Another Tesla S toy "car" goes up in smoke.
Why do electric cars catch fire?September 10, 2019
And how does the risk compare with conventional cars? Combustion. It's not just what makes cars go, it also makes them go up – in flames.The general news media is full of tales concerning electric vehicles (EVs) catching fire, but internal-combustion cars are also known to catch fire. Are EVs more prone to burst into flame than conventional cars?
And what causes fire in EVs? Can it be prevented in future designs of EV, taking all the red-alert risk out of these green-lit cars?
There's a growing perception that any item – not just cars – powered by lithium-ion batteries can be susceptible to 'thermal runaway', a state in which an exothermic process accelerates the build-up of heat (in a battery, specifically, in the case of EVs). Without any means of controlling the process, the battery could catch fire.
This has happened to batteries on-board airliners and in smartphones and laptop computers, so there's no reason EVs should be any different.
Nevertheless, modern EVs do come with thermal management systems, of one sort or another, to ensure batteries only operate in a safe, temperate environment. Car companies are also developing systems to limit the flow of electronics if there's any danger of overheating.
Despite this, however, EVs are still catching fire. And in some cases it's due to thermal runaway – but not all cases.
The causes and casesAlthough many presume that battery fires in EVs are directly related to thermal runaway, companies like Tesla have highlighted the other factors that have resulted in vehicle fires specifically involving any of the American brand's products.
These have included crashes, where damaged wiring has led to a short circuit, for instance, flash floods where saltwater has inundated the battery compartment and shorted the circuitry, and in one memorable case someone in the car firing a bullet into the battery pack through the floor.
If there's one major difference between the sort of fire that engulfs an EV and one that takes out an internal-combustion car, it's this: The EV fire doesn't necessarily break out immediately after a catastrophic event occurs. Warning lights in the panel and smoke from underneath the car are strong indicators the driver of an EV should pull over immediately and leave the car. There have been very few casualties associated with EV fires. Usually, fatalities and injuries have resulted from the preceding crash that led to the fire.
To illustrate the sort of lengthy timeframe for fire to gain a foothold in an EV, a Chevrolet Volt crash-tested by America's National Highway Traffic Safety Administration (NHTSA) some years ago burst into flames after nearly a month of sitting in a holding yard because the car's electrical system had not been isolated by the crash-test technicians.
It took three weeks for the damaged battery and electrical system to build up enough heat for the battery's chemicals to ignite.
A list of cases (not an exhaustive list) has been published in the Wikipedia entry for 'Plug-in electric vehicle fire incidents'. For the majority of incidents published the battery was not directly the cause of the fire.
What the US government saysNHTSA has conducted a study into vehicle fires that involved EVs with on-board lithium-ion batteries. In its report, NHTSA noted that car companies have developed 'current limiting devices' to reduce the likelihood of thermal runaway, but the safety authority also raises the concern that lithium-ion battery technology is far from mature.
Work being undertaken by car companies and battery manufacturers to extend the life and performance of Li-ion batteries, while also reducing size and weight, could lead to an upturn in vehicle battery fires.
That said, however, NHTSA is confident that EVs are inherently safer from fire risk than conventional cars, as observed in this paragraph:
Regarding the risk of electrochemical failure, the report concludes that the propensity and severity of fires and explosions from the accidental ignition of flammable electrolytic solvents used in Li-ion battery systems are anticipated to be somewhat comparable to or perhaps slightly less than those for gasoline or diesel vehicular fuels. The overall consequences for Li-ion batteries are expected to be less because of the much smaller amounts of flammable solvent released and burning in a catastrophic failure situation.
NHTSA, which attributes some vehicle fires to manufacturing defects and misuse or abuse, has categorised the various causes of fires in 'plug-in' vehicles (both plug-in hybrids and battery-electric vehicles) as follows:
• Electrical short, overcharging, or overdischarging,
• Exposure to high temperatures or charging at cold temperatures,
• Excessive shock, impact, compression (crush), or penetration,
• Corrosive and aggressive agents contaminating internal components (eg: saltwater)
• Excess cycling, electrochemical component breakdown, fracture and crack growth,
• Cumulative abuse and service causes,
• Errors in design, manufacturing, operation, and maintenance.
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