GM is pushing for a "depowering" protocol after accidents involving electric vehicles, to ensure EV battery packs are in a "safe low-power state". This is occuring in the wake of a controversy arising from a Chevy Volt that caught fire, in May 2011, three weeks after an NHTSA crash test. In mid-November NHTSA officials ran a new series of tests on Chevy Volt battery packs, tests which resulted in either fires or sparks in two of the three packs tested.
The NHTSA has awarded the Volt its highest five-star crash safety rating. The NHTSA tests includes a side impact test meant to simulate a collision with something like a telephone pole. The test also "rotates" the car (flips it over), to simulate a car rolling over as happens in some accidents. The tested car which caught fire, did so in a storage lot three weeks later. November's tests of the battery packs involved smashing the packs to create damage similar to the original test, and then rotating the battery pack. One of the packs caught fire a couple days later, and another of them started sparking once the pack was turned upside down. NHTSA is not aware of any roadway crashes that have resulted in battery-related fires in Chevy Volts or other vehicles powered by lithium-ion batteries. Still they are concerned over the fires, because they occurred in tests explicitly designed to replicate real world crash scenarios.
GM says that May's test "was conducted before GM had finalized its battery depowering procedure" and they are asserting that when a significant amount of "energy" remains in a battery pack it poses danger similar to the gasoline in a gas tank. The NHTSA test procedures for gasoline cars include draining gasoline from the gas tank, and GM asserts that in testing electric cars the protocol should include draining electricity from the battery pack.
This argument appears to be curious because batteries are not inherently flammable, while gasoline is an outright explosive it's so flammable. However when batteries are "short circuited" the energy discharge can be immense, can vaporise metal, or otherwise create spectacular effects which could concievably include a fire. Not because of the battery is flammable but because of the electrical discharge. Draining energy from the battery would limit the potential harm it could do.
GM has developed a post-accident protocol which includes "depowering" the battery pack. They have been using this protocol since July, have shared the protocol with the NHTSA and is working with other automakers in a Society of Automotive Engineers committee on the issue of post-crash battery pack safety.
The "depowering" process involves hooking up what GM calls a "load cell" to the battery pack. You can think of this as being similar to a gigantic light-bulb, and it's sole purpose is to discharge electricity from the pack under controlled conditions. In the event of an accident GM will learn of it almost immediately, thanks to the OnStar service in every Volt. As the "depowering" process is developed, stake-holders such as repair shops and towing companies will receive training but in the meantime GM will dispatch engineers to perform to depower any such packs.
The condition they have identified does not pose an initial risk for a crashed Volt. GM's assertion is that "the potential for an electrical fire from this condition should not exist until days after a severe crash". The NHTSA current guidance for responding to electric vehicle accidents remains unchanged, at this time. Occupants should exit safely (if they can) and move away from the vehicle, just as they would with a gasoline powered vehicle. Emergency responders should check for markings that the vehicle is electrically powered, and exercise cautions such as storing the vehicle out-doors and away from other vehicles.
See:
News roundup of the Chevy Volt battery pack fire controversy, as of Nov 28, 2011
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