Patent Publication Number: US-2023137286-A1

Title: Flexible aluminum busbar

Description:
TECHNICAL FIELD 
     The present invention relates generally to a busbar for an electric vehicle. More particularly, the present invention relates to a flexible busbar made of aluminum and configured with a low profile to enable the fabrication of a compact battery pack assembly. 
     BACKGROUND 
     Vehicles such as battery-electric vehicles (BEVs) and plug-in hybrid-electric vehicles (PHEVs) contain an energy storage device, such as a high voltage battery in a battery pack assembly, to act as a propulsion source for the vehicle. The battery may include components and systems to assist in managing vehicle performance and operations. The battery may also include one or more arrays of battery cells interconnected electrically between battery cell terminals by intercellular connectors. 
     Intercellular connectors, which may include a system of electrical conductors for collecting and distributing current, provides the means to efficiently distribute power to the vehicles&#39; various systems. A number of different types of Intercellular connectors including wires, cables, and busbars are commercially available. Busbars may have modular designs that allow for quicker and safer installation. 
     SUMMARY 
     The illustrative embodiments disclose a low-profile busbar for an electric vehicle battery pack assembly and a corresponding method. In one aspect, is disclosed. The busbar includes a body having a plurality of aluminum layers stacked together. The body has a first end, a second end opposite the first end, and a middle portion disposed between the first end and the second end, and the plurality of aluminum layers includes at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer and secured to the first aluminum layer with one or more laser welds at the first and second ends. The plurality of aluminum layers may be designed to have an offset at the middle portion such that the middle portion has a raised profile relative to a profile of the first or second ends. 
     The busbar may also be designed to have an aluminum layer thickness, an offset radius of curvature and an offset height that jointly provide a defined flexibility of the busbar in a plurality of axes, and a continuous current carrying capacity ranging from 100 A to 500 A. The busbar may further comprise Aluminum 1100. 
     In one aspect, another busbar is disclosed that includes a body including a plurality of aluminum layers stacked together with the body having a first end, a second end opposite the first end, and a middle portion disposed between the first end and the second end, The plurality of aluminum layers may include at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer and secured to the first aluminum layer with one or more laser welds at the first and second ends. The plurality of aluminum layers is configured to have a plurality of offsets at the middle portion such that the middle portion has a plurality of raised profiles relative to a profile of the first or second ends. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
     In one aspect, a method is disclosed. The method may include laser welding a plurality of aluminum layers together by preparing a busbar that includes at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer, generating an offset at a middle portion of the busbar such that a profile of the middle portion of the busbar is raised relative to a profile of a first or second end of the busbar, and laser welding, the first aluminum layer to at least the second aluminum layer at the first and second ends of the busbar. 
     The method may also include laser welding using a laser device having a wavelength of about 1070 nm, a welding speed of about 20 mm/s, a laser power of about 1400W, a scan width of about 3 mm and a scan frequency of about 200 Hz. In other implementations, the method may include laser welding using a laser device having a power density ranging from 170 Kw/mm2 to 180 Kw/mm2 and a welding speed ranging from 20 mm/s to 25 mm/s, with a busbar thickness ranging from 1 mm to 4 mm and an offset height ranging from 1 mm to 4 mm. 
     The method may also include further includes laser welding the busbar to one or more aluminum cell terminals of a battery pack. Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
     A system of one or more computers can be configured to perform particular operations or actions by virtue of having software, firmware, hardware, or a combination of them installed on the system that in operation causes or cause the system to perform the actions. One or more computer programs can be configured to perform particular operations or actions by virtue of including instructions that, when executed by data processing apparatus, cause the apparatus to perform the actions. 
     These and other features, and characteristics of the present technology, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced. Certain novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of the illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIG.  1    depicts a drivetrain and energy storage components in accordance with illustrative embodiments. 
         FIG.  2    depicts a diagram of a battery pack arrangement in accordance with an illustrative embodiment. 
         FIG.  3 A  depicts a perspective view of a busbar in accordance with an illustrative embodiment. 
         FIG.  3 B  depicts a two-dimensional view of a front end of a busbar in accordance with an illustrative embodiment. 
         FIG.  3 C  depicts a two-dimensional view of a busbar showing opposing forces. 
         FIG.  4    depicts layers of a busbar in accordance with an illustrative embodiment. 
         FIG.  5 A  depicts busbar connections to battery cells in accordance with an illustrative embodiment. 
         FIG.  5 B  depicts busbar connected to battery cells in accordance with an illustrative embodiment. 
         FIG.  6    depicts a perspective view of a busbar in accordance with an illustrative embodiment. 
         FIG.  7    depicts a perspective view of a busbar in accordance with an illustrative embodiment. 
         FIG.  8    depicts a sketch of cell connections in accordance with an illustrative embodiment. 
         FIG.  9    depicts a flow chart showing a method in accordance with an illustrative embodiment. 
         FIG.  10    depicts a flow chart showing method in accordance with an illustrative embodiment. 
         FIG.  11    depicts a flow chart showing a method in accordance with an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Batteries for which high current levels are needed may have thicker busbars compared to conventional busbar sizes of batteries that deliver comparatively lower currents. As the thickness of a busbar increases the material stiffness of the busbar, i.e., a measure of how the busbar bends under load while still returning to its original shape once the load is removed, may increase by a cubic factor. This may require increasingly larger offsets of the busbar to maintain flexibility. An effect thereof may be a reduction in volumetric efficiency of the batteries. More specifically, a representation of the moment of inertia of a rectangular beam is: 
     
       
         
           
             I 
             = 
             
               
                 bh 
                 3 
               
               12 
             
           
         
       
     
     where h is the thickness of the rectangular beam and b is the width of the rectangular beam. Further, the basic equation for deflection of a cantilever beam is: 
     
       
         
           
             
               δ 
               B 
             
             = 
             
               
                 FL 
                 3 
               
               
                 3 
                 ⁢ 
                 EI 
               
             
           
         
       
     
     At a given deflection, as the moment of inertia (I) increases, the force (F) may also need to increase by the same ratio. Since I is a function of h 3 , the thickness may have a cubed impact on the stiffness of the busbar. It may be desirable therefore, to have a bus bar with low stiffness so that it may be able to flex without exerting too large a force back on to the busbar-to-cell-terminal weld. 
     The illustrative embodiments recognize that connections between battery cells or modules may be essential parts of a battery pack assembly design that may affect thermal stability, electrical protection and volumetric energy density. Conventional intercellular connections may occupy excessively large volumes in battery pack assemblies. Connections such as those comprising wires, cables, lugs and even conventional busbars are susceptible to failure and short circuits when the cells even slightly dislocate during operation, for example, due to the heating and cooling of cells or vibrations of a moving vehicle combined with lack of flexibility in the connections. The illustrative embodiments further recognize that arbitrarily introducing offset structures in busbars to add flexibility may require increasing offset heights, and thus the vertical heights of the busbars, to values that unnecessarily increase the volume occupied by the busbars in the battery pack assembly, resulting in a corresponding decrease in volumetric energy density of the battery pack assembly. Further, any requirements for high continuous current capacities of the busbars than is conventional (e.g., a continuous current carrying capacity of 220 A or more) may require increasing a thickness of the busbar, which may result in decreased flexibility unless an offset with a high offset height is introduced. Thus, an incongruous relationship may exist between a vertical height of the busbar (i.e., thickness and/or offset height, and thus the busbar volume and volumetric energy density of the battery pack assembly), flexibility of the busbar (and thus safety and longevity of the battery pack assembly), and a continuous current carrying capacity of the busbar. That is, increasing a continuous current carrying capacity of the busbar from a base value may involve increasing a thickness of the busbar which may decrease flexibility of the busbar and thus decrease the safety of the pack. Countering this by increasing an offset height of the busbar, while possibly successful restoring the flexibility, may decrease the compactness and thus volumetric energy density of the pack. Further, it may be desirable to minimize a temperature rise of the bus bar during high continuous currents since the busbar may transfer said heat back into the cell and potentially damage it (e.g., in an illustrative embodiment, a maximum of at most 15° C. temperature rise on the busbar over ambient temperature may be tolerable). Further, it may be desirable to minimize a resistance of the busbar as the resistance may produce system losses during charging and discharging and cause the battery to lose energy efficiency. 
     The illustrative embodiments described herein are directed to a busbar  150  having a low profile configured to aid in the production of an optimal volumetric efficiency of a battery pack assembly that contains the busbar  150 . One or more embodiments provide a busbar  150  configured to have an aluminum layer thickness, an offset radius of curvature and an offset height that jointly provide a defined flexibility of the busbar in a plurality of axes, and a high continuous current carrying capacity of between 100 A to 500 A, or between 200 A to 250. In one or more embodiments the defined flexibility may refer to a minimum deflection of the busbar of 0.2 mm without needing to apply more than 300N each of opposing forces ( 336 ,  338  as shown in  FIG.  3 C ) on the cell terminal welds. 
     One of more embodiments further generate one or more offsets of the busbar  150  to occupy a minimized vertical profile in comparison to conventional flexible busbars, based on selecting a defined radius of curvature and offset height combination that jointly provide both the defined flexibility of the busbar  150  in a plurality of axes while maintaining or increasing volumetric energy density. 
     The busbars  150  may be configured electrically to handle not only high currents coming from the cells but also increasing voltage levels. Mechanically, the busbars  150  may be designed to be durable, capable of withstanding high levels of vibration, while simultaneously providing enough rigidity to keep the integrity of the battery pack assembly, especially those with cell-to-pack configurations, while also being flexible enough to cope with elastic, thermal and G-forces. In a cell-to-pack configuration, battery cells are arranged directly inside sidewalls without the use of separate battery modules to house the cells. Alternatively, the busbars may be used in battery modules without a cell-to-pack configuration. 
     One or more embodiments further employ a plurality thin aluminum layers or foils to form a body of the busbar  150  as described hereinafter by reference to the accompanying figures. Using a plurality of thin aluminum layers may allow and a small offset may allow the formation of a busbar  150  with a low profile that supports high current capacity needs. 
     In one or more other embodiments, a method of welding the plurality of thin aluminum layers, which have hitherto been unknown to be weldable is shown. The embodiments recognize that well designed busbars are essential components of a compact battery pack assembly and the selection of the busbars is not always as simple as one might think. It is an exceptionally arduous task in busbar designs to properly weld thin aluminum foils/layers together while maintaining structural integrity. Having generally described the busbar  150  and methods thereof, examples and systems will now be described in more detail. 
     Turning to  FIG.  1   , a schematic of a generalized electric vehicle system  100  in which a busbar  150  of a battery pack assembly  102  may be housed will be described. It will become apparent to a person skilled in the relevant art(s) that the concepts described herein are directed to busbars used in all electrified/electric vehicles, including, but not limited to, battery electric vehicles (BEV&#39;s), plug-in hybrid electric vehicles, motor vehicles, railed vehicles, watercraft, and aircraft configured to utilize rechargeable electric batteries as their main source of energy to power their drive systems propulsion or that possess an all-electric drivetrain. 
     The electric vehicle  120  may comprise one or more electric machines  140  mechanically connected to a transmission  128 . The electric machines  140  may be capable of operating as a motor or a generator. In addition, the transmission  128  may be mechanically connected to an engine  126 , as in a PHEV. The transmission  128  may also be mechanically connected to a drive shaft  142  that is mechanically connected to the wheels  122 . The electric machines  140  can provide propulsion and deceleration capability when the engine  126  is turned on or off. The electric machines  140  also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in the friction braking system. The electric machines  140  may also reduce vehicle emissions by allowing the engine  126  to operate at more efficient speeds and allowing the electric vehicle  120  to be operated in electric mode with the engine  126  off in the case of hybrid electric vehicles. 
     A battery pack assembly  102  stores energy that can be used by the electric machines  140 . The battery pack assembly  102  typically provides a high voltage DC output and is electrically connected to one or more power electronics modules  134 . In some embodiments, the battery pack assembly  102  comprises a traction battery and a range-extender battery. Cells  104  of the battery pack assembly  102  may be electrically coupled by busbars  150  described herein. One or more contactors  144  may isolate the battery pack assembly  102  from other components when opened and connect the battery pack assembly  102  to other components when closed. To increase the energy densities available for electric vehicles, a structure of the busbars  150  is configured to eliminate unnecessary use of space as described hereinafter. The battery pack assembly may also have a cell-to-pack configuration. For example, a battery pack configuration may include cells directly placed in an enclosure without the use of separate modules, with the enclosure also housing other hardware such as, but not limited to the power electronics module  134 , DC/DC converter module  136 , system controller  118  (such as a battery management system (BMS)), power conversion module  132 , battery thermal management system (cooling system and electric heaters) and contactors  144 . By minimizing a vertical height of the busbars  150  in a pack for which high continuous current carrying capacities relative to conventional packs are needed (e.g.,  220  A or more), a consolidated arrangement is provided that allows space otherwise occupied by unusually tall offsets in the busbars to be saved and a volumetric energy density increased without sacrificing flexibility and safety provided by the busbar  150 . 
     The power electronics module  134  is also electrically connected to the electric machines  140  and provides the ability to bi-directionally transfer energy between the battery pack assembly  102  and the electric machines  140 . For example, a traction or range-extender battery may provide a DC voltage while the electric machines  140  may operate using a three-phase AC current. The power electronics module  134  may convert the DC voltage to a three-phase AC current for use by the electric machines  140 . In a regenerative mode, the power electronics module  134  may convert the three-phase AC current from the electric machines  140  acting as generators to the DC voltage compatible with the battery pack assembly  102 . The description herein is equally applicable to a BEV. For a BEV, the transmission  128  may be a gear box connected to an electric machine  14  and the engine  126  may not be present. 
     In addition to providing energy for propulsion, the battery pack assembly  102  may provide energy for other vehicle electrical systems. A typical system may include a DC/DC converter module  136  that converts the high voltage DC output of the battery pack assembly  102  to a low voltage DC supply that is compatible with other vehicle loads. Other electrical loads  146 , such as compressors and electric heaters, may be connected directly to the high voltage without the use of a DC/DC converter module  136 . The low-voltage systems may be electrically connected to an auxiliary battery  138  (e.g., 116V battery). The illustrative embodiments recognize that due to the numerous components that make up the drivetrain of the electric vehicle being in contact with the battery pack assembly, and heating and cooling of cells of the battery pack assembly conditions, it is desirable maximize safety and longevity of the battery pack assembly through flexible busbars while making judicious use of space to enhance volumetric efficiency. 
     The battery pack assembly  102  may be recharged by a charging system such as a wireless vehicle charging system  112  or a plug-in charging system  148 . The wireless vehicle charging system  112  may include an external power source  106 . The external power source  106  may be a connection to an electrical outlet. The external power source  106  may be electrically connected to electric vehicle supply equipment  110  (EVSE). The electric vehicle supply equipment  110  may provide an EVSE controller  108  to provide circuitry and controls to regulate and manage the transfer of energy between the external power source  106  and the electric vehicle  120 . The external power source  106  may provide DC or AC electric power to the electric vehicle supply equipment  110 . The electric vehicle supply equipment  110  may be coupled to a transmit coil  114  for wirelessly transferring energy to a receiver  116  of the vehicle  120  (which in the case of a wireless vehicle charging system  112  is a receive coil). The receiver  116  may be electrically connected to a charger or on-board power conversion module  138 . The receiver  116  may be located on an underside of the electric vehicle  120 . In the case of a plug-in charging system  148 , the receiver  116  may be a plug-in receiver/charge port and may be configured to charge the battery pack assembly  102  upon insertion of a plug-in charger. The power conversion module  132  may condition the power supplied to the receiver  116  to provide the proper voltage and current levels to the battery pack assembly  102 . The power conversion module  132  may interface with the electric vehicle supply equipment  110  to coordinate the delivery of power to the electric vehicle  120 . 
     One or more wheel brakes  130  may be provided for decelerating the electric vehicle  120  and preventing motion of the electric vehicle  120 . The wheel brakes  130  may be hydraulically actuated, electrically actuated, or some combination thereof. The wheel brakes  130  may be a part of a brake system  122 . The brake system  122  may include other components to operate the wheel brakes  130 . For simplicity, the figure depicts a single connection between the brake system  122  and one of the wheel brakes  130 . A connection between the brake system  122  and the other wheel brakes  128  is implied. The brake system  122  may include a controller to monitor and coordinate the brake system  122 . The brake system  122  may monitor the brake components and control the wheel brakes  130  for vehicle deceleration. The brake system  122  may respond to driver commands and may also operate autonomously to implement features such as stability control. The controller of the brake system  122  may implement a method of applying a requested brake force when requested by another controller or sub-function. 
     One or more electrical loads  146  may be connected to the busbars  150 . The electrical loads  146  may have an associated controller that operates and controls the electrical loads  146  when appropriate. Examples of electrical loads  146  may be a heating module or an air-conditioning module. 
     The battery pack assembly  102  may be constructed from a variety of chemical formulations, including, for example, lead acid, nickel-metal hydride (NIMH) or Lithium-Ion.  FIG.  2    shows a schematic of the battery pack assembly  102  in a simple series configuration of N cells  104 . Other battery pack assembly  102 , however, may be composed of any number of individual battery cells connected in series or parallel or some combination thereof. The battery pack assembly  102  may have a one or more low profile, flexible busbars  150  connecting the cells  104 . The battery pack assembly  102  may also have controllers such as the Battery management system (BMS  204 ) that monitors and controls the performance of the battery pack assembly  102 . The BMS  204  may monitor several battery pack level characteristics such as pack current  208 , pack voltage  210  and pack temperature  206 . The BMS  204  may have non-volatile memory such that data may be retained when the BMS  204  is in an off condition. Retained data may be available upon the next key cycle. 
     In addition to monitoring the pack level characteristics, there may be cell  104  level characteristics that are measured and monitored. For example, the terminal voltage, current, and temperature of each cell  104  may be measured. A system may use a sensor module(s)  202  to measure the cell  104  characteristics. Depending on the capabilities, the sensor module(s)  202  may measure the characteristics of one or multiple of the cells  104 . Each sensor module(s)  202  may transfer the measurements to the BMS  204  for further processing and coordination. The sensor module(s)  202  may transfer signals in analog or digital form to the BMS  204 . In some embodiments, the sensor module(s)  202  functionality may be incorporated internally to the BMS  204 . That is, the sensor module(s)  202  hardware may be integrated as part of the circuitry in the BMS  204  and the BMS  204  may handle the processing of raw signals. 
     It may be useful to calculate various characteristics of the battery pack. Quantities such a battery power capability and battery state of charge may be useful for controlling the operation of the battery pack as well as any electrical loads receiving power from the battery pack. Battery power capability is a measure of the maximum amount of power the battery can provide or the maximum amount of power that the battery can receive for the next specified time period, for example, 1 second or less than one second. Knowing the battery power capability allows electrical loads to be managed such that the power requested is within limits that the battery can handle. 
     Battery pack state of charge (SOC) gives an indication of how much charge remains in the battery pack. The battery pack SOC may be output to inform the driver of how much charge remains in the battery pack, similar to a fuel gauge. The battery pack SOC may also be used to control the operation of an electric vehicle. Calculation of battery pack or cell SOC can be accomplished by a variety of methods. One possible method of calculating battery SOC is to perform an integration of the battery pack current over time. Calculation of battery pack or cell SOC can also be accomplished by using an observer, whereas a battery model is used for construction of the observer, with measurements of battery current, terminal voltage, and temperature. Battery model parameters may be identified through recursive estimation based on such measurements. The BMS  204  may estimate various battery parameters based on the sensor measurements. The BMS  204  may further ensure by way of the pack current  208  that a current of the cells  104  does not exceed a defined continuous current carrying capacity of the busbars  150 . 
       FIG.  3 A  shows a perspective view of a busbar  150  in accordance with one or more embodiments. The busbar  150  may comprise a body  310  that includes a plurality of thin aluminum layers  308  stacked together. The body  310  may have a first end  312 , a second end  314  opposite the first end, and a middle portion  316  disposed between the first end  312  and the second end  314 . The plurality of aluminum layers  308  includes at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface  324  of the first aluminum layer (in the X-Z plane as shown in  FIG.  3 A ). The aluminum layers  308  may be secured together with one or more laser welds  302  at the first and second ends. Further, the aluminum layers  308  may be configured to have an offset at the middle portion such that the middle portion has a raised profile  326  relative to a level profile  328  of the first or second ends as shown in  FIG.  3 B . The body  310  is configured to be thin enough to provide a low profile to the busbar without compromising on a continuous current carrying capacity of the busbar  150 . This may be achieved by providing a thickness of the individual aluminum layers  308 , an inner radius of curvature  318  and/or outer radius of curvature  320  (collectively referred to herein as an offset radius of curvature) and an offset height  306  that jointly provide a defined flexibility of the busbar in a plurality of axes, as well as a high continuous current carrying capacity of about 100 A to 500 A (e.g., between 200 A and 250 A, pulses at higher rates may be achieved). 
     As shown in  FIG.  3 B , forces (F) may be applied to the busbar  150  due to, for example, movement of cells relative to other, vibrations and thermal expansions and contractions. The busbar  150  may flex  330  along the top  332  of the offset  304  as well as around each of the offset radii of curvature. If the busbar was rigid, solid (one thick layer) and flat, when there is movement (e.g., 0.2 mm of movement) of the first and second ends relative to each other, the forces would be high enough, and in tension (along the length of the rigid, flat busbar) to cause bonds between cell terminals and the busbars and/or bonds between cell electrodes and cell terminals to sever. By introducing the offset, the forces exerted on the bonds may become reduced. The higher the offset height  306 , and the longer the length (in the X-direction) of the top  332  is, the more deflection the busbar experiences and the lower the forces exerted on the bonds are. In an illustrative embodiment, the top  332  is about 22 mm long in the X-direction. Further, as busbar technology advances, the continuous current carrying capacity of the busbars may get increasingly higher. Higher currents may require higher busbar thicknesses  334  and the thicker the busbar, the harder it may be for the busbar to flex. This may be mitigated by increasing the offset height  306  to maintain flexibility. For example, a solid busbar of 0.8 mm to 1 mm thick may require a tall offset of about 4-6 mm to prevent bonds/electrical connections from severing and a thicker solid busbar of about 2 mm for which high continuous current carrying capacities are desired may require an offset of about 8-12 mm or more to prevent bonds from severing. These high offsets relative to the busbar thicknesses may introduce additional unusable volume in a battery pack assembly and result in a decrease the compactness and volumetric energy density relative to the same assembly with flat busbars. Using a plurality of aluminum layers  308  to form the body  310  may aid in maintaining a low profile without compromising on flexibility as discussed herein. 
       FIG.  4    depicts aluminum layers  308  that may form the busbar according to one or more embodiments. Aluminum is lightweight and may be cheaply obtained. Moreover, unlike copper, aluminum may not require plating to weld together. Thin layers of aluminum foils/layers may be formed, stamped to obtain the offset  304  and then laser welded at the ends to hold the layers together prior to assembly into a battery pack or module. Stamping before welding may allow the individual layers to take form to prevent deformations in the busbar that may be caused by welding before stamping, i.e., by welding after forming the offset, the bus bar may hold its shape prior to assembly into the battery module or pack and prevent unnecessary stresses in the welds caused by forming after welding. A combination of the number of aluminum layers  308  and thickness of each layer, and thus of the busbar  150  may be chosen to maintain flexibility and low profile. The number of layers may range from 10 to 30 layers, or 15 to 20 layers. The thickness of each aluminum layer  308  may range from 0.05 mm to 0.2 mm or from 0.10 mm to 0.13 mm. The busbar thickness  334  may range from 1.2 mm to 4 mm or from 1.8 mm to 2.5 mm. Below is a table showing possible combinations of properties. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Busbar properties 
               
            
           
           
               
               
            
               
                 Property 
                 Value 
               
               
                   
               
               
                 flexibility of the busbar 
                 minimum deflection of the busbar of 0.2 mm 
               
               
                   
                 without needing to apply more than 300N 
               
               
                   
                 each of opposing forces on the cell terminal 
               
               
                   
                 welds 
               
               
                 radius of curvature 
                 0.5 mm to 5 mm; tighter range 1.0 mm to 
               
               
                   
                 2.0 mm 
               
               
                 offset height 
                 1 mm to 4 mm; tighter range 1.5 mm to 
               
               
                   
                 3.0 mm 
               
               
                 number of aluminum 
                 10 to 30; tighter range 15 to 20 
               
               
                 layers stacked together 
               
               
                 thickness of an 
                 0.05 mm to 0.2 mm; tighter range 0.10 mm 
               
               
                 aluminum layer 
                 to 0.13 mm 
               
               
                 busbar thickness 
                 1.2 mm to 4.0 mm; tighter range 1.8 mm to 
               
               
                   
                 2.5 mm 
               
               
                 ratio of offset height 
                 0.75:1 to 1.5:1 or 1:1 to 1.5:1 
               
               
                 to busbar thickness 
               
               
                 continuous current carrying 
                 100 A to 500 A; tighter range 200 A to 
               
               
                 capacity of busbar 
                 250 A 
               
               
                 thickness of topmost 
                 0.4 mm to 0.5 mm 
               
               
                 aluminum layer (FIG. 6) 
               
               
                   
               
            
           
         
       
     
     Combinations of these properties have been found to provide unexpectedly low profiles and volumes for busbars  150  for which high continuous current carrying capacities are desired without compromising on flexibility of the busbars  150  and thus safety and longevity of the battery pack assembly  102 . In some embodiments, each property may be selected such that none of the remaining properties fall outside the given ranges. In other embodiments a subset of the properties may be selected for a low-profile busbar design. 
     In one or more embodiments, the plurality of aluminum layers  308  comprise Aluminum  1100 . In an example, the busbar thickness  334  may be 2 mm and the offset may be between 1.5 mm and 2 mm. In another example, the aluminum layer thickness of each layer is about 0.005 inches (0.127 mm) and the number of aluminum layers stacked together is 16. Due to the relatively thin aluminum layers or foils being use, a laser welding step of the layers which maintains the structure of the layers may be arduous and even ineffective without the right combination of laser welding parameters. The illustrative embodiments thus disclose a laser welding method hereinafter. 
     The embodiments disclosed are not meant to be limiting and other variations and technical features may be readily apparent to one skilled in the art from the figures and descriptions. 
       FIG.  5 A  and  FIG.  5 B  illustrate a plurality of busbar  150  configured to connect a plurality of cells  104 . 
     Busbars may electrically couple cells  104  in series or parallel combinations. Busbars  150  (e.g., end busbar  512   d ) may also be configured to bolt end cells (e.g., end cell  510   d ) to an electrical isolation  514 . The low profile of the busbars may minimize the overall package space needed for height and width of busbars. The busbars  150  allow flexibility in multiple axes to accommodate cell to cell movement (from tolerances, vibration, cell growth during cycling). They may also minimize force on cell terminal welds and consolidation welds (inside the cell from electrode foils to cell terminal) during cell-to-cell movement.  FIG.  5 A  shows battery pack comprising cells  104  that include a cell A  510   a , a cell B  510   b , a cell C  510   c , an end cell  510   d . The battery pack may also include busbars  150 , and terminals  502  and an electrical isolation  514 . The busbars include busbar  1   512   a , busbar  2   512   b , busbar  3   512   c , and end busbar  512   d . The terminals include positive terminal A  504   a , a negative terminal A  504   b , a negative terminal B  506   a , a positive terminal B  506   b , a positive terminal C  508   a , a negative terminal C  508   b . By laser welding busbar  2   512   b  to negative terminal A  504   b  and positive terminal B  506   b  though the foils of the busbar and into the terminals, cell A  510   a  is connected to adjacent cell B  510   b  in a series connection as shown in  FIG.  5 B . Of course, cells and busbars may be arranged in a myriad of ways to obtain series and/or parallel cell connections. Further, both positive and negative terminals of cells may typically be made of aluminum. By laser welding aluminum of the busbar to aluminum of the terminals, instead of laser welding different materials together, the laser welding process to obtain the busbar-terminal laser weld  516  is made easier, stronger and more efficient. Further, no intermetallic layer (usually brittle) is generated between the aluminum of the busbar and the aluminum of the terminals unlike in conventional welds. In some embodiments, the end busbar  512   d  has a nut plate. A pack wire bus bar may get bolted through the busbar into the nut plate to clamp the end busbar to the pack wiring/busbar. 
     The busbar  150  may also include at least one aluminum layer of the plurality of aluminum layers has a thickness that is dissimilar from the thicknesses of remaining layers. For example, as shown in the enlarged section  602  of  FIG.  6   , the busbar  150  comprises a topmost aluminum layer  604  having a thickness that is different from a thickness of the remaining aluminum layers  606 . A thickness of the topmost aluminum layer  604  may range from 0.3 to 0.7 mm, or from 0.0.4 mm to 0.5 mm. A thicker top layer may make it easier to weld voltage sense taps (not shown) to the busbar  150  or offset  304  of the busbar  150 . Though a thicker top layer may reduce flexibility the busbar  150  may be still significantly more flexible than that of a solid 2 mm thick bus bar. 
     In one or more further embodiments, as shown in  FIG.  7   , the busbar has a body that includes a plurality of aluminum layers  308  stacked together, the body having a first end  312 , a second end  314  opposite the first end, and a middle portion  316  disposed between the first end and the second end. The plurality of aluminum layers includes at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer and secured to the first aluminum layer with one or more laser welds  302  at the first and second ends. The plurality of aluminum layers  308  may be configured to have a plurality of offsets  304  at the middle portion  316  such that the middle portion has a plurality of raised profiles  326  relative to a level profile  328  of the first or second ends. Further, the thickness of the busbar  150  maybe maintained throughout the length of the busbar. In the example of  FIG.  7   , the busbar  150  may connect 4 cell terminals  502 . 
     The busbar  150  may further be used to connect across two cell stacks as shown in  FIG.  8    wherein a first row of cells  802  is connected to a second row of cells  804 . 
     Turning now to  FIG.  9   , a laser welding method  900  is disclosed. The method  900  may begin at step  902  wherein a plurality of aluminum layers is obtained. In step  902 , the method may prepare a busbar comprising at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer. In step  904 , method  900  may generate an offset at a middle portion of the busbar such that a profile of the middle portion of the busbar is raised relative to a profile of a first or second end of the busbar. In step  906 , method  900  may laser weld the first aluminum layer to at least the second aluminum layer at the first and second ends of the busbar. The method  900  ends thereafter. 
       FIG.  10    shows a laser welding method  1000  using a laser device having a combination of welding techniques that may enable the laser welding of aluminum foils. In step  1002 , method  1000  may prepare a busbar comprising at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer. In step  1004 , method  1000  may generate an offset at a middle portion of the busbar such that a profile of the middle portion of the busbar is raised relative to a profile of a first or second end of the busbar. In step  1006 -step  1008 , method  1000  may laser weld, responsive to the generating, the first aluminum layer to at least the second aluminum layer at the first and second ends of the busbar. In method  1000  laser welds the aluminum layers together using a laser device having a wavelength of 1070 nm (+/−5%, or +/−10%), a welding speed of about 20 mm/s (or ranging from 20-30 mm/s or 20-25 mm/s), a laser power of about 1400W (+/−5%, or +/−10%), a scan width of about 3 mm (+/−5%, or +/−10%), a scan frequency of about 200 Hz (+/−5%, or +/−10%). A head with a beam spot diameter at focus of 100micron may also be used. The method  1000  ends thereafter. Of course, other defined wavelength and laser welding properties may be used as shown in the table below. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Wavelength and laser welding properties. 
               
            
           
           
               
               
            
               
                 Laser 
                   
               
               
                 Equipment 
                 Property 
               
               
                   
               
               
                 Co2 laser 
                 wavelength 10.6 micrometer, single mode or multi mode 
               
               
                 Fiber lasers 
                 wavelength between 500 nanometer and 2200 nanometer, 
               
               
                   
                 single mode or multi mode or beam shaping 
               
               
                 Disk Laser 
                 wavelength between 500 nanometer and 2200 nanometer, 
               
               
                   
                 single mode or multi mode or beam shaping 
               
               
                 Direct 
                 wavelength between 500 nanometer and 2200 nanometer, 
               
               
                 diode lasers 
                 single mode or multi mode or beam shaping 
               
               
                 Blue laser 
                 wavelength between 400 nanometer and 500 nanometer, 
               
               
                   
                 single mode or multi mode 
               
               
                 Welding 
                 Fix optics welding head 
               
               
                 head 
                 Scanning optics/beam oscillation optics 
               
               
                   
               
            
           
         
       
     
     In some other embodiments, the method recognizes that factors that may be considered during welding include material composition, thickness, power density and welding speed. The method recognizes that increasing a level of the power density may require a corresponding increase in the welding speed for the same material/thickness of the aluminum layers to be welded together. A laser power density of between 175 Kw/mm2 to 180 Kw/mm2 may be suitable for welding thin aluminum layers together. This may be accompanied by a welding speed of between 20 mm/s to 25 mm/s for a busbar of about 2 mm thickness having an offset height of between 1.5 mm to 2 mm. As shown in  FIG.  11   , in step  1102 , method  1100  may prepare a busbar comprising at least a first aluminum layer and a second aluminum layer stacked on one longitudinal surface of the first aluminum layer. In step  1104 , method  1100  may generate an offset at a middle portion of the busbar such that a profile of the middle portion of the busbar is raised relative to a profile of a first or second end of the busbar. In step  1106 , method  1100  may obtain a defined combination of laser power density and welding speed for welding the plurality of aluminum layers that have a defined total thickness together. The method may then laser weld, the first aluminum layer to at least the second aluminum layer at the first and second ends of the busbar. In step  1108 , method  1100  may laser weld the aluminum layers together using a laser device configured with the defined combination. The defined combination may include a power density ranging from 170 Kw/mm2 to 180 Kw/mm2 or 175 Kw/mm2 to 180 Kw/mm2 and a welding speed ranging from 20 mm/s to 25 mm/s. This may be used for a 2 mm aluminum busbar with an offset ranging from 1.5 mm to 2 mm. The method  1100  ends thereafter. Methods  900 ,  1000  and  1100  may be performed wholly or partly by a user such as a welder or computer or a combination of a welder and a computer. The computer may be part of a system having a processor and software configured to perform some or all the steps herein. The software may be stored in a non-transitory computer-readable storage medium and loaded into a memory. Control logic, when loaded and executed by the processor, causes the computer, to perform all or some of the some of the methods described herein. 
     Although the present technology has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred implementations, it is to be understood that such detail is solely for that purpose and that the technology is not limited to the disclosed implementations, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present technology contemplates that, to the extent possible, one or more features of any implementation can be combined with one or more features of any other implementation.