; ultrasonic metal welding; electrical resistance; temperature rise

1. Introduction Lithium-ion (Li-ion) electrochemistry-based secondary batteries are now widely used for electriﬁcation of automotive vehicles due to several advantages, including high energy density, low self-discharge and portability [1,2]. Greenhouse gases emission legislation has driven the automotive industry to develop fully electric vehicles (EV), hybrid or plug-in hybrid electric vehicles (HEV/PHEV) with low carbon emissions [3,4]. To assist in meeting emission legislation, automotive manufacturers are moving towards lightweight applications, intelligent automation or adapting new powertrain technology based on hybrid or pure secondary batteries. Furthermore, Li-ion batteries are progressively being used for large-scale energy storage systems for grid applications [5]. As a consequence, there is substantial demand for battery manufacturing which involves a large number of individual cells to be connected either in series or parallel to deliver the required power and driving range [6]. For example, the widely used pouch cell format uses a tab-to-busbar connection as the electrical interconnect. Based on the module design, a number of pouch cells are to be connected within a module and several modules are arranged in a battery pack or storage unit. Therefore, a large number of tab-to-busbar joints are inevitable, and each must provide electrically and thermally suitable connections in which vehicles are often exposed to harsh driving conditions [7]. The busbar plays an important role in providing desired electrical and thermal characteristics combined with mechanical strengths. In general, the selection of busbar material and its thickness are largely based on the current carrying capacity, mechanical and electrical characteristics and cost [8]. In addition, busbar material and thickness play a vital role in avoiding excessive heat generation at the tab-to-busbar interconnects. Copper and aluminium are the widely used as busbar materials across the electrical, power, electronic

World Electric Vehicle Journal 2019, 10, 55; doi:10.3390/wevj10030055 www.mdpi.com/journal/wevj World Electric Vehicle Journal 2019, 10, 55 2 of 13 or automotive industries [9–11]. Traditionally, copper has been used for busbars due to its excellent mechanical and electrical properties. However, more recently aluminium has been used especially for lightweight applications. Lewchalermwong et al. [12] conducted a study on selection of busbar material based on electrical resistance and thermal expansion of both copper and aluminium busbars. However, extensive study considering the tab-to-busbar joints needs upmost research attention to avoid excessive resistance or heat generation at the tab-to-busbar interconnects. One of the widely accepted joining methods to connect the pouch cell tab to the busbar is ultrasonic metal welding (UMW) due to several advantages, including joining of dissimilar materials of varying thickness, joining of highly reflective and conductive material, multiple stack-ups and low thermal input during the welding [6,13]. Li-ion battery packs for General Motors’ (GM) Chevy Volt and the Nissan LEAF are currently being manufactured using UMW where the pouch cell tabs are connected with busbars [6,14]. Research has been conducted on understanding the ultrasonic metal welding joining mechanism [15–18], microstructure and material properties [19–21], thin to thick material joining (i.e., varying stack-ups) [22], process robustness and optimisation [13,23,24], and mechanical and vibrational behaviours of the welded joints [25,26], which are reported in the literature. In spite of UMW’s extensive use and wide research, there are a few critical areas which are yet to be addressed, including electrical resistance and thermal behaviour of the joint. Little research can be found in the area of busbar-to-busbar connections. Published work mainly considers the mechanical joints, such as nuts and bolts [27,28], focuses on evaluation of contact resistance at varying contact pressure, and even identifies the effect of coating materials on the busbar [10]. However, UMW joint-based electrical and thermal characterisation is missing from the literature. An attempt has been made by Brand et al. [29] to measure the electrical resistance at the UMW joint and they observed that maximum tensile load corresponded to lower electrical resistance when ultrasonic welding was employed to join two brass (CuZn37) test samples. Additionally, Das et al. [30] reported the electrical resistance change and temperature rise at battery tab joints between the Al/Cu[Ni] tab to single Cu busbar assembly. However, extensive electrical and thermal characterisation of ultrasonic welded joints considering different busbar materials with varying thicknesses are missing from the literature. Combining the ultrasonic welded joints and busbar variation, limited work has been reported so far to characterise their critical-to-quality behaviours, including mechanical strength, electrical resistance and temperature rise at the joint. Therefore, this study focuses on tab-to-busbar ultrasonic welded joints to characterise their critical-to-quality behaviour. The remainder of this paper is arranged as follows: Section2 provides the details of experimental investigation including materials, ultrasonic welded joint preparation, and set-ups for mechanical, electrical and thermal characterisation; the results and discussions are made in Section3; and conclusions are drawn in Section4.

2. Experimental Details The experimental plan was developed based on the joining requirements for diﬀerent busbar thicknesses. Material selection, ultrasonic metal welded sample preparation, test set-ups for mechanical strength, electrical resistance and thermal characterisation are described in this section.

2.1. Tab and Busbar Materials Typically, a pouch cell consists of two terminal tabs that protrude through seals to allow external connection [31]. In general, nickel-coated copper (Cu[Ni]) and aluminium (Al) are widely accepted tab materials for pouch cells and act as the negative and positive terminals, respectively. In this study, 0.3 mm Cu[Ni] and Al tabs were used for the experimental investigations. The selection of bus bar material and corresponding thickness depends on module capacity, thermal management and current carrying capacity. Copper and aluminium are the most commonly used materials for busbar applications in electrical equipment [9] and their general properties at 20 ◦C based on the International Annealed Copper Standard (IACS) are listed in Table1. Although copper is better in World Electric Vehicle Journal 2019, 10, 55 3 of 13 terms of electrical resistivity, tensile strength and thermal conductivity, Pryor et al. [9] argued that due to higher density of copper, when weight is taken into consideration, aluminium has a conductivity that is approximately 1.85 times that of copper. As a consequence, for applications where weight is a concern (e.g., automotive electric vehicle applications), aluminium may be the better choice. Copper may be the better alternative when size and space are important. However, a comparative study of diﬀerent busbar materials/thicknesses considering mechanical strength, electrical resistance and thermal behaviour of joints using ultrasonic metal welding is missing from the literature.

Table 1. A comparison of copper and aluminium properties [9,32]. IACS: International Annealed Copper Standard.

Properties Copper (Cu) Aluminium (Al) Electrical resistance (nΩ mm) 17.2 28.3 Cross section for same conductivity (% IACS) 100 156 Weight for same conductivity (% IACS) 100 54 Thermal conductivity (W/(m K)) 397 230 · Temperature coeﬃcient of resistivity (per ◦K) 0.0039 0.004 Density (g/cm3) 8.91 2.7 Tensile strength (N/mm2) 200–250 50–60

The main objectives of this study were to conduct a comparative study between aluminium and copper busbars when they are used for pouch-cell-based battery pack manufacture. Joint behaviour is characterised in terms of mechanical strength, electrical resistance and temperature rise. To identify the joint characteristics, Cu and Al busbar materials were chosen with varying thicknesses. The details of tab and busbar materials, their speciﬁcations and corresponding thicknesses are listed in Table2. In general, the Ni-coated copper is commonly used as tabs, and nickel coating of approximately 2 µm was used for this experimental investigation. The copper tabs are generally connected with cell electrodes and Ni coating is mainly used for corrosion resistance, whereas the copper busbar is externally connected to the tabs. Both busbar and tab sample coupon dimensions were 100 mm in length by 25 mm in width [21], as shown in Figure1b.

Table 2. Tab and busbar materials used for experimental investigations.

Material Details Type Material Speciﬁcation Thickness [mm] Aluminium (Al) AW1050A-H18; BS EN546 0.3 Upper material-Tabs Ni-coated Copper (Cu[Ni]) CW004A-H040; BS EN1652 (C101Sl) 0.3 Aluminium (Al) AW1050A-H14; BS EN485 1.0, 1.5, 2.0, 2.5 Lower material-Busbar Copper (Cu) CW004A-H065; BS EN1652 (C101HH) 1.0, 1.5, 2.0 World Electric Vehicle Journal 2019, 10, x 4 of 13

Figure 1.Figure(a) Ultrasonic1. (a) Ultrasonic metal metal welding weldingjoining joining set-up; set-up; (b) ( ba )pictorial a pictorial example example of ultrasonic of ultrasonic welding to welding produce tab-to-busbar interconnect in lap configuration; and (c) T-peel test set-up for mechanical to produce tab-to-busbar interconnect in lap conﬁguration; and (c) T-peel test set-up for mechanical strength characterisation. strength characterisation. Figure 1b shows the typical arrangement of placing the ultrasonic weld nugget (i.e., 10 mm × 5 mm) at the centre of 25 mm overlap between tab and busbar. This lap configuration was used for both electrical and thermal characterisation. The T-peel test was used to evaluate joint strength with respect to the ultrasonic welding parameters. It can be observed from the literature that researchers have used welding pressure, welding time and amplitude as the most influencing weld parameters to achieve satisfactory weld strength and weld quality [23,24]. In this study, 0.3 mm Cu[Ni] and Al tabs were welded to 1.0 mm Cu and Al busbars, respectively, to identify satisfactory process parameters by varying welding pressure, welding time and amplitude of ultrasonic vibration. The process parameters and corresponding levels are given in Table 3. The trigger mode time was set at 0.2 s which allowed converting of the traversing pressure to welding pressure. The welding pressure was varied from 0.5 to 4.0 bar with incremental increases of 0.5 bar while amplitude of vibration and welding time were set at 35 µm and 0.25 s, respectively. These parametric values were chosen based on pilot tests. Five levels of amplitude were chosen from 30 to 50 µm with increments of 5 µm while welding pressure was kept constant at 1.5 bar and welding time at 0.25 s. Similarly, the welding time was increased from 0.15 to 0.55 s at 0.15 s intervals when welding pressure and welding amplitude were held at 1.5 bar and 50 µm, respectively.

Table 3. Level of process parameters for experimentation.

Process parameter Value Welding pressure (bar) 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 Welding amplitude (µm) 30, 35, 40, 45, 50 Welding time (s) 0.15, 0.25, 0.35, 0.45, 0.55

2.2. Tab-To-Busbar Joints Using Ultrasonic Metal Welding As ultrasonic metal welding is most suitable for thin to thick material, the tab was used as the upper part and the busbar as the lower during joining, i.e., the sonotrode oscillation with defined amplitude propagated from the tab to the busbar which was supported by the stationary anvil. Ultrasonic metal welding was performed using a Telsonic MPX ultrasonic welder as shown in Figure1a operating at 20 kHz, having 6.5 kW maximum power and allowable amplitude variation from 30 to 60 µm. Figure1b shows the typical arrangement of placing the ultrasonic weld nugget (i.e., 10 mm × 5 mm) at the centre of 25 mm overlap between tab and busbar. This lap configuration was used for both electrical and thermal characterisation. The T-peel test was used to evaluate joint strength with respect to the ultrasonic welding parameters. It can be observed from the literature that researchers have used welding pressure, welding time and amplitude as the most influencing weld parameters to achieve satisfactory weld strength and weld quality [23,24]. In this study, 0.3 mm Cu[Ni] and Al tabs were welded to 1.0 mm Cu and Al busbars, respectively, to identify satisfactory process parameters by varying welding pressure, welding time and amplitude of ultrasonic vibration. The process parameters and corresponding levels are given in Table3. The trigger mode time was set at 0.2 s which allowed converting of the traversing pressure to welding pressure. The welding pressure was varied from 0.5 to 4.0 bar with incremental increases of 0.5 bar while amplitude of vibration and welding time were set at 35 µm and 0.25 s, respectively. These parametric values were chosen based on pilot tests. Five levels of amplitude were chosen from 30 to 50 µm with increments of 5 µm while welding pressure was kept constant at 1.5 bar and welding time at 0.25 s. Similarly, the welding time was increased from 0.15 to 0.55 s at 0.15 s intervals when welding pressure and welding amplitude were held at 1.5 bar and 50 µm, respectively.

Table 3. Level of process parameters for experimentation.

Process parameter Value Welding pressure (bar) 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 Welding amplitude (µm) 30, 35, 40, 45, 50 Welding time (s) 0.15, 0.25, 0.35, 0.45, 0.55

2.3. Set-Up for Mechanical Strength Characterisation The output variable chosen to represent the weld quality was the maximum load obtained from the T-peel tests. These T-peel tests were performed to evaluate the effects of welding process parameters on the joint strength. The test set-up for the T-peel test is shown in Figure1c where the samples (i.e., open end of tab and busbar) were bent by 90◦ in opposite directions allowing them to be held by a lower static grip and upper moving grip. Using an Instron 3367 static test frame with 30 kN load cell, a test speed of 20 mm/min was applied to perform the T-peel tests in order to minimise the unexpected dynamic effect for weld failure [20]. The peak load during the T-peel test was recorded in order to evaluate the mechanical strength of the weld and further used as a measure of weld performance. To obtain preferred process parameters with satisfactory T-peel load, tests were performed using the samples produced as listed in Table3. Each test variant was repeated three times, and the average of the three replicates was used for analyses. Relying on the load-displacement characteristics, Das et al. [13] defined three weld conditions which were under-weld, good-weld and over-weld. Furthermore, they explained these three categories using weld cross-sectional images where under-weld showed un-bonded gaps between the mating surface, good-weld exhibited uniform bonding along the weld cross-section, and over-weld was excessive thinning with a tab broken zone. This weld classification was used in this study while evaluating the weld quality using T-peel tests. World Electric Vehicle Journal 2019, 10, x 5 of 13 over-weld. Furthermore, they explained these three categories using weld cross-sectional images where under-weld showed un-bonded gaps between the mating surface, good-weld exhibited uniform bonding along the weld cross-section, and over-weld was excessive thinning with a tab brokenWorld Electric zone. Vehicle This Journal weld2019 classification, 10, 55 was used in this study while evaluating the weld quality using5 of 13 T-peel tests. 2.4. Set-Up for Electrical and Thermal Characterisation 2.4. Set-Up for Electrical and Thermal Characterisation In order to evaluate the electrical resistance and temperature rise at the joint, the lap configuration In order to evaluate the electrical resistance and temperature rise at the joint, the lap samples, as shown in Figure1b, were produced using the parameters corresponding to maximum configuration samples, as shown in Figure 1b, were produced using the parameters corresponding strength in the T-peel test. The schematic representation and test set-up for electrical and thermal to maximum strength in the T-peel test. The schematic representation and test set-up for electrical characterisation are shown in Figure2. The sample was mounted in a fixture using brass blocks which and thermal characterisation are shown in Figure 2. The sample was mounted in a fixture using brass were connected to the power supply and voltage sensor. An additional voltage sensor was placed blocks which were connected to the power supply and voltage sensor. An additional voltage sensor to measure the potential difference across the joint when current was applied. The resistance was was placed to measure the potential difference across the joint when current was applied. The calculated using the induced voltage due to the application of current. As a result of resistive heat loss, resistance was calculated using the induced voltage due to the application of current. As a result of the weld area heated up and the corresponding temperature rise was measured using a thermal camera resistive heat loss, the weld area heated up and the corresponding temperature rise was measured placed directly above the weld location. A FLIR T440 thermal camera was employed to record the using a thermal camera placed directly above the weld location. A FLIR T440 thermal camera was joint temperature during the current application. It has been reported that Li-ion pouch cells are often employed to record the joint temperature during the current application. It has been reported that Li- exposed to aggressive automotive duty cycles, such as a race cycle, which are dominated by a high ion pouch cells are often exposed to aggressive automotive duty cycles, such as a race cycle, which current application up to 300 amp [33,34]. As the purpose of this study was to characterise the joint are dominated by a high current application up to 300 amp [33,34]. As the purpose of this study was behaviour based on the busbar thickness, a high value of current, i.e., 250 amp, was passed through to characterise the joint behaviour based on the busbar thickness, a high value of current, i.e., 250 the joint for 60 s to visualise the temperature rise. As a result of this current application, the resistance amp, was passed through the joint for 60 s to visualise the temperature rise. As a result of this current change coupled with temperature rise at the joint area was captured. application, the resistance change coupled with temperature rise at the joint area was captured.

FigureFigure 2. TestTest set-up for electrical and thermal characterisationcharacterisation of tab-to-busbar joints ( a)) schematic view; and ( b)) experimental set-up.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Joint Joint Strength Strength Behaviour Behaviour InIn order order to to obtain obtain the the satisfactory satisfactory ultrasonic ultrasonic welding welding process process parameters for 0.3 0.3 mm mm Cu[Ni] Cu[Ni] and and Al tabs for different different busbar materials and thicknesses, as as shown shown in in Table Table 22,, thethe mechanicalmechanical strengthstrength ofof the weld was evaluated by performing T-peel T-peel te tests.sts. Initially, Initially, 0.3 0.3 mm mm Al Al tabs tabs with with a a 1.0 1.0 mm mm Al busbarbusbar and and a a 0.3 mm Cu[Ni] tab tab with a 1.0 mm Cu busbar were considered for process parameter selection.selection. The The maximum maximum T-peel T-peel loads loads under under varying varying welding welding pressure pressure for for both both the the 0.3 0.3 mm mm Al Al tab tab to to 1.0 mm Al busbar and the 0.3 mm Cu[Ni] tab to 1.0 mm Cu busbar are plotted in Figure3 3a.a. InIn thethe casecase of of 0.3 0.3 mm mm Al Al tab tab to to 1.0 1.0 mm Al Al busbar busbar joints, joints, the the T-peel T-peel strength strength was was above above 200 200 N N in in between between 1 1 andand 3.5 bar. However, However, beyond 2.5 bar the joints showed excessive Al tab deformation and and were were not recommended for further evaluation. The highest T-peel load and acceptable top surface deformation were obtained at 1.5 bar welding pressure. Similarly, for 0.3 mm Cu[Ni] tab to 1.0 mm Cu busbar joints, welding pressure of 1.5 bar produced the highest T-peel strength. Therefore, for both the 0.3 mm Al tab to 1.0 mm Al busbar and 0.3 mm Cu[Ni] tab to Cu busbar joints, 1.5 bar was chosen as the welding World Electric Vehicle Journal 2019, 10, x 6 of 13 recommended for further evaluation. The highest T-peel load and acceptable top surface deformation were obtained at 1.5 bar welding pressure. Similarly, for 0.3 mm Cu[Ni] tab to 1.0 mm Cu busbar joints, welding pressure of 1.5 bar produced the highest T-peel strength. Therefore, for both the 0.3 World Electric Vehicle Journal 2019, 10, 55 6 of 13 mm Al tab to 1.0 mm Al busbar and 0.3 mm Cu[Ni] tab to Cu busbar joints, 1.5 bar was chosen as the welding pressure when varying busbar thicknesses were used. The effect of amplitude on T-peel strengthpressure for when both varying the joint busbar configurations thicknesses are were plotted used. in The Figure effect 3b. of In amplitude general, under-welds on T-peel strength were producedfor both the at jointlow value configurations of amplitudes are plotted (e.g., at in 30/35 Figure µm)3b. and In general, gradually under-welds shifted towards were producedgood-welds at aslow amplitude value of increased amplitudes [13]. (e.g., For at both 30/35 configurations,µm) and gradually an amplitude shifted towardsof 50 µm good-welds produced joints as amplitude with the highestincreased T-peel [13]. strength For both and configurations, was selected an for amplitude electrical ofand 50 thermalµm produced characterisation. joints with A the steady highest increase T-peel instrength T-peel andload was was selected observed for with electrical increasing and thermal welding characterisation. time, as shown A in steady Figure increase 3c, especially in T-peel forload the 0.3was mm observed Cu[Ni] with tab increasingto 1.0 mm weldingCu busbar time, joint, as shownwhere a in 0.55 Figure s welding3c, especially time resulted for the 0.3 in mmthe highest Cu[Ni] averagetab to 1.0 T-peel mm Cu load busbar of 261 joint, N. In where contrast, a 0.55 T-peel s welding load timewas resultedslightly reduced in the highest for 0.3 average mm Al T-peel tab to load 1.0 mmof 261 Al N. busbar In contrast, joints T-peelat 0.55 loads due was to slightlyexcessive reduced deformation for 0.3 mmand Alan tabover-weld to 1.0 mm condition Al busbar [13]. joints As ata balance0.55 s due between to excessive under-weld deformation and over-weld, and an over-weld a 0.35 s welding condition time [13 ].was As preferred a balance betweenfor producing under-weld joints ofand 0.3 over-weld, mm Al tabs a 0.35 to varying s welding thicknesses time was preferredof Al busbars. for producing The preferred joints ofwelding 0.3 mm pressure, Al tabs to welding varying amplitudethicknesses and of Al welding busbars. time The are preferred listed weldingin Figure pressure, 3d which welding was used amplitude for producing and welding samples time for are electricallisted in Figureand thermal3d which characterisation. was used for producing samples for electrical and thermal characterisation.

(a) Effects of welding pressure (b) Effects of amplitude 350 300 300 250 250 200 200 150 150 100 100 50 50

Maximum T-peel Load [N] Load T-peel Maximum 0 [N] Load T-peel Maximum 0 0.5 1 1.5 2 2.5 3 3.5 4 30 35 40 45 50 Welding Pressure [bar] Amplitude [µm]

(c) Effects of welding time (d) 350 0.3 mm Al tab 0.3 mm Cu[Ni] 300 to Al Busbar tab to Cu busbar 250 Welding Pressure 200 1.5 1.5 (bar) 150 Welding Amplitude 50 50 100 (µm) 50 Welding Time (sec)

Maximum T-peel Load [N] Load T-peel Maximum 0.35 0.55 0 0.15 0.25 0.35 0.45 0.55 Welding time [sec]

0.3 mm Al tab to 1 mm Al busbar 0.3 mm Cu[Ni] tab to 1 mm Cu busbar FigureFigure 3. MaximumMaximum T-peel T-peel load load obtained obtained from from (a) welding (a) welding pressure pressure variation; variation; (b) amplitude (b) amplitude variation; variation;(c) welding (c) timewelding variation; time variation; and (d) and parameters (d) parameters selected selected for producing for producing samples samples for electrical for electrical and andthermal thermal characterisation. characterisation. 3.2. Joint Electrical Resistance Behaviour 3.2. Joint Electrical Resistance Behaviour Due to application of 250 amp current through the joints, the induced voltage across the joints was Due to application of 250 amp current through the joints, the induced voltage across the joints measured using the voltage sensors as shown in Figure2. Based on the induced voltage, the electrical was measured using the voltage sensors as shown in Figure 2. Based on the induced voltage, the resistance was determined at the joint area for two different tab-based configurations, i.e., 0.3 mm Al electrical resistance was determined at the joint area for two different tab-based configurations, i.e., tabs and 0.3 mm Cu[Ni] tabs to different busbars. As current was applied for 60 s, it was observed that 0.3 mm Al tabs and 0.3 mm Cu[Ni] tabs to different busbars. As current was applied for 60 s, it was the resistance was slowly increasing within the period of current application due to heat generation, observed that the resistance was slowly increasing within the period of current application due to although the gradient of the profile depends on the busbar material and corresponding thickness. heat generation, although the gradient of the profile depends on the busbar material and Figure4a shows the electrical resistance change of the 0.3 mm Al tab to di fferent thicknesses of Al/Cu corresponding thickness. Figure 4a shows the electrical resistance change of the 0.3 mm Al tab to busbar joints due to application of current. Under a uniform flow of electric current, the electrical different thicknesses of Al/Cu busbar joints due to application of current. Under a uniform flow of resistance is expressed as R = ρl/A, where ρ represents the resistivity, l is the length and A is the cross-sectional area [30,35]. Furthermore, Das et al. [30] reported detailed theoretical analysis and simulation methodology for electrical and thermal characterisation based on a single tab-to-busbar joint. World Electric Vehicle Journal 2019, 10, x 7 of 13 electric current, the electrical resistance is expressed as R = ρl/A, where ρ represents the resistivity, l is the length and A is the cross-sectional area [30,35]. Furthermore, Das et al. [30] reported detailed theoretical analysis and simulation methodology for electrical and thermal characterisation based on World Electric Vehicle Journal 2019, 10, 55 7 of 13 a single tab-to-busbar joint. In this experimental investigation, voltage sensors were placed across the joints, keeping the lengthIn of this measured experimental length investigation,constant. Similarly, voltage identical sensors ultrasonic were placed welding across parameters the joints, were keeping used the to producelength of uniform measured joints length using constant. different Similarly, thicknesses identical of busbars ultrasonic to avoid welding variation parameters during were welding used operation.to produce However, uniform jointsthe cross-sectional using different area thicknesses depends on of the busbars thickness to avoid of the variation busbar. As during the thickness welding ofoperation. the busbar However, increases, the it cross-sectionalis expected to reduce area depends the resistance on the thicknessof the measured of the busbar. joint. ItAs is worth the thickness noting thatof the the busbar starting increases, resistance it is gradually expected decreases to reduce with the resistance increasing of thickness the measured of the joint. busbar It is due worth to the noting fact thatthat thethe startingresistance resistance decreases gradually with increasing decreases with cross-sectional increasing thicknessarea as ofthe the current busbar value due to remains the fact constant.that the resistance For example, decreases the starting with increasing resistance cross-sectional values were 0.089, area as0.087, the current0.080 and value 0.065 remains mΩ for constant. the 0.3 mmFor example,Al tab to the1.0, starting1.5, 2.0 and resistance 2.5 mm values Al busbars, were 0.089, respectively. 0.087, 0.080 Comparatively and 0.065 mΩ lowfor change the 0.3 mmin starting Al tab resistanceto 1.0, 1.5, was 2.0 and observed 2.5 mm for Al Cu busbars, busbars, respectively. e.g., 0.084, Comparatively0.083 and 0.080 low mΩ change were obtained in starting from resistance the 0.3 mmwas observedAl tab to for1.0, Cu 1.5 busbars, and 2.0 e.g., mm 0.084, Cu busbars. 0.083 and Accordingly, 0.080 mΩ were lower obtained busbar from thickness the 0.3 mmgave Al higher tab to change1.0, 1.5 andin resistance 2.0 mm Cu values busbars. due Accordingly, to the application lower busbarof current. thickness For example, gave higher a 0.034 change mΩ in resistance resistance increasevalues due was to obtained the application for the of0.3 current. mm Al tab For to example, the 1.0 mm a 0.034 Al busbar mΩ resistance joint. These increase resistance was obtained change valuesfor the decreased 0.3 mm Al with tab toincreasing the 1.0 mm busbar Al busbar thickness, joint. e.g., These 0.024, resistance 0.021 and change 0.016 m valuesΩ resistance decreased changes with wereincreasing obtained busbar from thickness, the 0.3 mm e.g., Al 0.024, tab to 0.021 1.5, and2.0 and 0.016 2.5 m mmΩ resistance Al busbar changes joints. wereSimilar obtained behaviour from was the also0.3 mm observed Al tab for to the 1.5, 0.3 2.0 mm and Al 2.5 tab mm to varying Al busbar thicknesses joints. Similar of Cu behaviourbusbar joints. was The also percentage observed change for the in0.3 resistance mm Al tab values to varying for varying thicknesses thicknesses of Cu busbarof Al and joints. Cu busbars The percentage are shown change in Figure in resistance 4b. It is valuesworth notingfor varying that similar thicknesses changes of Alin andresistance Cu busbars value were are shown obtained in Figure from the4b. 2.0 It is mm worth Al busbar noting (i.e., that 26.30%) similar andchanges the 1.5 in resistancemm Cu busbar value (i.e., were 26.92%) obtained or fromthe 2.5 the mm 2.0 mmAl busbar Al busbar (i.e., (i.e.,24.33%) 26.30%) and and2.0 mm the 1.5Cu mmbusbar Cu (i.e.,busbar 23.54%) (i.e.,26.92%) joints. or the 2.5 mm Al busbar (i.e., 24.33%) and 2.0 mm Cu busbar (i.e., 23.54%) joints.

(a) 0.3 mm Al tab to varying Al/Cu busbars 0.15

0.12

0.09 Resistance (mΩ) Resistance 0.06

0.03 0 102030405060 Time (sec)

1.0 mm Al 1.5 mm Al 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu

(b) Change in resistance 40 30 20

% Change % 10 0 1.0 mm Al 1.5 mm Al 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu Busbar thickness

Figure 4. (a) Electrical resistance proﬁles for 0.3 mm Al tab to Al/Cu busbars; and (b) the percentage Figure 4. (a) Electrical resistance profiles for 0.3 mm Al tab to Al/Cu busbars; and (b) the percentage (%) changes in resistance. (%) changes in resistance. Similarly, joints of the 0.3 mm Cu[Ni] tab to varying thicknesses of Al/Cu busbars have incremental change in electrical resistance due to the application of current which are shown in Figure5. In the case of 0.3 mm Cu[Ni] tab-to-busbar joints, it was observed that electrical resistance at the start of the World Electric Vehicle Journal 2019, 10, x 8 of 13

Similarly, joints of the 0.3 mm Cu[Ni] tab to varying thicknesses of Al/Cu busbars have World Electric Vehicle Journal 2019, 10, 55 8 of 13 incremental change in electrical resistance due to the application of current which are shown in Figure 5. In the case of 0.3 mm Cu[Ni] tab-to-busbar joints, it was observed that electrical resistance currentat the start application of the current was almostapplication the same was foralmost joints the having same for a busbar joints having thickness a busbar of (i) 1.5thickness mm Al of and (i) 1.01.5 mm Al Cu and (i.e., 1.0 0.057 mm m CuΩ ),(i.e., (ii) 0.057 2.0 mm mΩ Al), (ii) and 2.0 1.5 mm mm Al Cu and (i.e., 1.5 0.050mm Cu mΩ (i.e.,) and 0.050 (iii) m 2.5Ω) mmand Al(iii) and 2.5 2.0mm mm Al and Cu (i.e.,2.0 mm 0.046 Cu m (i.e.,Ω). 0.046 Furthermore, mΩ). Furthermore, it is evident it fromis evident Figure from5a that Figure the gradient5a that the of gradient resistance of changeresistance decreases change withdecreases the increasing with the increasing thickness ofthickness the busbar, of the and busbar, withhigher and with thicknesses higher thicknesses of busbar, theof busbar, electrical the resistance electrical changeresistance proﬁle change exhibited profile a plateau.exhibited In a lineplateau. with In starting line with resistance, starting the resistance, range of resistancethe range of increase resistance (i.e., increase the diﬀerence (i.e., the in difference resistance in values resistance at the values end and at startthe end of currentand start application) of current graduallyapplication) decreased gradually with decreased increase ofwith busbar increase thickness of busbar for the samethickness materials. for the For same example, materials. maximum For incrementalexample, maximum increase incremental of 0.013 mΩ increasewas obtained of 0.013 for m theΩ was 0.3 mmobtained Cu[Ni] for tab the to 0.3 1.0 mm mm Cu[Ni] Al busbar tab to joint, 1.0 andmm thereafter,Al busbar joint, the range and thereafter, of resistance the increase range of was resistance gradually increase decreased was gradually to 0.010, 0.006 decreased and 0.005 to 0.010, mΩ for0.006 1.5, and 2.0 0.005 and 2.5 mΩ mm for Al 1.5, busbars, 2.0 and respectively. 2.5 mm Al busbars, In the case respectively. of 0.3 mm In Cu[Ni] the case tab of to 0.3 varying mm Cu[Ni] Cu busbar tab joints,to varying ranges Cu ofbusbar resistance joints, increase ranges of were resistance 0.0089, increase 0.0053 and were 0.0049 0.0089, m Ω0.0053for 1.0, and 1.5 0.0049 and 2.0mΩ mm for 1.0, Cu busbars,1.5 and 2.0 respectively. mm Cu busbars, The percentage respectively. changes The in percentage resistance changes for a 0.3 mmin resistance Cu[Ni] tab for to a varying0.3 mm busbarsCu[Ni] aretab to shown varying in Figurebusbars5b. are A shown maximum in Figure of 20.52% 5b. A increasemaximum in of electrical 20.52% resistanceincrease in waselectrical obtained resistance from thewas 0.3obtained mm Cu[Ni] from the tab 0.3 to 1.0mm mm Cu[Ni] Al busbar tab to 1.0 joint mm and Al lowerbusbar values joint and were lower obtained values from were 2.5 obtained mm Al busbarsfrom 2.5 ormm 2.0 Al mm busbars Cu busbars. or 2.0 mm The Cu maximum busbars. absoluteThe maximum values absolute of electrical values resistance of electrical obtained resistance at the endobtained of current at the application end of current from application all the tab andfrom busbar all the combinations tab and busbar are combinations tabulated in Table are tabulated4. It can be in observedTable 4. It thatcan be an observed average electrical that an average resistance electric obtainedal resistance from a obtained 0.3 mm Cu[Ni] from a tab0.3 ismm 0.6 Cu[Ni] times lowertab is than0.6 times a 0.3 lower mm Al than tab, a irrespective0.3 mm Al tab, of busbar irrespective selection. of busbar selection.

(a) 0.3 mm Cu[Ni] tab to varying Al/Cu busbars 0.09

0.06 Resistance (mΩ) Resistance

0.03 0 102030405060 Time (sec)

1.0 mm Al 1.5 mm Al 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu

(b) Change in resistance values 40 30 20

% Change % 10 0 1.0 mm Al 1.5 mm Al 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu Busbar thickness

Figure 5. (a) Electrical resistance proﬁles for 0.3 mm Cu[Ni] tab to Al/Cu busbars; and (b) the percentage Figure 5. (a) Electrical resistance profiles for 0.3 mm Cu[Ni] tab to Al/Cu busbars; and (b) the (%) changes in resistance. percentage (%) changes in resistance.

World Electric Vehicle Journal 2019, 10, 55 9 of 13

Table 4. The maximum values of electrical resistance obtained from all tab and busbar combinations. World Electric Vehicle Journal 2019, 10, x 9 of 13 Al Busbar Cu Busbar Tab Material Table 4. The maximum1.0 mm values of 1.5 electrical mm resistance 2.0 mm obtained 2.5 from mm all tab 1.0 an mmd busbar 1.5combinations. mm 2.0 mm Al Busbar Cu Busbar 0.3 mm Al TabTab Material 0.124 0.111 0.101 0.080 0.110 0.105 0.099 0.3 mm Cu[Ni] Tab 0.0761.0 mm 0.066 1.5 mm 0.057 2.0 mm 2.5 0.052 mm 1.0 mm 0.066 1.5 mm 0.056 2.0 mm 0.051 0.3 mm Al Tab 0.124 0.111 0.101 0.080 0.110 0.105 0.099 0.3 mm Cu[Ni] Tab 0.076 0.066 0.057 0.052 0.066 0.056 0.051 3.3. Joint Thermal Behaviour 3.3. Joint Thermal Behaviour Li-ion battery degradation is one of the biggest challenges in electric vehicle industries, and one of the mainLi-ion causes battery for degradation battery aging is one/degradation of the biggest is elevatedchallenges temperature in electric vehicle [36]. industries, Hunt et al. and [37 one] argued that tabof the temperature main causes risefor battery is more aging/degradation alarming than battery is elevated surface temperature temperature [36]. Hunt rise, et and al. [37] they argued concluded that tabthat jointtab temperature cooling rather rise is than more surface alarming cooling than battery would surface be equivalent temperature to rise, extending and they the concluded lifetime of a that tab joint cooling rather than surface cooling would be equivalent to extending the lifetime of a pack by 3 times or reducing the lifetime cost by 66%. The temperature proﬁles of tab-to-busbar joints pack by 3 times or reducing the lifetime cost by 66%. The temperature profiles of tab-to-busbar joints are reported in Figure6. All the tests were performed with the same initial start temperature of 25 C are reported in Figure 6. All the tests were performed with the same initial start temperature of 25 °C ◦ and theand maximumthe maximum temperature temperature obtained obtained atat the end of of the the tests tests were were different, diﬀerent, which which was wasdue to due the to the combinationcombination of tab of tab and and busbar busbar materials materials along along with their their associated associated thicknesses. thicknesses.

0.3mm Al tab to Al busbar joints 0.3mm Al tab to Cu busbar joints 100 60

60 40

Temperature (°C) Temperature 40 Temperature (°C)

20 0 102030405060 20 Elapsed Time (sec) 0 102030405060 1.0 mm Al 1.5 mm Al Elapsed Time (sec) 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu (a) (b) 0.3mm Cu[Ni] tab to Al busbar joints 0.3mm Cu[Ni] tab to Cu busbar joints 100 60

60 40

Temperature (°C)40 Temperature (°C) Temperature

20 0 102030405060 20 Elapsed Time (sec) 0 102030405060 1.0 mm AL 1.5 mm AL Elapsed Time (sec) 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu (c) (d)

FigureFigure 6. The 6. The temperature temperature rise rise profilesprofiles due due to to applic applicationation of 250 of 250amp ampcurrent current for 60 s for through 60 s through (a) a 0.3 (a) a 0.3 mmmm Al Al tab tab toto didifferentfferent Al busbar joints; joints; (b ()b a) 0.3 a 0.3 mm mm Al Altab tab to different to different Cu busbar Cu busbar joints; joints; (c) a 0.3 (c) mm a 0.3 mm Cu[Ni]Cu[Ni] tab totab di toff differenterent Cu Cu busbar busbar joints; joints; and and ((dd)) aa 0.3 mm mm Cu[Ni] Cu[Ni] tab tab to todifferent different Cu Cubusbar busbar joints. joints. World Electric Vehicle Journal 2019, 10, 55 10 of 13 World Electric Vehicle Journal 2019, 10, x 10 of 13

SimilarSimilar to to the the change change in in electrical electrical resistance, resistance, th thee temperature temperature at at the the joint joint increases increases due due to to heat heat generationgeneration and and subsequent subsequent electric electricalal resistance resistance change. change. For Forexample, example, the highest the highest temperature temperature of 96.18 of °C96.18 was◦ Cmeasured was measured when a when 250 amp a 250 current amp was current passed was through passed the through 0.3 mm the Al 0.3 tab mm to 1.0 Al mm tab toAl 1.0busbar mm jointAl busbar for 60 joints. With for an 60 increase s. Withan in increasebusbar thickn in busbaress, the thickness, maximum the temperature maximum temperature measured at measured the joint reducedat the joint due reduced to a number due toof areasons, number including of reasons, initial including resistance, initial thermal resistance, conductivity, thermal thermal conductivity, mass andthermal electrical mass resistance and electrical change. resistance For example, change. ma Forximum example, temperatures maximum obtained temperatures for the obtained 0.3 mm forAl tabthe to 0.3 1.5, mm 2.0 Al and tab to2.5 1.5, mm 2.0 Al and busbar 2.5 mm joints Al were busbar 80 joints.27, 67.83 were and 80.27, 59.97 67.83 °C, and respectively. 59.97 ◦C, respectively.In general, copperIn general, busbar-based copper busbar-based joints were joints not heated were not as much heated as as Al much busbar-based as Al busbar-based joints due joints to higher due tothermal higher conductivitythermal conductivity and lower and resistance lower resistance change. change.For instan Force, instance, the temperature the temperature rise at the rise 0.3 at themm 0.3 Al mm tab Alto 2.0tab mm to 2.0 Al mm busbar Al busbar joint was joint 67.83 was °C, 67.83 which◦C, whichis consid is considerablyerably higher higherthan the than temperature the temperature rise at risethe 0.3at themm 0.3 Al mmtab to Al 2.0 tab mm to 2.0Cu mmbusbar Cu joint busbar (i.e., joint 50.07 (i.e., °C). 50.07 Figure◦C). 6a,b Figure shows6a,b the shows temperature the temperature responses forresponses the 0.3 mm for the Al 0.3tab mmto varying Al tab tothicknesses varying thicknesses of Al and Cu of busbar Al and joints, Cu busbar respectively. joints, respectively. The maximum The temperaturemaximum temperature obtained from obtained the 2.5 from mm theAl 2.5busbar mm jo Alint busbar (i.e., 59.97 joint °C) (i.e., is 59.97 higher◦C) than is higher the measured than the temperaturemeasured temperature from any of from the Cu any busbar of the joints. Cu busbar Therefore, joints. the Therefore, performance the performance of copper busbar of copper was busbarbetter whenwas better Al was when used Al as was tabs. used The as performance tabs. The performance of 0.3mm Cu[Ni] of 0.3mm tab Cu[Ni]to Al and tab Cu to Albusbar and Cujoints busbar are plottedjoints are in plottedFigure in6c,d, Figure respectively.6c,d, respectively. As copper As copperis electrically is electrically and thermally and thermally a better a betterconductor conductor than Al,than the Al, temperature the temperature rise risewas was lower lower for forCu[Ni] Cu[Ni] tab-to-busbar tab-to-busbar joints. joints. For For example, example, the the maximum maximum temperaturetemperature measured measured for for the the 0.3 0.3 mm mm Cu[Ni] Cu[Ni] tab tab to to 1.0 1.0 mm mm Al Al busbar busbar joint joint was was 66.96 66.96 °C,◦C, which which was was significantlysigniﬁcantly lower lower than than the the temperature temperature obtained obtained fr fromom the the 0.3 0.3 mm mm Al Al tab tab to to 1.0 1.0 mm mm Al Al busbar busbar joint joint (i.e.,(i.e., 96.18 96.18 °C).◦C). A A comparative comparative percentage percentage (%) (%) chan changege in intemperature temperature values values obtained obtained from from 0.3 0.3mm mm Al andAl and Cu[Ni] Cu[Ni] tab-to-busbar tab-to-busbar joints joints are aregiven given in Figure in Figure 7. Similar7. Similar to electrical to electrical resistance resistance responses, responses, the averagethe average temperature temperature increase increase from from the the 0.3 0.3 mm mm Al Altab tab was was 0.6 0.6 times times higher higher than than the the 0.3 0.3 mm mm Cu[Ni] Cu[Ni] tab,tab, irrespective irrespective of of busbar busbar selection. selection.

Change in temperature values 300

250

200

150

100 %Change

0 1.0 mm Al 1.5 mm Al 2.0 mm Al 2.5 mm Al 1.0 mm Cu 1.5 mm Cu 2.0 mm Cu 0.3mm Al tab to busbar joints 0.3mm Cu[Ni] tab to busbar joints

FigureFigure 7. 7. TheThe percentage percentage (%) (%) changes changes in in temperature temperature du duee to to application application of of 250 250 amp amp current current for for 60 60 s s throughthrough different different tab-to-busbar assemblies. 4. Conclusions 4. Conclusions This study compared the mechanical strength, electrical resistance and thermal behaviours of This study compared the mechanical strength, electrical resistance and thermal behaviours of tab-to-busbar joints. The critical-to-quality criteria joint criteria, i.e., mechanical strength, electrical tab-to-busbar joints. The critical-to-quality criteria joint criteria, i.e., mechanical strength, electrical resistance and temperature rise were evaluated for 0.3 mm Al and Cu[Ni] tabs to varying thicknesses resistance and temperature rise were evaluated for 0.3 mm Al and Cu[Ni] tabs to varying thicknesses of Al and Cu busbar joints. Joint mechanical strength was measured using the T-peel tests to find of Al and Cu busbar joints. Joint mechanical strength was measured using the T-peel tests to find the the preferred joining parameters for 0.3 mm Al tabs to busbars (i.e., 1.5 bar welding pressure, 50 µm preferred joining parameters for 0.3 mm Al tabs to busbars (i.e., 1.5 bar welding pressure, 50 µm welding amplitude and 0.35 s welding time) and 0.3 mm Cu[Ni] tabs to busbars (i.e., 1.5 bar welding welding amplitude and 0.35 s welding time) and 0.3 mm Cu[Ni] tabs to busbars (i.e., 1.5 bar welding pressure, 50 µm welding amplitude and 0.55 s welding time). These UMW process parameters were pressure, 50 µm welding amplitude and 0.55 s welding time). These UMW process parameters were used to produce samples for electrical and thermal characterisation. This paper significantly explores used to produce samples for electrical and thermal characterisation. This paper significantly explores the following areas: the following areas: • Electrical resistance change was decreasing with the increasing thickness of busbars, and with a higher thickness of busbar, the electrical resistance change profile exhibited a World Electric Vehicle Journal 2019, 10, 55 11 of 13

Electrical resistance change was decreasing with the increasing thickness of busbars, and with • a higher thickness of busbar, the electrical resistance change proﬁle exhibited a plateau. It was observed that equivalent resistance changes were obtained from the 0.3 mm Al/Cu[Ni] tab to 1.5 mm Al busbar and 1.0 mm Cu busbar. The electrical resistance at the start of the current application and the rate of change of electrical • resistance provide additional information on the possible interchangeability of busbar materials. For example, in the case of 0.3 mm Cu[Ni] tab-to-busbar connections, equivalent electrical resistance was obtained from 1.0 Cu and 1.5 mm Al busbars, 1.5 mm Cu and 2.0 mm Al busbars or 2.0 mm Cu and 2.5 mm Al busbars. The temperature rise proﬁles showed that with an increase in busbar thickness, the maximum • temperature measured at the joint was reduced. The average temperature increase from the 0.3 mm Al tab was 0.6 times higher than the 0.3 mm Al tab, irrespective of busbar selection.

This paper provides guidelines for suitable busbar selection using UMW to facilitate better battery pack manufacturing. Furthermore, these results can be used for modelling and simulation to evaluate whole module performance.

Author Contributions: Conceptualisation, A.D.; Formal analysis, A.D. and A.B.; Funding acquisition, D.W.; Investigation, A.B.; Methodology, A.D.; Project administration, D.W.; Supervision, Iain Masters; Validation, A.B.; Writing – original draft, A.D.; Writing – review & editing, I.M. Funding: This research has been partially supported by WMG Centre High Value Manufacturing (HVM) Catapult and Research Development Fund from The University of Warwick, UK. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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