Optimal Interconnection for Cylindrical Battery Systems in Electric Vehicles

Mar 04, 2020

Reliability

Battery packs have been used in automotive and industrial applications for many years. Traditionally, these battery packs use a relatively small amount of large battery cells, typically lead-acid type battery cells. A battery pack design that uses a small amount of large battery cells has several disadvantages. For example, if one of the cells fails, a substantial percentage of the overall energy capacity is lost. In addition, such a battery pack has a poor thermal cooling efficiency.

As we know, the performance of batteries for electric vehicles depends on the power and energy capacity of the battery pack. Considering all battery technologies, the lithium-ion cylindrical cell batteries, in particular, have received great attention after Tesla sold its first Model S a few years ago using cylindrical cell technology for their battery pack. It has been proved that the battery packed using cylindrical cells provide the highest energy density, high working voltage, long durability, reliability as well as susceptibility to heat during discharge and recharge cycles.

However, such battery packs tend to be expensive, due to a large number of cells being connected - sometimes-even thousands of cells. Costs are also driven by the complexity involved in interconnecting such a large amount of cells using conventional interconnection techniques e.g. wires, cables or a metal strip welded to cell terminals. Moreover, conventional cell interconnections (welded wires or strips) are susceptible to failure when the cells dislocate slightly during operation, for example, due to vibration within a moving vehicle. That is why the new interconnection technique using the busbar as current collectors have been introduced to overcome these challenges.

Electrically, connections must not only handle high current coming from the cells but also increasing voltage levels, as cells of the future will be up to 5.0 V per cell. As a result, the impact on the clearance and creepage distance for the electrical insulation will be significant.

Mechanically, current collector busbars for battery packs must be durable, capable of withstanding high levels of vibration and provide rigidity to keep the integrity of the battery module assembly while being flexible enough to cope with thermal and G-forces.

The connections' performance also depends on the used composite materials to construct the current collector busbars. To connect the cylindrical cells, manufacturers typically use aluminum conductors in various thicknesses from 0.25 to 1.0 mm. Aluminum busbars are attractive for the battery cell connection because they provide reliable electrical performance while saving total battery pack weight, since aluminum busbars are typically 50% lighter than copper busbars. However, for equivalent electrical / thermal performance, the cross-section of an aluminum busbar is greater than that of a copper busbar with a 0.5 mm copper conductor replacing a 1 mm aluminum conductor. Copper busbars offer excellent solutions where space is limited, while aluminum busbars enable efficient energy distribution with weight and cost savings.

Thermal management is a challenge that the correct busbar can assist with, especially for cylindrical cell connections where the busbar may connect hundreds of cells to make a complete module. For the cylindrical cell, connections to the positive and negative electrodes can be made either from the top and bottom or only from the top. The disadvantage of top and bottom design is that it may prevent the efficient operation of either an air-based or a liquid coolant based cooling system that is used to remove heat generated by the cells during operation. That is why some battery packs and carmakers select only the top connection of the battery cell, despite the fact the current collector busbar is more complex. These current collectors are high precision stamped and have very tight tolerances, they are formed down to cell level for laser weld or wire bond attachment to positive and negative terminals. The top and bottom collectors are separated by adhesive backed insulation film that is attached to it. The insulation film is also used on the outside of collectors to seal/laminate the busbar construction in a single structure.

Another aspect is the connection between the cells and the current collector busbar. It is important to minimize the contact resistance in order to reduce the electrical loss that generates heat at connection points along a busbar. To minimize contact resistance, it is recommended to use laser welding or ultrasonic wire bonding technology to connect the groups of battery cells to a collector for assembling the large, high-power battery packs for electrical vehicles. Laser welded and wire bonded connections can also be made as part of an automated assembly process to minimize manufacturing costs in large volume production.

These types of current collector busbars typically have a large surface and in addition, to collecting the current from the cell, it can be accommodated with voltage and temperature sensing lines Battery Management Systems (BMS) in an electric vehicle´s battery pack.

Overall, we have to remember that the electrical cell connection is an essential element of the battery pack design and the selection of the current collector busbar for the battery cell is not always as simple as one might think. If you have any design questions or require assistance for the optimization of your design using current collector busbars for your application, Rogers’ PES experts are available to help. Please contact us, if you have any questions. 

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