E-Mobility Engineering | September/October 2023 51 Battery welding | Insight nicely together. The fusion process melts both materials, and as they resolidify they form a brittle, intermetallic compound which typically falls apart under vibration or stress.” Design for manufacture is another challenging area that Boyle points to, particularly the positioning of positive and negative terminals, which can be on the same or opposite sides of the cell, top and bottom for example. “Having terminals on opposing sides requires a complicated flip of the battery to weld both sides,” he explains. “When terminals are on the same side, the current collection plate needs to be intricately designed to allow access to the battery cans, and the landing of the outer ring is often critical to achieving a good weld.” Wide tolerances in manufacture also present problems, he adds, particularly of the cells themselves, as varying terminal heights and positions make automated welding more difficult. The need for speed Boyle also notes that high demand usually translates into a need for highspeed production, which can limit the number of welding technologies considered or require large capital investment in multiple machines. Resistance welding on a production line, for example, typically involves a pair of electrodes moving down onto the workpiece, making the weld, moving (indexing) up, along and down again in a cycle that repeats roughly every second. “However, if you use a laser, particularly with a scanning head, it can scan through a range of eight or nine cells – depending on the pack, cell size and field size of the scanning head – in about the same time and then index to the next group of cells,” he says. He adds that laser welding is a little more complicated because it is a noncontact process that doesn’t physically push the workpieces together, yet requires intimate contact in order to make the weld. That makes tooling and fixtures a necessity, so a balance has to be struck. “If high-speed production is not the be-all and end-all for the customer, they opt for resistance welding because it provides that contact force, reliability and repeatability,” he says. “If high speed is required, then companies will go for laser. “Each company has to weigh the pros and cons. A laser requires a larger initial investment, whereas resistance welding equipment costs less but uses consumable electrodes. And if you do need speed, you will be bulk-buying multiple resistance welding machines.” fusion joint, whereas soldering and conductive adhesives are prone to melting or weakening at high temperatures. Also, mechanical fasteners carry a significant weight penalty, so they tend to be restricted to some dissimilar metal joints and to prototype battery systems. Further, welding provides significant cost advantages over conductive adhesives and fasteners, and is a much more repeatable, rapid and accurate process than soldering, Mark Lakis at Wiegel says. Material issues At the highest level, reliability and process speed are the main engineering challenges in welding battery contacts and structures, he notes. Carr concurs, and cites cost and customers’ desire to use the latest materials, such as grades of aluminium that are superlight but hard to weld and seal shut. “Copper seems to be more prevalent, as its cost has remained stable and its extra conductivity is a bonus because it reduces losses,” he says. “We see a demand for highly controllable weld penetration, the highest seam quality with little or no spatter as well as the highest throughput possible,” Franks notes. In general, Boyle says, manufacturers want light, highly conductive, increasingly efficient battery packs, and they often choose materials such as copper and aluminium, both of which are difficult to weld, making joining current collectors to cell cans a challenge. “Depending on the battery design and materials, the metallurgy of the welds prevents some joints from being made with sufficient strength to handle the current,” he says. “One of the more difficult combinations for any type of welding is steel and aluminium,” he explains. “Cylindrical cells are often made of cold-rolled, nickel-plated steel, to which manufacturers want to weld an aluminium tab because it is lightweight and very conductive. “But steel and aluminium don’t play Weld penetration depth is a key parameter for determining the quality of a join, regardless of the welding technique used. This image was captured through a digital microscope (Courtesy of Keyence)
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