General Motors is offering free loan cars to Chevrolet Volt owners who are worried about their vehicles bursting into flame.The American National Highway Traffic Safety Administration announced on Friday, November 25, 2011, that the US government was investigating fires involving the Volt's lithium-ion batteries.A Volt battery pack being monitored after a crash-test caught fire last Thursday. The agency says another recent crash test saw a battery give off smoke and sparks and there wasa another fire at a test centre in Wisconsin back in June, 2011.GM says its electric cars are safe but it will contact owners of the more than 5000 Volts sold in North America since December of 2010 to reassure them. It will also offer loan cars to make sure Volt owners are satisfied and confident in their purchase. GM has not put a time limit on how long customers can keep the loan cars.VOLT GOOD FOR GM'S IMAGEThe Volt can cover about 60km on battery power before a small petrol generator kicks in to run the car and charge the battery pack and has helped Chevrolet's public image. GM is keen to protect that goodwill. The company has promoted the car extensively as a first step toward independence from imported oil and it has helped to counter GM'S "gas-guzzling" image left over from years of selling mainly light trucks and sport utility vehicles.Mary Barra, GM's senior vice-president for product development, said both fires reported by the NHTSA occurred seven days to three weeks after the crash tests and could have been prevented if the battery charge had been drained as GM has called for in its post-crash procedures.She said only a few Volts had crashed on public roads; none had caught fire and no battery pack had been compromised."We don't think there's an immediate fire risk," said GM North American president Mark Reuss, who was also on the conference call. "This is a post-crash activity."GM has said the NHTSA wasn't aware of the post-crash procedures at the time of the June fire. Barra said that in all the incidents, the battery cell was not involved in the fires, only the electronics within the battery. She would not be more specific until the NHTSA investigation was over.POST-CRASH PROCEDUREIn the Volt's system, Lithium-ion battery cells are assembled into a battery pack, and coolant is pumped between the cells. In the June fire at a test site in Burlington, Wisconsin, coolant leaked from the battery and crystallised and that could have been a factor in the fire, GM said. The fire came three weeks after a side-impact crash test and was severe enough to cause several other vehicles parked nearby to catch fire as well.Reuss said GM won't sell any Volts outside North America until it makes sure emergency responders, scrap yards and dealers have been trained to discharge the batteries after a severe crash.In the US, GM is notified of any severe Volt crash through its OnStar safety system and it sends a team to the car within a day to drain the battery charge to prevent any fire.