E-Mobility Engineering | September/October 2023 65 Busbars and interconnects | Focus copper and aluminium. While copper is still ‘king’, one expert reports that interest in aluminium and copper-clad aluminium is starting to grow, while tin, silver and nickel are all common plating materials. As application requirements become more challenging, and production volumes needed to meet market demand grow, manufacturing techniques are moving towards greater automation and higher levels of control. That means it is essential to manage process variability and reproducibility. Manufacturing evolution The experts we consulted note that while the techniques for manufacturing busbars have not changed much over the years, the equipment used has evolved significantly. For example, servo-driven stamping presses and CNC bending equipment have increased accuracy and efficiency. On the other hand, no single manufacturing technology has won out, and battery systems rely on numerous technologies within the pack. In addition to stamping and bending, these can include forming techniques such as photochemical machining or etching, plus the creation of flexible busbars built up from thin layers of coated copper formed in a stack and covered with an insulating coating. Where possible, however, CNC-bent busbars are progressively replacing stamped ones, which reduces waste and capital expenditure. With the feasibility of mass production being particularly critical in automotive applications, this trend towards CNC bending is becoming significant. CNC bending works with aluminium as well as copper conductors, in both bare metal and plated forms. The technology is flexible, economical in terms of materials and relatively easy to automate, according to a company specialising in the technology. Busbars are often made from thick wire that is both rigid and flat, the latter meaning it is wider than it is thick, and it routinely has to be bent across its width as well as through its thickness, or even twisted. CNC bending can handle copper and aluminium conductor material that already has its insulating coating in place, making automation easier. The process is flexible, because the angles and directions through which the wire is bent or twisted, and the Conductor material selection is a fundamental task, in which weight, cost and conductance are key factors to be balanced, according to the electrical, industrial and power management provider. Busbar packaging is another hurdle, as space is limited in battery disconnect units and batteries because of creep and clearance requirements. It is also important to offset the applied stress at the electrical interfaces owing to the different rates of thermal expansion and contraction of modules, and to ensure that batteries can be serviced or repaired. “Huge cost savings can be realised if individual cells can be swapped, rather than having to replace an entire cell array,” the company’s expert says. With busbars, one of the most important considerations is electrical insulation, which is critical to the integrity of the whole electrical system, both in normal operations and in mitigating the effects of thermal runaway, for example, according to an engineer from a major chemicals company. There are many different approaches on the market, the most prominent being extrusion coating with alternatives, including direct overmoulding and the securing of busbars to housings using insulated clips. There are also many classes of insulating material, with the choice between them being affected by different battery concepts and busbar geometries. Further, the busbars and their insulation need to be designed to handle high currents and voltages, and to dissipate heat efficiently, especially during fast charging. “A deep understanding of the application and its requirements, and knowledge of advanced materials, are needed,” the company’s expert says. “This is the only way to make the right materials decision and is also relevant in terms of scalability and costs – factors that are important for mass production.” Most current collectors are aluminium, with interconnects being nickel or aluminium alloy. Busbar materials will vary, depending on application, between Stamped copper busbars emerge from a machine at a precision metal stamping and ultrasonic welding specialist, which has patented a means of stamping variable and compound thickness parts (Courtesy of Wiegel)
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