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04.2 What is thermal runaway?

All EV traction battery fires start with thermal runaway

Thermal runaway occurs when a battery cell short circuits & starts to heat up uncontrollably.

Lithium ion batteries contain a large amount of energy in a very small space. Under normal operation, they rapidly convert chemical energy to electrical energy.

Here, we take a look at two examples, firstly a lab test showing thermal imaging of battery cells going into thermal runaway, then an e-bus fire in China showing thermal runaway & a vapour cloud explosion

We'll then look at how thermal runaway happens & progresses.

Watch lithium ion batteries in thermal runaway

Big thanks for this video to: George Brilmyer

This video shows 5 x cylindrical lithium ion battery cells, forced into thermal runaway in test conditions. Please note:

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Gases venting from battery cells

Gases ignite explosively as thermal runaway occurs

Second cell ignites (others follow)

How does thermal runaway happen & progress?


Multiple battery cells create a module

As we've previously seen, an EV traction battery is constructed using individual cells to create a module, with multiple modules creating a pack. You can review this on the What is a traction battery? page.

Cylindrical lithium ion battery cell under normal operation

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When a battery short circuits, the cell heats up

If a cell is abused, e.g. by heating, crushing, penetration or overcharge, chemical reactions replace the normal electrochemical reactions: the former generate heat and toxic & flammable gases. The heat speeds up these exothermic reactions producing more heat and gases. 

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Cell affected by short circuit

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Heating starts to affect other cells

Cell heating will continue until the rise in temperature exceeds the heat that can be dissipated to the cell's surroundings. This released heat will start to affect other nearby battery cells.

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In some cases, cell ignition will occur

As oxygen mixes with the vapour cloud & heat continues to build, the battery cell may ignite, causing surrounding cells to do the same.

Ignition will occur anywhere between seconds & minutes of the white vapour cloud showing

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Vapour cloud explosion is a risk

In the right circumstances, the white vapour cloud of toxic flammable gases will deflagrate (explode) without warning.

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If it occurs, vapour cloud explosion is a high risk to emergency responders


The cell goes into thermal runaway

When the generation of heat becomes self-sustaining - the heat releases energy, & the energy in turn releases more heat - the cell is experiencing thermal runaway. When thermal runaway occurs, the cell is undergoing an unstable chemical reaction that is hard to bring under control.

At some point, the separator structure collapses and the electrodes touch, causing an internal short circuit and masses of heat, catapulting the cell to ever higher temperature.  


Eventually, the gases are vented, either via blast caps on cylindrical and prismatic cells or when pouch cells burst. Initially, heavy metal dust particles from the cathode will present as a dark cloud, which is followed by a white vapour cloud as the gases take with them fine droplets of the solvent.

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Heavy metal particles present as a dark cloud, followed by a white vapour cloud of toxic flammable gases

Suppression challenge; burning battery cells contained within a battery module & casing

Previously, we looked at how lithium ion cells are contained with a battery module, & multiple modules create a battery pack.​


External battery casing

Battery modules containing individual battery cells

As the individual lithium ion cells experiencing thermal runaway are contained with several layers of metal (usually aluminium) casings, it can be almost impossible to direct water directly onto the seat of the fire.​

Later, we look at the challenges of extinguishing battery cells.

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