EV battery safety - what are the risks?

27 February, 2024 By: Toby Hagon

Although rare, the incidence of battery fires in electric vehicles (EV) still has some people worried, so what are the real risks?

Catch up with the news and you’d be forgiven for thinking electric vehicles are prone to spontaneously combusting.

Social media supercharges the sentiment, with dozens of videos of lithium-ion batteries spectacularly exploding and Teslas being barbecued.

It’s a hot topic as Australians continue to buy more EVs – 87,430 were sold in 2023 - something likely to accelerate following the government’s recent announcement of a vehicle emissions standard.

Many parts of an EV are no different to a car powered by fossil fuels.

The seats, windscreens, steering system and body panels are all identical to what we still call regular cars.

A key difference is that most EVs contain lithium-ion batteries, which because of their relative newness, has resulted in them receiving added scrutiny. They have also been a source of legitimate concerns over the batteries and the materials in them.

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Of course, a liquid-fuelled car with a full fuel tank has a lot of stored energy as well. But over more than a century of building and living with cars powered by liquid fuels, the associated risks have become better managed and also more widely accepted.

Lithium-ion battery risks

Storing large amounts of energy often brings with it a fire risk. Whether it’s petrol, diesel or gas, these fuels all have the potential to catch fire and burn fiercely.

But the large amounts of energy in the lithium-ion batteries used in EVs make the issue bigger in an electric car than it is in our laptops and smartphones.

EVs have hundreds of kilograms of batteries. Those batteries can not only catch fire but are also far more difficult to extinguish than other types of fires.

Emma Sutcliffe is the director of EV FireSafe, a research organisation that is partially funded by the Government and has the support of car makers such as Tesla and emergency services around the country.

She says EV fires typically start following damage or a manufacturing defect.

“The risk of battery-firing increases with … collision, submersion for extended periods in water - and that has to be days - or if it's been recalled by the manufacturer due to fire risk.”

Even then, the instances of fires from the battery of an electric vehicle are extremely low, according to statistics collated by EV FireSafe and research organisations from around the world.

Major battery manufacturers and car makers put batteries through rigorous test procedures that simulate every climate and road scenario around the world, from arctic temperatures and sheeting rain to searing heat and high humidity across bitumen, snow, mud, dust, corrugations and water crossings. They then test in the real world to verify the results.

EVs also have advanced battery management systems designed to monitor the temperature, voltage and performance of each cell, with safeguards while charging and discharging to minimise the chance of failure and faults. They also adjust the temperature of the battery pack for efficiency and safety.

“The battery management systems (in an electric car) are smart,” says Sutcliffe. “The battery cells are incredibly high quality.”

Sutcliffe says in Australia there have been six electric vehicle battery fires since 2010. Considering there are almost 200,000 EVs on the road, it’s a lower number than those news reports may have you think.

“One was an arson attack. One incident was two Teslas in a garage and in both those incidents the garage had burnt down around the cars and taken the cars with them. And then we've had a high-speed collision and … the most recent one where it ran over the tail shaft of a truck.”

The low numbers gel with others from research agencies around the world, including the UK-based not-for-profit Thatcham Research, which stated in a 2023 report: “The risk of fire increases with vehicle age for all fuel types in the UK, but the analysis shows that EV fires are less common than petrol and diesel car fires even when taking vehicle age into account.”

Collating reliable stats on much older EVs may take longer given the relative infancy of the technology.

And EV FireSafe’s numbers don’t include fires involving EVs that didn’t include the battery pack or vehicles not in “normal operation”, such as the damaged battery pack that had been removed from an EV and left in the Sydney weather, subsequently burning and igniting nearby cars.

It’s all in the battery chemistry

Like any battery, lithium-ion batteries have a cathode and anode and electrolyte to carry electrons between them.

It’s that electrolyte (typically a liquid or gel) in a single battery cell that can cause issues when it gets hot (solid state batteries have solid electrolyte that has a much lower fire risk).

According to Sutcliffe the electrolyte can vapourise “and that's highly toxic and highly flammable”.

A damaged battery exposed to the elements can also create its own hydrogen and oxygen, one reason lithium-ion batteries can flare up under water. With thousands of cells in an EV, the problem can soon spread.

“The heat is so much that the cells next door will start to heat up as well,” says Sutcliffe.

“So then the next cell goes and then the next cell goes and so on and so forth, which we call thermal propagation.” It’s often referred to as thermal runaway.

What is thermal runaway?

Thermal runaway is “an unstable chemical process”, explains Sutcliffe and in most instances it occurs as a result of damage.

“You're typically having to do some pretty major damage to that battery for it to go into thermal runaway,” says Sutcliffe, explaining that once it happens heat is the enemy.

“What happens is a battery cell short circuits and starts to heat up. Once it reaches about roughly 160 degrees (Celcius) then basically we're at the point of no return. And what happens then is the battery will actually burst open, so if it's a cylindrical battery, it has a little safety valve at the top and that'll shoot off.”

She says there can be a noise like a gunshot or hissing, some of the early signs that thermal runaway may be occurring. Sutcliffe says thermal runaway can be unpredictable and last for hours or days.

As a former firefighter, she says the processes of extinguishing fires are usually familiar.

“We put the wet stuff on the red stuff, and it goes out,” she says.

But fire departments are having to think differently with electric vehicles.

“We're going to have to change our way of thinking and how we manage these incidents.”

Sutcliffe says one school of thought currently being considered for lithium-ion batteries is to let them burn.

“It goes against the grain for firefighters to not try and put something out, but actually if we are able to let an electric vehicle burn out and just sit there watching it go and protecting any exposures, that's actually a really good outcome.”

Her reasoning is that the cells burn out “and we're left with scrap metal”.

That then removes the risk of reignition, something that’s important in dealing with a damaged or unstable battery pack.

“Secondary ignition doesn't occur very often but it's a risk to our secondary responders; our tow truck drivers, salvage yards, those kinds of things.”

Sutcliffe references an imminent test to be conducted overseas in which identical EV battery packs are burnt. One will be left to burn out and the other will be doused with the view to extinguishing it, in turn dealing with longer term thermal runaway and its ability to reignite.

The idea of the test is to determine which will be easier to deal with and which will cause fewer issues.

“We think it's going to be the one we let burn out because we let it burn hot and fast to get through all those cells. We end up with scrap metal … and we don't get any secondary ignition risk.”

Beware of toxic gases

The gases emitted by a lithium-ion battery fire are more toxic to humans and animals than some other types of fires.

“From a fire-fighting perspective we always would say – it doesn’t matter what it is – we're always in breathing apparatus, we’re always protecting our respiratory system.”

Of course, such protections aren’t available to bystanders and those who happen to be in the wrong place at the wrong time.

“In terms of toxicity of EV versus internal combustion, we know that the early off-gassing from lithium-ion batteries is very toxic,” says Sutcliffe.

She refers to an e-skateboard fire in an apartment overseas that resulted in fatalities. Locally, she says EV FireSafe is aware of tow truck drivers that have been hospitalised due to smoke inhalation.

Fortunately, thermal runaway in EV batteries is uncommon, to the point where Sutcliffe says it’s difficult to instigate in a test environment.

“When you talk to people who are doing testing with cars … the conversation has been how do we get it to go into thermal runaway?”

She says even with heat and mistreatment it’s difficult to induce thermal runaway in an EV.

What precautions are required?

None of which means we should ignore the potential risk with EV battery fires. But this is where common sense again comes into play.

In short, Sutcliffe says if an EV has been damaged – particularly its battery pack – be wary.

“If you've been in a collision don't charge it up … don't come home after a flooding event and try to put your car on charge or drive it.”

But, equally, don’t panic. EVs have high quality batteries and sophisticated management systems that mean they can be left on charge for days.

“The chances of [a fire] happening spontaneously are so low,” says Sutcliffe. “We typically don't see the kind of fireball that people like to make out on social media.”

Think smart

Equally, you don’t have to kit out your home, although as a basic precaution Sutcliffe recommends having interconnected smoke alarms so that if something happens in the garage, you’ll know about it sooner in the house.

But she says not to bother buying a fire blanket or a fire extinguisher.

“Don’t buy a fire extinguisher that says it can extinguish lithium-ion battery fires because they can’t.”

Instead, inspect any damage on your car.

“Just be aware of your car. If you’ve got any concerns about it, put it out on the driveway and call for help.”

Just as importantly, Sutcliffe says people need to be cautious of the batteries in cheap personalised transport.

“I would strongly suggest people do not charge their e-bikes, e-scooters overnight – always supervise that charging - same with your power tools.”