Suppliers of future electric vehicle battery systems are working through a series of complex issues to make EVs a reality while maintaining safe operations.
Polymer composite electric vehicle battery enclosure designers already face "difficult and challenging" requirements as legislation in China and Europe hold automakers to higher design standards.
"When it comes to lithium-polymer and lithium-ion batteries, they are somewhat unstable," Ian Swentek, senior application development engineer at Hexion Inc., said during a Feb. 18 Center for Automotive Research webinar.
"You can't over- or undercharge them," Swentek said. "They are susceptible to thermal runaway from the battery pack itself or from external events such as a short circuit, heating from outside environments such as collision with a battery or gasoline-propelled vehicle."
While "no one wants their vehicle to ignite or burn," he said, if the batteries "do experience a high-temperature condition, they are self-burning and self-oxygenating. You can't put out the fire with traditional materials and the goal of passenger safety and protection is really around buying enough time for occupants to safely leave the vehicle."
Two key requirements for polymer composite enclosures pose the challenge for designers.
In China and Europe, enclosures must be able to withstand the "bonfire test," a two-minute burn event from a gasoline-based fuel source.
Under regulations in China, all new EV battery enclosures must also pass a test for thermal runaway events, a condition "either from an electrical ignition source, some puncture source or perhaps from a thermal event," Swentek said.
"If that happens, because of how closely packed the cells are, and even if one cell goes off, that poses a very high internal risk," he said.
"Within the first few seconds of a thermal runaway, a lithium-polymer battery could heat up to 2,200° C (3,992° F) with average internal temperatures in the batteries around 1,400° C (2,552° F)."
That's "enough to almost melt glass," he added.
EV battery enclosure covers must provide fire protection while also preventing moisture, dust or debris from entering the enclosure, Swentek said.
Load-bearing trays have "varying designs," he said. "Depending upon if it's a full-battery-electric vehicle or a plug-in hybrid vehicle, you'll have battery pack weighing anywhere between 80 and 400 kilograms."
That weight includes the battery's circuitry requirements to maintain temperature and its electronic controls.
The tray also acts as a "crash protector," making them "harder to design for" as it has "more requirements," Swentek said. "We're not able to get away with heavily filled polymer composites anymore. We have to start looking at nonfilled or at even continuous-fiber-reinforced polymers.
"Metals can be all right at addressing some of these mechanical requirements, but they've become quite tricky to address the combined mechanical and thermal protection," he said. "Some of these combined requirements are where polymers really shine because they can be designed and manufactured as a hybrid, holistic solution.
"We want the material to be corrosion resistant; we want it to be lightweight for assembly and service. We don't want the battery packs to be so thermally conductive that it's experiencing very cold temperatures in the winter. We want it to always stay around room temperature both for their life cycle and for safety," he said. "[Polymers] can address all of these requirement simultaneously without having to think of things like extra coatings, blankets, extra assembly steps and types of production steps."