The burning question: why do EV battery fires happen?

Saturday, 18 September 2021

You have probably seen a headline along the lines of “EV bursts into flame in garage” and thought to yourself that you might just stay away from EVs because of it.

But it really shouldn’t make you think that, because you might also have seen EV proponents crying out that it rarely makes the news when an ICE vehicle catches on fire, which happens far more often. Which is very true.

But the reason EV battery fires catch our attention is not for the type of car it is, but for the type of fire it is: it is actually very rare, but EV batteries do catch fire. And they make the headlines because, when they do, the result is often something rather nasty.

The current lithium-ion battery fires create intense heat, toxic fumes, the fire-dowsing water is contaminated, and (as if that wasn’t bad enough), battery fires burn longer and they can spontaneously reignite days after being seemingly extinguished.

Due to the severity of the fires, the actual cause of some fires are still a mystery, however the main reasons lithium-ion batteries can catch fire (whether in a vehicle or an electronic device – remember the Samsung Galaxy Note 7?) boil down to either improper manufacturing, damage, or poor design, both physically or in the operating software. But first let’s explain the batteries themselves.

In the automotive world, battery cells come in a variety of shapes, designs (cylindrical, prismatic and pouch-type) and sizes, however, they each generally have three key elements: electrodes, electrolyte and separator. The electrodes store the lithium, the electrolyte carries trillions of charged lithium ions between electrodes and the separator keeps the positive electrode from coming in contact with the negative electrode, all very simple really.

As with virtually all technology, batteries have progressed a lot over time and yet the fundamentals stay the same – you are dealing with high amounts of energy in a concentrated mass. Lithium-ion batteries are arguably at the forefront of modern safety and reliability standards but still, the heat generated by their cells, combined with other ‘component issues’ has caused well-publicised recalls.

In one such recent EV recall, key components within the batteries were damaged (the anode tab was torn and the separator was folded) and these defects, however unusual, were cited as able to increase the risk of fires.

But despite this apparent volatility, battery fires in EVs are actually exceedingly rare: it’s been argued that EVs are only around a tenth as likely to catch fire as an ICE-powered vehicle, and experts agree that pound for pound, electric vehicles catch fire less often than ‘traditional’ vehicles, but the duration, intensity of those fires and the fact that they can catch fire when charging (ie; often at home in a garage) make them headline-worthy occurrences still.

While design or manufacturing faults are largely to blame for most of the recent seemingly spontaneous EV battery fires, there are other causes of combustion.

Heat

Battery-cell pack overheating should really fall under a manufacturer fault category, as not only do these packs need to be fully insulated for optimum functionality and safety (heat control plays a massive part in getting the most out of your EV range and performance) but conversely, an overheated battery pack will either shut the vehicle down or worse, cause it to ignite.

Software issues

On the whole, software issues in EVs tend to cause the vehicle to stop rather than explode, but it’s still an area of concern for automakers. For example, should the system not recognise a fully-charged battery it may keep sending it power, and therefore heat - one manufacturer’s solution was a software update that limited the battery capacity to 90 per cent.

Environmental

It’s been noted that climate conditions may have been responsible for a small number of EV fires, in particular mountains or coastal areas of America. It was concluded that these environments resulted in harsh conditions including large temperature swings, high humidity and elevated levels of dust and particulates leading to failure modes resulting in fires.

Damage

Vehicle/battery cell damage speaks for itself. Should the cell pack be compromised due to a crash or shunt, chemical instability and therefore a fire can be a real possibility. So have it checked out after an accident, also note that should you buy a secondhand EV, pay particular attention to the battery and its housing.

But why are they so bad when they do catch on fire? EV batteries are very high voltage, with 400v and 800v becoming the norm. To the untrained eye, the seemingly unexplained ignition of a dormant vehicle looks more like spontaneous combustion, however the internal process it goes through is called a “thermal runaway”

Unlike petrol (where just one single reaction occurs) in a battery fire multiple steps occur. Essentially, an uncontrolled, cascading loop of violent chemical reactions releases a tremendous amount of energy and heat, as the individual battery cells warms up it drives through the rest of the battery in a kind of domino effect.

What makes this worse is the fact that as the thermal runaway accelerates, the burning battery’s stored energy essentially creates its own fuel. It’s a chemical fire and doesn’t necessarily need oxygen, making it far harder to extinguish than a petrol fire.

The fires emit combustible and harmful gases such as hydrogen fluoride and thanks to its internal thermal runway these fires won’t die down, in fact they will self-perpetuate and reignite, in some cases up to a week later.

All of this might sound like a good reason to avoid EVs altogether, but as mentioned earlier, although the fires are worse, EVs are still far less likely to catch fire than an ICE-powered vehicle anyway. So don’t let the headlines scare you off – your current ICE vehicle is still far more likely to burst into flames.