Feature: Batteries
Figure 1: Ultrasonic wirebonding is an ambient-temperature (‘friction weld’) process, so doesn’t run the risk of damaging the cell. Bond integrity/quality is the product of the applied physical force and ultrasonic energy (i.e., power over time). These parameters are controlled to ensure the bond takes, and can even be adjusted to account for some amount of surface contamination
Ensuring EV battery pack reliability at the cell interconnect
level By John Govier, Sales Director, and Jim Rhodes, Technical Sales Director, Inseto
T
he move away from using combustion engines in cars, bikes, buses, trucks and so on is called electromobility (e-mobility) and requires batteries to store and release power. The properties of a battery or battery pack are therefore largely responsible for setting the
vehicle’s performance. Storage capacity defines range, the rate at which power can be released sets acceleration, and the rate at which a battery pack can be charged determines its charge time. Typically, a battery pack in a car comprises multiple battery modules, connected using bars, bolts and heavy-gauge cables,
24 April 2023
www.electronicsworld.co.uk
arranged in parallel or series combinations to produce the desired power characteristics. Each module can contain anywhere between a few and over a thousand cells. For example, the Tesla Model S Plaid has a 99kWh total capacity, of which 95kWh is usable. It has five identical power modules, each containing 22 rows (series connections) of 72 cells wired in parallel (within each row). This equates to 1,584 cells per module and 7,920 for the pack. Each cell is a Panasonic lithium- ion 18650-type, slightly larger than a standard AA cell. Not surprisingly a vehicle’s battery pack accounts for much of
its cost – for a BEV (battery EVs) that can be over a third. With material costs about the same across the industry, battery pack manufacturers want to make their products as cost-effectively as possible. However, they can’t compromise on durability or safety; for battery management systems, the ISO 26262 functional safety standard applies.
Connections In the Tesla battery example, connecting the cells in parallel is achieved with busbars, which can be done in one of two ways: • Laser welding: Each busbar is physically connected to the cells’ respective terminals. Here, tooling can be a problem when it comes to cell height tolerances. Also, as it is a traditional weld process, the objective is to heat metals until they fuse together, which causes localised heat from the welding process to penetrate the negative terminal, altering the cell’s chemistry and even leading to catastrophic thermal runaway. Note that cell positive terminals are ‘floating’, hence they are less vulnerable because of their air gap.
• Ultrasonic wirebonding (Figure 1): The process is already dominating power electronics manufacturing as a flexible and
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