US regulators are investigating the safety of batteries used to power electric vehicles after a Chevrolet Volt caught fire following a routine crash test.The National Highway Traffic Safety Administration (NHTSA) said that it has asked other manufacturers who produce electric cars how they handle lithium-ion batteries. The request also includes recommendations for minimising fire risk.NHTSA said it does not believe the Volt and other electric vehicles are at greater risk for fire than petrol-powered engines.Jim Federico, General Motors chief engineer for electric vehicles, said: "First and foremost, I want to make this very clear - the Volt is a safe car.'THE VOLT IS SAFE'"We are working cooperatively with NHTSA as it completes its investigation. However, NHTSA has stated that based on available data, there's no greater risk of fire with a Volt than a traditional gas-powered car," he said.The Volt is a plug-in electric hybrid that also has a petrol engine. The Volt crash test was conducted in May at a facility in Wisconsin. The fire did not break out until more than three weeks later.GM said it was not aware of any other Volt fires. A senior NHTSA official said the agency has received no consumer complaints about fires involving GM or other electric cars.Both GM and NHTSA conducted follow-up tests and could not repeat the fire. The agency plans additional electric car battery tests with Energy Department experts in the US in coming weeks.A range of new electric vehicles, including the Volt and the upcoming sedan from Tesla, are powered by a similar same kind of lithium-ion battery..NO LEAF FIRESThose batteries deliver the power and range that electric vehicles require, but the current generation of lithium-ion batteries also has a tendency to overheat.Nissan, who produces the Leaf, states there no reports of fires. Nissan spokesman David Reuter said, "The Nissan Leaf battery pack has been designed with multiple safety systems in place to help ensure its safety in the real world."All of our systems have been thoroughly tested to ensure real-world performance. To date, the more than 8000 Nissan Leafs driving on the US roads have performed without reported incident," he said.NHTSA said the Volt side-impact test damaged the battery pack, but the exact reason for the fire is not yet clear.The NHTSA said: "Apparently, there was some cell activity, latent activity, that resulted in the fire. That cell activity we don't know."The Volt carries a 181.4kg lithium-ion battery in a T-shape that sits across the rear and runs down the middle of the car between the passenger seats.American consumers have been slow to embrace electric and rechargeable vehicles, in part because of their additional cost. Pure electric vehicles like the Leaf also have limited range before recharging.The Volt has a petrol-powered 1.4-litre engine to provide additional range after it has run about 64km on a fully-charged battery. GM has sold nearly 5000 Volts.