E-Mobility Engineering | November/December 2023 29 run them at multiple SoCs to see how energetic the reaction is at different levels, including the worst-case scenario at 100%,” Dr Flannery says. “Even then, the module self-extinguishes within 20 seconds and the pack keeps working.” In Xerotech’s cell testing lab, several racks of cell cyclers from Neware run endurance tests, each machine interfacing with eight cells at a time, with machines typically stacked in groups of eight per rack. Multiple different chemistries are tested simultaneously in each cycler, given that they all are 21700 cells and therefore compatible with the same machines, even if some are 4.9 Ah and others are next-generation 5.5 and 5.8 Ah. Environmental cycling rigs in the cell testing lab run several hundred cells at the same time in individual chambers with liquid-cooled backplates. These cool and heat the cells at different gradients (from -10 to over 60 oC) while running varying C-rate profiles, to characterise and map their operating behaviours. An environmental test chamber here enables further testing below -40 oC. “We’re at about 10% of our testing capacity now, but we’ll need to grow that capacity over the next few years to serve the range of cell technologies, customers and pack supply quantities we’re aiming for,” Dr Flannery says. “Some of our cyclers are for longterm endurance cycling, and those only go up to 5 A or 1C discharge, but we have high-performance cyclers that go up to 30 A per channel and 5-6C. When we go up to 4680 cells, or make more use of high-power 21700s like Molicel’s products, we’ll use our biggest cyclers, which do 50-100 A per channel.” Key standards for cell testing include UN 38.3, which Xerotech follows for every new cell type it receives, along with all the standard homologation tests even if the cell supplier claims compliance with them. “We do long-term cycling of new cell types, running them for a year at different rates, dynamics, temperatures and gauge their behaviour across every SoC and SoH,” Dr Flannery comments. “We don’t need that for compliance, but we want to optimise the DoD, other configuration points, and warranties we make available to customers.” Actual cell performance testing is relatively short, lasting around a month. It maps voltages, currents, direct current internal resistance and other parameters across time and states Xerotech will also consider steels, aluminium and coatings on a caseby-case basis, given the potential for corrosion risks owing to paint failures. “Bolting the pack and BDU lids means debonding and opening them for maintenance is very easy,” Dr Flannery says. “There is a dispensed adhesive seal, but it doesn’t get in the way, and although our housings are IP67-rated, we’re capable of IP69K and we’ll have that rating at some point.” Testing and analysis Once the module stack is lowered into its pack housing, end-of-line tests are carried out. These include load cycling, leak testing, isolation monitoring, cycles in test chambers across different temperatures and humidities, and external shock and vibration tests before it is packaged and shipped. When validating a new pack design or configuration, key destructive testing includes nail penetration tests. “We’ll also detonate a cell in a module that’s sandwiched above and below by additional modules, to measure crossmodule propagation,” Dr Flannery notes. “In automotive EVs, there’s a low risk of cross-propagation because their modules typically sit apart on a flat, wide bed. But off-highway packs are more block-like because of how those vehicles are shaped, so it’s an important parameter for us to check.” For validating lower-level systems, module tests on Xerotherm’s extinguishing capabilities start with using a wire around a cell in the middle of a module to heat that cell to failure. Once the cell goes into runaway, test instrumentation on fluid flow and temperature can sense the speed at which the adjacent duct melts and releases coolant onto the cell, half-asecond being the average time taken to release the water-glycol. “We’ll often use the highest power and energy density cells we can get to run those tests, to characterise the most extreme failures, and we’ll The Hibernium packs are bolted together to enable straightforward debonding for maintenance, inspection and repairs
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