Most of the lingering concerns that drivers have about electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) can be traced back to their batteries—in particular, cost and capacity. Now add safety to that list.

That's because Chevrolet's Volt is catching fire, though not in the way General Motors wanted. In three National Highway Traffic Safety Administration (NHTSA) assessments dating back to May, the PHEV Volt's lithium-ion battery packs burst into flames following tests designed to measure the vehicle's ability to protect occupants from injury in a side collision. As a result, the NHTSA has launched a safety-defect investigation of the Volt.

During the initial crash test in May, the Volt sustained damage to its battery and a ruptured coolant line. When the car burst into flames more than three weeks later, the NHTSA fingered the lithium-ion battery pack as the culprit. The agency re-created the May test three times this month to better study what happened. In each of the battery tests, the Volt's battery was impacted and rotated to simulate a side-impact collision into a narrow object such as a tree or a pole followed by a rollover. Two of those tests resulted in battery fires, although none of the fires broke out immediately upon impact at the crash site. In one case, the flames started hours after the test. The other battery caught fire a week after it had been tested.

Lithium-ion battery fires are nothing new, but until now they have been more a problem for makers of cell phones, MP3 players and laptops than car companies. As Scientific American reported in August 2010, the usual cause of lithium-ion battery fires has been "thermal runaway," a chemical reaction that could start from excessive overheating, then potentially cause a cell to catch fire or explode. Researchers at Sandia National Laboratories' Battery Abuse Testing Laboratory concluded at the time that although even extreme driving conditions are unlikely to trigger those problems in automobiles, a crash could, and so could a sudden overcharge—for example, if lightning struck a charging port while a car was being recharged.

The fact that the Volt fires were traced back to lithium-ion batteries is especially troubling for the future of EVs and PHEVs, which use an electric motor for propulsion and batteries for electricity storage. The energy in the batteries provides all main and auxiliary power onboard the vehicle. Continued improvements in charging capacity and lower pricing are widely seen as crucial to mass-market acceptance, and neither will come easily. For instance, EVs and PHEVs need to cut battery costs from $500 to $800 per kilowatt-hour (kWh) to less than $400 per kWh by 2020 to be competitive with gas-powered vehicles, according to a report released earlier this year by the International Energy Agency.

The NHTSA claims it 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, the agency acknowledges concern that its tests, designed to replicate situations that drivers may actually face on the road, resulted in fire. The NHTSA released a statement last week that provides little comfort, indicating that Chevy Volt owners whose vehicles "have not been in a serious crash do not have reason for concern."

In the meantime, the agency advises drivers and passengers of lithium-ion battery-powered vehicles involved in collisions to exit those vehicles as quickly as possible and stay away until emergency responders arrive. The NHTSA advises those responders to first disconnect the battery from the vehicle circuits if possible. Damaged EVs should then be taken to an open area for storage rather than parked inside a garage while awaiting repairs.

Image: GM put its Chevy Volt on display at April's New York International Auto Show. This stripped-down model displayed the gas–electric hybrid's lithium ion battery (1), charger port, electric drive unit and engine generator. Courtesy of Larry Greenemeier