The US National Highway Traffic Safety Administration (NHTSA) is eyeing the safety of lithium-ion batteries in plug-in electric vehicles after a General Motors Chevrolet Volt caught fire - at an NHTSA test centre, a media report citing people familiar with the probe said.

Four people familiar with the inquiry told Bloomberg News the regulators have asked automakers, including GM, Nissan Motor and Ford, that sell or have plans to sell vehicles with lithium-ion batteries about the batteries’ fire risk. LG Chem supplies the Volt batteries.

An agency official told the news agency a Volt caught fire while parked at an NHTSA centre in Wisconsin, three weeks after a side-impact crash test on 12 May.

“I want to make this very clear: the Volt is a safe car,” Jim Federico, GM’s chief engineer, said in a statement sent to Bloomberg. “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 [petrol]-powered car.”

LG Chem “is fully aware of the situation and is working closely with GM and NHTSA on the investigation,” the Seoul- based company said in a statement e-mailed to Bloomberg.

NHTSA’s normal practice is to open formal auto safety investigations, which it publicises, following consumer complaints. In this case, it had no such complaints and decided to probe based on its observations from the one fire, which occurred in a Volt it bought for the test, the official said.

In June, GM and NHTSA both crashed a Volt and couldn’t replicate the May fire, Greg Martin, a spokesman for the automaker, told the news agency. GM has safety procedures for handling the Volt and its battery after an accident. Had those been followed, there wouldn’t have been a fire, he said.

“There are safety protocols for conventional cars,” Martin said. “As we develop new technology, we need to ensure that safety protocols match the technology.”

NHTSA and the US energy department this week plan to test Volt battery modules that have been removed from the cars to see if they can replicate the condition that led to the fire, the NHTSA official said. The agencies will study the batteries immediately and continue to observe them in the coming weeks, he said.

Carmakers have engineered electric vehicles using lithium batteries to withstand serious accidents because the element is flammable, Sandy Munro, president of Munro and Associates, an engineering consulting firm in Troy, Michigan, said. Lithium batteries could catch on fire if the battery case and some of the internal cells that store electricity are pierced by steel or another ferrous metal, he added.

“Lithium burns really hot,” Munro told Bloomberg. “But it doesn’t happen often. You have to do something pretty dramatic to make it catch fire.”

If a lithium battery is pierced by steel, a chemical reaction will start raising the temperature and can result in a fire, he said. If the piercing is small, that reaction can take days or weeks to occur.

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