Every fire needs fuel, oxygen and heat. Remove any one and there is no fire. So the first engineering question about any battery is simple: what inside it can burn?
Two electrolytes, two answers
A lithium-ion cell — any variant, including lithium iron phosphate — carries its electrolyte as lithium salts dissolved in organic carbonate solvents. Those solvents are flammable. They are the fuel that sustains a battery fire once one starts, and in thermal runaway the cell generates its own heat to keep the reaction going.
A lead-acid cell carries dilute sulphuric acid in water. Water does not burn. There is no fuel inside the cell, and no self-sustaining reaction to feed. This is not a design feature that can fail or a management system that can lapse — it is the chemistry, true on the best day and the worst.
Thermal runaway, stated fairly
Thermal runaway is a chain reaction: heat from one failing cell drives its neighbours into the same state. Lithium iron phosphate deserves its reputation as the most stable mainstream lithium chemistry — its runaway onset is materially higher than older chemistries, and that is genuine progress. But higher is not never: abuse testing places LFP runaway onset in the region of 200–270 °C, and the vented gas is hydrogen-rich and flammable. Stability is relative. Absence of the mechanism is a different thing — and flooded lead-acid has no self-propagating runaway mechanism at all. Its energy release is bounded by its own chemistry, and its free electrolyte acts as a heat sink.
What a certificate does — and does not — promise
A recent Indian case study describes a 1 MWh certified storage system — IEC 62619, UL 9540A — in which a single cell failed internally, climbed from 28 °C to over 100 °C in about a minute, and propagated. The suppression system operated exactly as designed. The installation was still a total loss, because aerosol and gaseous agents remove flame, and runaway is driven by heat. The case study’s own conclusion bears repeating: certification does not eliminate the possibility of failure. Certificates are necessary. They are not the same thing as demonstrated non-propagation — which is the test that confined spaces should ask for.
Why confinement changes the arithmetic
On the surface, a battery fire has somewhere to go and firefighting has room to work. In a basement plant room, a container, or an underground roadway, neither is true — and a fire that resists conventional extinguishing agents and can reignite hours later is a different risk category in a space you cannot leave quickly. That is why the safety case for confined installations should start from what can burn, not from what usually doesn’t.
Questions engineers ask us
Is lead-acid completely without fire risk?
No battery is without hazard. Lead-acid evolves hydrogen on charge — a real, named hazard with a written standard (IEC 62485) and a fivefold design margin, managed by ventilation for over a century. The distinction is between a hazard you calculate and ventilate, and a fuel you carry.
Does a BMS prevent thermal runaway?
A BMS reduces the probability and catches many precursors. It cannot arrest an internal cell short that outruns it — the case above had cell-level monitoring. Prevention and mitigation are different words for a reason.
Where does lithium’s fire behaviour matter least?
Open, engineered, attended sites with modern suppression and space to lose — which is where most successful large lithium installations live. The calculus tightens as the space closes in.
Related reading: OPzS flooded cells for critical infrastructure · mining batteries · the companion post on hydrogen and the battery room