E-Mobility Engineering | November/December 2023 27 Xerotech battery system | Dossier The cross-flowing nature of the current enables Xerotech to use the same standard aluminium as in most of its pack housings – AL 5254, rather than AL 1000, which others might use in their busbars – to enhance production simplicity and cost-effectiveness. It also enables high current densities in the modules, the company noting that its smallest module outputs around 1500 A continuously and 2000 A at peak. “And that’s without using copper busbars or any special preparations,” Dr Flannery adds. “We can effectively redline a Molicel P45B, the most powerful cell in the world, with 2000 A peak for 3-4 minutes, essentially discharging at 20C, which is probably double the power of eVTOL aircraft. “Achieving those current flows is difficult with single-sided wire bonding and busbar configurations, as that gives a lot of pinch points on the current path, leading to localised heat generation and other problems.” After an optical inspection for QC, the cells are joined to the busbars at both ends using ultrasonic wire bonding. “That basically melts the micro-buses protruding from the aluminium bars to fix them to the cells,” Dr Flannery says. “Ultrasonic welding is quick, reliable and robust. It scuffs and abrades the oxide layers, so you don’t get intermetallic compounds forming at the interface, as with laser welding, and it’s cleaner than hot-plate welding. “A wire bond intrinsically functions as a fuse on top of the cell, and it detaches easily in response to a crush impact. All that said, we’ll probably switch to laser welding for our next-generation platform when we move to 4680 or similarly larger cylindrical cells, as it will mean higher throughput, and we’ll have a new busbar design that will need to be laser-welded in place.” The ducts are then expanded with water to their operating volume, the force of which pushes the cells to bottom-out in their holders, rigidly locking them in place without adhesives or UV-curing glues. The cells can then be potted with their foam compound to lock the cells in place permanently while still ensuring the ducts have enough room for full contact and hence thermal transfer with the cell walls. The polyurethane is a two-part foam, dispensed into the module by a dynamic mixer head. “It has the consistency of olive oil when injected, and it’s a selflevelling foam with delayed action, so it levels and spreads out through the box quickly before expanding into a foam structure,” Dr Flannery says. He adds that Xerotech is also looking into new polyurethanes that will provide greater rigidity and density than the current foam, to enhance the structural integrity and thermal runaway performance in the Hibernium packs. Pack assembly Once the foam sets, the top and bottom module covers are installed and glued on, and the module is validated optically to confirm it is finished. The required number of modules for a pack are stacked on top of each other and separated by silicone seals unique product and don’t make your own machines, it’s very hard to understand what you need to change to optimise your manufacturing throughput.” After the plastic sub-assemblies and module housing parts are joined by welding and adhesives, the cells are slotted into the separator plates installed at the bottom of the module. With those bottom plates holding the cells’ bases, an upper separator plate is placed on top, so the cells are held in place at both ends. Busbars and welding Before this assembly stage, both separator plates will have been bolted to their module’s busbars, which are machine-cut in-house from sheet aluminium. Hibernium’s busbars are cut as linear pieces that run the length of each module, with current cross-flowing across them. The cell power take-off points run along their sides as parallel connections, with the main power takeoffs enabled through the L-shaped geometry of the bar, through which the busbars also serve as structural members for the modules. “And we split these linear busbars down the middle, to give our two different voltage configurations: 8S or 16S,” Dr Flannery says. The cells are ultrasonically welded to the busbars for fast and reliable wire bonding; laser welding is likely to be used in the future
RkJQdWJsaXNoZXIy MjI2Mzk4