66 September/October 2023 | E-Mobility Engineering length of material that passes through the tool before the bend is made, are controlled by software, says the CNC bending company’s expert. That means multiple parts with different lengths and configurations of bends and twists can be produced on the same tooling, so long as the dimensions of the stock are within the range it can accommodate, and changes to the design can be implemented as quickly as they can be put into the software. In contrast, with most common alternatives of stamping, the sizes and shapes of the parts are dictated by the tools installed in the stamping machines, and any changes to the busbar design require expensive new tooling. The stamping process starts with a flat sheet from which the tool cuts thin strips; the rest of the sheet, about 50%, is waste. While waste copper and aluminium are routinely recycled, the process requires energy. With CNC bending there is almost no waste, the expert adds, although some of the insulation on the ends of the busbar has to be removed by mechanical or laser stripping to leave clean metal for electrical connections. Insulation choices The choice of insulating coatings is wider, with options including the polyamides PA6, PA66 and PA12, polyether ether ketone, polybutylene terephthalate, polyphthalamide (PPA) and polyphenylene sulphide (PPS), all of which are thermoplastics. For applying electrical insulation, there are several widely used techniques on the market, including direct overmoulding, extrusion coatings and clipped housings, in which insulated mounting clips provide a stand-off distance between busbar and housing. So far, there is no clear trend towards one particular technology, according to the expert from the chemicals company. “If anything, the extrusion coating is slightly preferred for longer connections, which will probably disappear due to battery architecture optimisation,” he says. “We see manufacturers choosing between extrusion and injection moulding, but many are not choosing just one process because they see the need for expertise in different manufacturing processes. “Companies need to master multiple processes to handle diverse customer requirements. It comes back to being flexible enough to make safe, highperformance parts for different types of battery architecture.” He adds that the challenge with such insulating coatings is that metals and thermoplastics do not naturally adhere to each other, so considerable expertise is required to ensure the bonds between them will survive thermal shock testing. “We have our own moulding technology and equipment for thermal shock testing, and for thermal runaway equivalent testing,” the expert says. “Sometimes we show parts at trade shows or in meetings where the parts failed the testing. “Why show a part that failed? Well, we have plenty of samples of perfect parts, but we also want people to recognise that we have ‘walked the walk’ in doing the tests and going through trial and error and trial again, so that they don’t have to.” That also involves partnering with manufacturers and collaborating on design, he adds. Reliable connections Making connections within and between interconnects, current collectors and busbars can be seen as a straightforward process, both in routing the conductors and ensuring reliable electrical contacts. However, as battery and powertrain technology develops, and increasing energy and power levels require higher standards of reliability and safety, the connection technology has to meet them. At the same time, battery and powertrain integrators are also looking to reduce costs and cycle times. Such connections are typically made using techniques such as bolting and welding, and there is now also the choice of new types of mechanical connectors that support automated assembly, says the expert from the electrical, industrial and power management company. With rapid market access a priority, cell interconnection technologies have been kept relatively simple with the use of plastic overmoulded solutions and wire harnesses. With more recent demands for performance, integration, weight and optimisation, however, laminated busbars have been gaining in popularity, according to the power transfer, management and materials specialist. These are engineered components consisting of layers of fabricated copper separated by thin dielectric materials and laminated into a unified structure. In battery interconnects, wire bonding and ribbon bonding appear to be the most common means of making Focus | Busbars and interconnects All the bends and twists of a bar such as this are done in a single operation after programming a CNC wire bending machine (Courtesy of Wafios)
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