“As someone who assisted Nissan during the creation of its battery plant, I would, if I could afford one, have a Nissan Leaf tomorrow,” he says. “We don’t need to be worried about the small incidence of fires involving electric vehicles but we do need to be aware. A lithium ion battery stores a huge amount of energy in a very small space. Since 2008, the adoption of such batteries has outstripped our appreciation of their risks. We’re running to catch up but we will do.”
As part of his campaign to improve EV fire risk awareness among first responders, Christensen has, so far, presented to 30 of the UK’s 50 fire services, as well as to fire services in Europe, Australia and New Zealand. He begins each talk by describing the structure of a lithium ion battery cell. A sliver of aluminium, called the cathode, is coated with a mixed-metal oxide ink. It’s partnered by a slice of copper coated with graphite called the anode. In between them is a fragile, perforated plastic separator soaked in an organic solvent that contains a small quantity of additives whose identity is, troublingly, known only to the cell manufacturer. Depending on whether the battery is being charged or discharged, the lithium ions move either from, or to, the cathode and anode.
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 over 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.