Why do electric cars catch fire?

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 catching fire, but cars with an internal combustion engine are also known to erupt into balls of flames.

We see it on the side of freeways – and the nightly news – all too often.

So, are EVs more prone to burst into flames than conventional cars?

And what causes fire in EVs? Can it be prevented in future designs, taking all the red-alert risk out of these green-lit cars?

There’s a growing perception that any electronic device – not just cars – powered by lithium-ion batteries is susceptible to ‘thermal runaway’, a state in which an exothermic process accelerates the build-up of heat in the battery. And, 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.

Thermal runaway occurs when the battery is exposed to excessive heat or any of the cells are penetrated and damaged, causing an internal short circuit.

The net result is the leakage of electricity sparks a chemical reaction that generates more heat than can be dissipated, exponentially increasing the chemical reaction until the flammable solvents in the battery cells ignite and catch fire.

Unlike petrol-powered cars, which often erupt immediately when fuel is ignited as a result of an accident, the process which causes a battery fire can be much slower and therefore diagnosed earlier to prevent injuries or fatalities.

Nevertheless, most modern EVs are fitted with some form of thermal management to ensure batteries only operate in a safe, temperate environment.

In the most basic scenario, some EVs simply vent the cooled air-conditioned air around the battery pack to maintain a stable temperature while bigger, more powerful units now feature their own liquid cooling circuit.

Car companies are also developing systems to limit the flow of electricity when recharging if there’s any danger of the battery overheating.

Despite this, EVs are still catching fire. Why?

Although many presume that battery fires in EVs are directly related to thermal runaway, companies like Tesla have highlighted other factors that have resulted in vehicle fires among its own fleet of vehicles.

These have included crashes, where damaged wiring has led to a short circuit, for instance, as well as flash floods where salt water has inundated the battery compartment and shorted the circuitry.

And in one memorable case, someone in the car fired 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: a battery fire doesn’t necessarily break out immediately after a catastrophic event occurs.

Warning lights in the dash 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 physical 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.

Among the most recent examples in Australia concerns the Hyundai Kona Electric, which was subject to a national recall after it was found that its lithium-ion battery posed a fire risk. Hyundai said the battery may have internal damage, and that the battery management system (BSM) control software may cause an electrical short circuit after charging.

Owners were still able to drive the car while arranging for battery diagnosis and, if necessary, repair at an authorised dealer, but were asked not to park the vehicle in a garage or other confined space and to only recharge the battery up to 90 per cent of its capacity.

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.

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Any fire is a dangerous event, and a battery blow-up is no different with some uniquely harmful elements.

When a battery catches fire, the flammable electrolytes in the cells can create more than 100 different organic chemicals, including carbon monoxide and hydrogen cyanide – both of which are extremely toxic to humans.

It is therefore highly recommended to move as far away as possible and allow only fire crews wearing professional respiratory equipment to attend to the conflagration.

Battery fires also take a lot of energy to be extinguished. If allowed to burn out, there’s the time it takes, the inconveniences to traffic and the harmful emissions emitted into the area.

Extinguishing a battery fire is obviously quicker but it requires a lot more water than a normal vehicle fire, which necessitates additional resources and also introduces a lot more contaminated water into the drainage system.

Even when it appears the battery fire has been extinguished, it can flare up again, days or even weeks later, which poses another risk if the vehicle has been relocated to a storage facility or junkyard without the battery being isolated properly.

NHTSA conducted a study into vehicle fires that involved EVs with on-board lithium-ion (Li-ion) batteries in 2012.

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 raised the concern that lithium-ion battery technology was far from mature at the time of the investigation.

It acknowledged the work being undertaken by car companies and battery manufacturers to extend the life and performance of lithium-ion batteries, while also reducing size and weight, could lead to an upturn in vehicle battery fires.

That said, NHTSA was 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: salt water)
• Excess cycling, electrochemical component breakdown, fracture and crack growth
• Cumulative abuse and service causes
• Errors in design, manufacturing, operation and maintenance