Thermal runaway in EVs

In an EV, many small battery cells make up a battery module, many battery modules go into making a large battery pack, which in turn get put into an electric car.

A myth about battery packs in EVs is that they’re more likely to catch fire than fuel tanks in ICE cars, and that these fires also can’t be put out easily.

EV Firesafe is an Australian EV fire research company. The EV FireSafe June 2023 presentation (PDF, 34.3 MB) shows that only 387 passenger plug-in EVs globally have had a battery fire between 2010 and June 2023. Whereas in comparison to 2020 and 2021 there were over 2,900 petrol or diesel car fires in NSW.

International research conducted in Sweden reported that EVs are twenty times less likely to catch fire than a petrol or diesel car. In this study, EV fires were reported at 0.004% of the total EV fleet whereas, ICE cars were 0.08% of the total ICE fleet. This showcases that it’s less likely for an EV to catch fire when compared to fires related to ICE cars.

Thermal runaway is where a damaged battery results in a rapid increase of battery cell temperature and pressure, which can in turn catch fire when accompanied by the release of flammable gas. This process known as ‘thermal runaway’ happens in the individual battery cells that make up an EV’s battery pack. 

Batteries in an EV can become unstable when the battery cells have sustained damage. The damage can be caused by being hit by an object, for example when involved in a traffic accident, making the chemicals inside the battery cell volatile.

According to Emma Sutcliffe, the Project Director of EV Firesafe, thermal runaway in EVs can occur when:

• A battery cell is physically damaged.
• The battery cell is of poor quality.
• The battery management system is poor quality.
• The battery’s been submerged in water, particularly salt water.

In general, the thermal runaway process in a battery cell looks like this:

1. A cell in a battery is damaged.
2. The battery cell short circuits.
3. The battery cell starts heating up causing pressure to rise.
4. The internal battery cell temperature rises above 170°C.
5. The battery cell bursts and catches fire.

After the initial cell within the battery bursts from the increase in pressure, other nearby cells start heating up. These cells in turn burst and heat up neighbouring cells, and so the thermal runaway process occurs.

Thermal runaway can be difficult to extinguish once it begins. This is because the process can rapidly move from cell to cell inside the battery module, and inevitably the larger battery pack. If your EV catches fire, don’t try to put the fire out yourself, call emergency services immediately on 000.

As rare as a thermal runaway is in batteries, several methods have been developed to help emergency responders control fires in EVs. According to the EV FireSafe June 2023 presentation, there are three common techniques used by emergency responders are: dousing the battery, letting the battery burn out and submerging the vehicle.

Emergency responders can cool an EV’s battery in thermal runaway simply by constantly pouring water over the car and battery. This is very similar to what emergency responders do when extinguishing a common house fire. This is the recommended method by all manufacturers to control a thermal runaway event.

The second method is to let the EV battery burn until all the chemicals inside the battery have been exhausted. This method can only be used if the EV is in a safe place that won’t bring harm to others as it continues to burn.

As people are more likely to have motor vehicle accidents in populated and high-use areas, this isn’t the method recommended by EV manufacturers. Although with this method, the thermal runway will eventually be brought under control.

The third method is to fully submerge the EV in water. This can be done by setting up flood barriers around the EV and filling the pool with water or by placing the entire car into an already-built container. The EV remains inside the pool until the fire is extinguished. This can take as long as 10 days, if not more in some cases.

The water will cool the battery and helps control the spread of the fire. However, like with letting the fire burn out, submerging an EV to control thermal runaways isn’t recommended by EV manufacturers.

Fortunately, EV fires are quite rare and thermal runaway only makes up a small proportion of total EV fires. With battery technology in EVs only getting better, the methods to control the very unlikely thermal runaway events in EVs are also improving.

These improvements will help make our roads safer with EVs, as there continues to be greater EV adoption.

For customers with battery EVs who are concerned with battery fires and thermal runaway events, Allianz Comprehensive Car Insurance provides cover for thermal runaway so you can enjoy eco-friendly motoring.