Xerotech battery system | Dossier E-Mobility Engineering | November/December 2023 23 outside of the BDU for isolating the BDU from the rest of the pack and hence safe swapping of contactors (which it notes is otherwise very straightforward). “Mechanically, the MSD breaks the power connections at the middle of the pack,” Dr Flannery says. “That temporarily reduces the voltage output by half, which is good from a fusing perspective and also means we can work freely on the BDU without needing any protective systems because the electrical system is completely dead. “It also incorporates a high-voltage interlock loop [HVIL], as do all our HV connectors, and there’s also an active HVIL circuit that goes out to the broader system.” As well as preventing high-voltage accidents, the MSD also enables untrained personnel to safely disconnect loads during accidents that can cause thermal events, such as fluids splashing on or immersing the pack. Other key electrical safety functions are embedded in the BMS. One is overcurrent protection, which is achieved via a dualredundant shunt (one primary shunt, one secondary) for current monitoring, and pre-programmed current limits. At one of these, a warning is delivered to the driverfacing external system, and at an upper limit the BMS cuts off the power supply. “Those BMS functions are pretty well-known and understood, but most commercially available MSDs are totally unusable for large, heavy-duty applications,” Dr Flannery cautions. “In the MSD we’re using, the biggest fuse is 630 A, but the actual continuous current rating on the plug receptor is 300 A. That’s fine for general automotive as road EVs only exceed a few hundred amps in very short durations, but if you’re running a 300 kWh pack continuously, you’re likely to be pulling 500-700 A for 2 hours, so we’d be really interested in trying out an MSD in which all subcomponents are actually rated to handle those kinds of currents. “The Gigavac system for hypercars is potentially interesting, but otherwise there’s a real gap in the EV market for someone to make a 1000 A-rated MSD for not just bigger stuff like construction and mining EVs, but even for high-performance, high-discharge road EVs like hypercars.” water, making it a low-weight solution. “Potting compounds aren’t new to the battery industry – Tesla started it with their ‘blue goo’, a silicone that they fully potted the modules in,” Dr Flannery says. “But the disadvantage of those material types is their weight, and the structural pink foam in their Model 3 encasing their 4680 cells isn’t even fire-rated – it burns. “Silicone cell encapsulants exhibit a similar density to water, and perhaps slightly greater weight. Furthermore, by their chemical nature they’re much more expensive than polyurethanes, as manufacturing them is a much more complex and involved process. They also generate a lot more contaminants for production lines; you really don’t want silicone off-gassing and contaminating metal surfaces.” Naturally the polyurethane is also chosen for its strong thermal insulation properties, which stem from the open but air-filled nature of its cellular structure, and help contain heat from passing between cells during normal operations as well as thermal runaways. It is also fire-retardant, and after extrusion into each module it is doped to minimise its capacity to burn. On top of fires and thermal runaways, the safety of Hibernium’s high-voltage output has been another engineering focus for Xerotech. “It’s important to understand that all a battery can do in a fault condition is open its contactors,” Dr Flannery notes. “It can’t reduce power, and it has no control over the load demand it’s being subjected to. “When you open an HV contactor under load, it’s a very energetic event. DC arc welding is about 10-30 V, so compare that to the 600-800 V, high-current output of an HV pack and you can maybe imagine how dangerous that is.” Contactors can therefore open under load between one and five times before being effectively destroyed. Accordingly, to enable prudent servicing of the Hibernium contactors, Xerotech has designed a manual service disconnect (MSD) mechanism on the The cooling system has been simulated in CFD, using Ansys Fluent, Ansys Mechanical, Altair SimSolid and some 2D and 1D modelling software
RkJQdWJsaXNoZXIy MjI2Mzk4