Batteries and Fuel Cells
Wired for performance T
As the popularity of e-mobility grows, even the most sophisticated battery packs are produced using well-established manufacturing processes to deliver reliability and safety. Jim Rhodes, technical sales director of Inseto, explains wirebonding
he DfT’s Transport and Environmental Statistics 2021 Annual report included the statistic that domestic transport accounted for 27 per cent of the UK’s greenhouse gas emissions in 2019. The report also provided a reminder that combustion engine-powered vehicles emit gases (such as NOx) and substances (particulate matter) that don’t have a significant greenhouse effect but do have health consequences.
The increased use of electro mobility (e-mobility - the partial or full use of electric power within a drivetrain) in vehicles under development and already on our roads, stands to reduce emissions. An essential component of a vehicle, whether pure EV or hybrid, is the battery. It is core to the vehicle’s performance in terms of range and acceleration. Also, while there are many on-going discussions over battery electric versus hydrogen, a vehicle with a fuel cell will most likely be a ‘hybrid’ and include a high-voltage battery pack for storing energy recovered through regenerative braking, for example. It is also worth noting that the battery pack of a pure EV accounts for more than one third of its total cost. And with the e-mobility market (which includes e-bikes, e-buses, e-trucks etc.) growing at the rate it is, the focus is on the manufacture of
Of these, the latter method is more popular because there is no risk of overheating and damaging the cell, so let’s talk more about wirebonding. The wire tends to be high purity aluminium, with a diameter of between 0.2 and 0.5mm. To accommodate high currents, wires can be placed in parallel.
Wirebonding is an ambient temperature (‘friction weld’) process, so does not run the risk of damaging the cell, plus the wire can act as a fuse.
reliable, durable and safe battery packs as cost-effectively as possible.
A battery pack used in something the size of a car comprises several battery modules. These are connected using bars, bolts or heavy gauge cables in parallel and series combinations to produce the desired energy and power characteristics. Each module contains several battery cells and, in many cases, dedicated power and thermal management systems. The cells are typically cylindrical lithium-ion (Li-ion) batteries, as they boast a high energy density (250 to 270kWh/kg). Within a module, dozens of cells are placed in
Aluminium ribbon can also be used to connect cells to busbars.
parallel, achieved by connecting their terminals to busbars, usually made of aluminium or steel.
The two most popular methods for connecting cells to busbars are: • Laser welding. Each busbar is placed in physical contact with the respective terminals of all cells to which it is to be connected. Tooling can be an issue to account for any cell height tolerances. Also, as it is a traditional weld process, the objective is to heat metals until they fuse together. Here, there’s a risk that localised heat from the welding process penetrating the negative terminal can alter the cell chemistry and lead to catastrophic thermal runaway. NB: cell positive terminals are ‘floating’, so less vulnerable because of the air gap. • Ultrasonic wirebonding. The process is already dominating power electronics manufacture 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. The process copes far better with tolerances in cell height (relative to the busbar) and any rework, should it be necessary, is far easier than re-doing a weld, which exposes the cell’s terminal to a high temperature for a second time.
20 April 2022 Components in Electronics
Manufacturing advantages aside, there are two other reasons for using wire: • Safety. If a single cell fails, it can act as a short across the others (wired in parallel) and, as Li-ion cells contain flammable electrolytes, they can ignite or explode. By matching the cross-sectional area and length to a maximum desired current, each cell’s bond wire can act as a fuse. A shorting cell will effectively self-isolate. • Durability. 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 aluminium wire can be specified to adapt the material properties to fine-tune wire softness/ flexibility (annealing).
Where e-mobility in the automotive sector is concerned, keeping costs as low as possible is of great importance. The use of well-established manufacturing processes is the solution. For the interconnection of cells, wirebonding has the edge over laser welding. That said, equipment manufacturers are striving to overcome the thermal implications (and busbar design considerations) required to make laser welding an affordable and more attractive alternative to wirebonding. It will though mean foregoing the peace of mind derived from having an electrical fuse in line with each cell and place more burden on (and trust in) the power and thermal management systems.
www.inseto.co.uk www.cieonline.co.uk
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