24 November/December 2023 | E-Mobility Engineering If a cell should fail Every cell has a coolant duct on one of its sides, and foam around all its others. The extent of encapsulation provides shock, NVH and penetration safety at the near-cell level, and importantly it can prevent heat propagation on a cellby-cell basis. “Containing failure to a single cell is the goal for us, and that strategy and target are additional reasons we’ve gone with cylindrical cells,” Dr Flannery comments. “Do a hotwire test on a large pouch or prismatic cell and you’ll see how violently and aggressively it fails. A 21700 cell has about 18 Wh of electrical energy inside, and a multiple higher when considering the chemical energy, while a large prismatic cell can go upwards of 1-2 kWh; the less energy per cell, the easier a failure is to contain.” If a single-cell thermal runaway event should occur, the foam is sufficient in some cases to contain it to that cell, depending on the severity of the heat or flame produced, and will otherwise slow the spread of heat to its neighbours more than automotive-standard requirements on passive propagation resistance. “And all the cells along a given busbar are connected in parallel. If you lose a cell, you’ll still read the same voltage,” Dr Flannery says. “You’ll have lost some capacity, and there’ll be a rupture in the cooling system, so it’s still end of life for the pack. “But for aerospace, defence and other integrators who require their vehicles to be ‘fail-operational’ [designed to maintain normal operations in the event of a system failure] for a certain period, minimising or substantially slowing damage during catastrophic events like that is important. Thermal events will initiate for us as often as they do for our competitors using the same cells, but our internal module architecture will slow or contain those events far better than conventional approaches. “For non-fail operations though, we tell customers to get the pack out of the EV as quickly as possible and submerge it in water for a week to prevent reignition later on.” Also, the coolant duct’s plastic melts and breaks apart at 100 oC, causing the water-glycol to flow out, extinguish and drown any burning or off-gassing cell material. “That passively enables either total fire suppression, or far more than 15 minutes of delay before smoke ingress into the passenger cabin,” Dr Flannery observes. “The cell that fails will melt the duct precisely where it touches, and our tests have found that in most instances, the foam encapsulates both the cell and its nearest portion of the coolant duct well enough that water-glycol will flow in a contained way, around only the failing cell, not any others.” The water-glycol fills the tight space between the failing cell and its surrounding foam within a few seconds, creating a pressure equilibrium such that the liquid does not flood the entire module, negating the need for valves to shut off flows within a row or module of cells. The module’s housing plastic is selected for being inexpensive, highly mouldable and a V-0 ABS. If burning, it self-quenches within 10 seconds, and no flaming debris drips off. Also, the foam encapsulant surrounding most cell walls helps prevent fire from touching the module enclosures to begin with. The cells also have several mechanical safety features. Layers stacked inside the top cover include a positive temperature coefficient (PTC) device. This is a ring that, with rising heat, generates an increased resistance, limiting the current and reducing the chances of an electrical fault. Close to the PTC is a current interrupt device, which consists of two plates in the cell cover. During internal venting, the top plate bursts upwards inside the cover (as a passive mechanical response to high pressure, heat or overvoltage), breaking the electrical connection inside the cell. Also, if a catastrophic impact should occur that risks an arcing event from the modules, the wire bonds from cell to busbar are designed to break, cutting all power connections across every module. Xerotech integrates WL Gore’s PolyVent XL (also called PMF 200542) at the pack level to serve as a breathable membrane that maintains ambient air pressure inside the pack regardless Dossier | Xerotech battery system The first room in Xerotech’s production facility is dedicated to injection moulding of plastic parts and CNC machining of mould tools and jigs
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