OPINION: An electric vehicle recently burst into flames on the Auckland Harbour Bridge. On the same night, there was also a fire on a bus (presumably an internal combustion vehicle). The probability of a fire in your vehicle, irrespective of the type, is unusual but not new or exceptional. Simple owner precautions can minimise that risk.
There are three types of electric vehicles: battery-electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs) and hybrid electric vehicles (HEVs). All involve electric motors, but the two hybrid varieties include a petrol combustion motor as well. The liquid medium inside a lithium-ion battery is flammable at room temperature and the cathode of the battery is a source of oxygen, so if internal parts of the battery or the terminals are damaged by shock or collision, the possibility of ignition occurs.
US comparative data for the year 2021, per 100,000 sales, indicates that about 1530 petrol internal combustion vehicles caught fire, and this compares with 3475 hybrid cars and only 25 battery-electric vehicles. Clearly, electric cars are much safer in this respect than either petrol-combustion cars or hybrids.
The majority of BEVs and PHEVs use various forms of lithium-ion batteries, while most HEVs use nickel-metal hydride batteries. For the latter, the main challenges are their high cost, high self-discharge and heat generation at high temperatures, and the need to control hydrogen loss. The limited life and safety concerns relating to overheating are all current research and development priorities for manufacturers of lithium-ion batteries.
Lithium-ion batteries also usually incorporate a mix of cobalt, manganese, nickel, graphite and other primary components. A variant involving lithium iron phosphate has been shown to have the lowest temperature rise, and so is probably the safest amongst the numerous lithium-ion battery types.
How safe are EV batteries? The Listener has previously reported concerns about lithium-ion batteries catching fire. Even when lithium-ion batteries are operating normally, they generate a considerable amount of heat, and without significant cooling, they are at risk of catching fire. To avoid this possibility, most EVs have extensive cooling systems that use liquid coolant to withdraw this heat away from the batteries.
Up to 2021, there were no universal regulations to minimise “thermal runaway”, as overheating is called. However, China, which produces many electric and hybrid vehicles, has introduced regulations that involve new standards that relate to the battery, the EV itself and buses. This move to regulate thermal runaways is very likely to encourage other governments to follow suit.
China’s battery regulations emphasise an improvement in battery safety regarding thermal diffusion, external fire, mechanical shock, simulated collision, thermal and humidity cycling, external short circuit, over-charging and overheating. This covers the main causes of thermal runaways for such fires.
Professional firefighters in the US have been reported as saying that handling a burning PHEV or HEV is not exceptionally difficult. This claim needs to be treated with some caution. The main risk appears to be managing the possibility of reignition and firefighters in many countries are now receiving special training to deal with such fires.
There are some simple things owners can do to minimise the possibility of fires in their hybrid or BEV cars:
- Minimise exposure to high ambient heat by parking your car under cover with adequate ventilation.
- Avoid charging your battery to 100% capacity because this can cause overheating. Around 80% is better.
- Take care driving your car over potholes as sudden physical shock may lead to internal damage to the battery, which may cause thermal runaway.
- If your car is involved in a collision, get the internal components of your battery checked for damage which may cause overheating.
- Allow your electric or hybrid vehicle to cool down after a long trip before recharging the battery.
Ralph Cooney is emeritus professor of chemistry at the University of Auckland.
