What does Thermal Runaway mean?

Explanation

Thermal runaway is a rare but serious failure mode in lithium-ion batteries. It occurs when a cell overheats beyond a critical point, triggering internal chemical reactions that generate even more heat. This can cascade to neighboring cells, potentially causing a fire. It is the primary safety concern with lithium-ion battery technology.

Modern EVs have extensive safeguards against thermal runaway. The battery management system continuously monitors cell temperatures and voltages, shutting down charging or driving if anything approaches dangerous levels. Battery packs include physical barriers between cell groups, cooling systems, and venting mechanisms designed to contain or slow a thermal event.

EV battery fires are extremely rare compared to internal combustion engine fires. Statistical data consistently shows that EVs catch fire at a much lower rate per vehicle-mile than petrol cars. When thermal runaway does occur, it is almost always the result of severe physical damage (a major crash), a manufacturing defect, or a failure of the cooling system.

For official definitions, visit National Renewable Energy Laboratory.

Where you'll see this

• In vehicle specifications

Common confusion

Media coverage of EV fires creates an outsized perception of risk. EV battery fires are statistically much rarer than petrol car fires. Modern battery design and monitoring systems make thermal runaway extremely unlikely during normal use.

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