Thermal runaway is where the heat a lithium-ion cell generates exceeds the heat it can dissipate, and the reaction accelerates itself. Once it starts it cannot be stopped from outside; the only thing you can do is prevent it spreading to neighbouring cells.
How the chain starts
There are four typical triggers:
- Mechanical damage: puncture or crushing tears the separator and creates an internal short circuit.
- Electrical abuse: overcharge, over-discharge or high current.
- Thermal abuse: an external heat source takes the cell above its critical temperature.
- Manufacturing defect: a metallic particle left inside the cell eventually pierces the separator and causes an internal short.
Once triggered, the decomposition reactions inside the cell follow one another, the temperature rises and the reaction speeds up — positive feedback.
Why conventional extinguishing does not work
The fire triangle cannot be broken here:
- Oxygen does not come from outside. Decomposition of the cathode material produces the cell's own oxygen, so smothering-based systems cannot stop the reaction.
- The fuel is inside the cell. The electrolyte is a flammable organic solvent and cannot be reached from outside.
- The heat feeds itself. The reaction is exothermic.
For that reason gaseous suppression systems (clean agent, CO2) do not extinguish a BESS fire. They may knock down the flame temporarily, but once the agent disperses the cell is still hot and reignites.
The right target is cooling, not extinguishing. Water is the most effective coolant thanks to its heat capacity and latent heat of vaporisation. The aim is to keep the neighbours of the burning cell below the thermal runaway threshold — that is, to break propagation.
Cell-to-cell propagation
When a single cell enters runaway its surface temperature climbs to several hundred degrees. The neighbouring cell receives that heat by conduction and radiation; if it passes its critical threshold, it enters runaway too. Spacing within the module, thermal barriers and cooling design all exist to break that chain.
NFPA 855 accepts the risk and takes a performance-based route: the results of large-scale fire testing (UL 9540A) at module and rack level show the system's real propagation behaviour, and the design follows that data.
Four design headings
| Heading | Purpose |
|---|---|
| Separation and barriers | Spacing between units and racks, fire-rated compartmentation |
| Water cooling | Sprinkler or deluge keeping neighbouring cells below the threshold |
| Gas detection | Early detection of the flammable gas released before flaming |
| Explosion control | Deflagration venting or explosion prevention |
The fourth heading is often skipped and is critical: if the gas mixture released during runaway collects in an enclosed space, an explosive atmosphere forms. Explosion, not fire, is the largest source of structural damage.
Practical consequences on site
- Putting a conventional gaseous system in a BESS room and calling the protection done is wrong.
- The water supply duration must be long; cooling can take hours.
- Cells carry a reignition risk for days after the fire, and the response plan must allow for it.
- Gas detection gives earlier warning than a smoke detector.
Frequently Asked Questions
Why can thermal runaway not be extinguished with a gaseous system?
Cathode decomposition produces the cell's own oxygen and the fuel is inside the cell. A smothering system cannot stop the reaction; once the agent disperses the cell is still hot and reignites.
What does water do in a BESS fire?
Water is used to cool, not to extinguish. The aim is to break propagation by keeping the neighbours of the burning cell below the thermal runaway threshold.
What approach does NFPA 855 take?
A performance-based one. UL 9540A large-scale fire test results show the system's real propagation behaviour, and separation, cooling, detection and explosion control are designed from that data.
Why is explosion control needed in a BESS room?
Flammable gas released during runaway forms an explosive atmosphere if it collects in an enclosed space. The largest source of structural damage is explosion, not fire.
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Download MEP Calc on the App StoreNFPA 13 (2025) · NFPA 15 · NFPA 855 · BS EN 12845:2015+A1:2019 · FM Global DS 5-33 · FM Global DS 4-1N. Definitions are for information only.