Thermal runaway and propagation
What actually happens, and how packs are designed to contain it.
- Explain thermal runaway and propagation at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real pack
Overview
Thermal runaway is a self-sustaining chain of exothermic reactions inside a cell. Modern pack design accepts that a single cell may fail and focuses on stopping the neighbours joining in.
In depth
- 1Trigger
Internal short, overcharge, crush or extreme heat pushes a cell past roughly 80–120 °C.
- 2SEI breakdown
The protective layer decomposes and exothermic reactions begin.
- 3Separator collapse
Above roughly 130 °C the separator melts and the internal short grows.
- 4Venting
The cell vents hot, flammable gas through its burst disc.
- 5Propagation
Neighbouring cells heat up — barriers, spacing and cooling decide whether they follow.
A battery fire needs very large volumes of water for cooling, or containment until it burns out. Evacuate and call professionals.
Key takeaways
- Runaway is self-sustaining once separators fail.
- Containment focuses on stopping propagation.
- Escape time, not zero risk, is the regulated target.
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