Then the professor gives his audience of firefighters their first shock. Full, a cell contains 4.2V of charge, but even when empty, it still holds 2.5V. A Nissan Leaf has from around 192 cells in 24 modules and a Tesla Model S more than 7000 in 16 modules. That’s a lot of energy when the car’s power indicator says it has none.
Full or ‘empty’, the risk of this energy escaping in an uncontrolled fashion is what some scientists believe leads to ‘thermal runaway’, when heat and gases fuel even higher temperatures and still more gases, including hydrogen and oxygen, in a self-fulfilling loop until the cells begin to burn and burst. A toxic vapour cloud develops, bringing with it the risk of deflagration. Once thermal runaway has started, no battery management system or circuit breaker can stop it. “A battery fire can be controlled but it cannot be extinguished,” says Christensen.
He has demonstrated in tests how perforating or otherwise damaging a battery pack, as in a crash, can cause it to catch fire. “If an EV’s battery case is dented, you have to assume it’s dangerous,” he says.
Battery packs have been known to catch fire through overheating and while being charged. More worrying, a battery fire can erupt spontaneously, contamination of even just a single cell during its manufacture being one possible explanation. “Even the most experienced and careful manufacturers have defective electric cells passing through their very careful quality control systems,” says Christensen.
A battery flame is like a blowtorch that will quickly ignite anything in its path, which is why Christensen wants councils and other organisations to consider EV safety risks in underground car parks, as well as bus depots where vehicles are parked side by side. “In Germany, three bus depots have gone up in flames in the past six or so months,” he says. “Tunnels, ferries, car parks, cargo ships transporting EVs – all the places you find electric vehicles should be considered a safety risk and the appropriate steps taken.”
He’s worried about classic cars being converted to run on used lithium ion batteries, too. “Nobody really knows how safe used lithium ion batteries are and no standard test has yet been devised to tell us,” he says. “Some batteries re-enter the market having been removed in illegal chop shops. How safe are they? There’s a lot of research into lithium ion battery safety, but everyone needs to link up, because right now we’re at the bottom of a very steep learning curve.”