Electromobility (e-mobility) – the move away from using internal combustion engines in cars, bikes, buses, trucks, and so on – requires batteries to store and release power. The properties of a battery, or rather a battery pack, are therefore largely responsible for setting the vehicle’s performance. Storage capacity defines range, and the rate at which power can be released sets acceleration. In addition, the rate at which the battery pack can accept power determines charge times – in, say, battery EVs (BEVs) and plug-in hybrid EVs (PHEVs), and how much power can be recovered through regenerative braking – in, for example, HEVs, PHEVs and range-extended EVs (REEVs).
Typically, a battery pack in a car comprises multiple battery modules, connected using bars, bolts and heavy-gauge cables, arranged in parallel or series combinations to produce the desired energy and power characteristics. Each module can contain anywhere between a few and over a thousand cells. For example, the Tesla Model S Plaid has 99kWh total capacity, of which 95kWh is usable. It has five identical power modules, each containing 22 rows (i.e., series connections) of 72 cells placed in parallel (i.e., 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, so larger in diameter and length than a standard AA cell.
Not surprisingly, a vehicle’s battery pack accounts for much of its cost – for a BEV that can be more than one third. With material costs more or less the same across the industry, all battery pack manufacturers want to make their products as cost-effectively as possible. However, they can’t compromise on durability or safety; for battery-pack battery management systems (BMS), 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 contacted 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 can cause localised heat from the welding process to penetrate the negative terminal, altering the cell’s chemistry and potentially lead 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 robust method of making electrical interconnects in hybrids, switches and regulators. Also known as ‘friction welding’, there is minimal localised heating to the wire or battery surface, with the process coping far better with tolerances in cell heights (relative to the busbar). In addition, there are several industry specifications relating to wirebonding and bond quality that are being adopted within the automotive sector. For example, MIL-STD-883E, Notice 4, Method 2011.7 is a test to measure bond strengths; see Figure 2.
Since durability and safety are key here, ultrasonic wirebonding has an edge over welding. Depending on the vehicle’s intended environment of operation, the battery pack may be subjected to significant vibration and mechanical shock. Any interconnect technology used at the cell level must withstand the external forces expected, to ensure a good operational lifetime.
The bond wire tends to be high purity aluminium, with a diameter of between 0.2 and 0.5mm, and has a degree of softness and flexibility (annealing). It’s worth noting that multiple bonds can be made side by side to accommodate high currents.
As for safety, with a suitable diameter, a bond wire can act as a fuse and a failing/shorting cell will effectively self-isolate, thus reducing the risk of fire or explosion.
In practice
In the automotive sector in particular, keeping manufacturing costs down is an imperative, and this applies to its suppliers, too. For example, Steatite’s Power Business Unit has recently taken delivery of an Asterion EV wire bonder; see Figure 3. Steatite specialises in the creation of custom battery packs, which often need to be of a particular size and shape. The new bonder helps the company establish electrical connections using wirebonding in a much shorter time it takes spot welding. In addition, as a result of the very low resistance of the wire in the wirebonding, battery packs can offer high discharge capabilities and better performance.
Steatite’s engineers have been successfully spot- and arc-welding battery pack components for several years. Spot-welding in particular is suitable for most of the company’s products, in which some parts are up to 3mm thick. Wirebonding is therefore a complement to welding in making most of the battery packs.
On a general note, for any high-value manufacturing process, the ability to rework process steps to improve assembly yield is important, especially in the initial prototyping and pilot production stages. In this respect, wire-bonding has the edge, since failing or weak bonds can be easily reworked. Moreover, wire bonders like the Asterion EV can automatically perform wire pull tests to verify the bond has taken.
Reworking a failed or imperfect weld is more problematic as there will be more surface material to remove and the cell will be exposed to another temperature process as it is rewelded. Also, depending on its design, and once in place, a busbar might not allow access to individual joints for rework purposes.
In summary, welding and wirebonding both have key roles in the construction of EV battery packs. As for which process to use, this depends on the pack architecture, ease of access to the parts to be connected, whether or not fuses are required, the ability to accommodate reworks, volumes being manufactured, production costs (including time) and the end application.
By John Govier, Sales Director, and Jim Rhodes, Technical Sales Director, Inseto
