Why do lithium batteries catch fire, and how should you store them?
Because of thermal runaway — a self-sustaining chemical reaction where heat produces more heat, faster than it can escape. It is rare but genuinely difficult to stop once started, which is why the precautions are about prevention rather than firefighting.
The mechanism. Lithium cells contain a flammable liquid electrolyte and store a great deal of energy in a small space. If a cell is damaged, overcharged, overheated or manufactured with a defect, an internal short can develop. That short heats the cell; the heat breaks down the electrolyte and the separator between electrodes; that produces more heat and flammable gas; and above a threshold the process accelerates on its own. Temperatures rise extremely fast, the cell vents, and in a multi-cell pack one failing cell can trigger its neighbours.
Why these fires are different: they produce their own oxygen from the cathode material, so smothering is ineffective; they release toxic gases; and they can reignite hours later after appearing extinguished. Water is used primarily to cool surrounding cells rather than to extinguish.
The common causes, in rough order: physical damage, particularly puncture or crushing; cheap or counterfeit chargers and cells lacking protection circuitry; charging damaged or swollen batteries; heat, including leaving devices in cars or in direct sun; water damage; and manufacturing defects.
What to do:
Buy certified products and avoid unbranded replacement cells and chargers — this is where the great majority of incidents originate, particularly with e-bikes and e-scooters.
Do not charge unattended overnight, and charge away from exit routes and bedrooms — the specific advice fire services give about e-bikes.
Stop using any swollen, hissing or hot battery immediately, and do not puncture it.
Store at around 40–60% charge in a cool dry place for long-term storage; full charge and heat together accelerate ageing and risk.
Never put batteries in household waste or recycling — crushed cells in bin lorries and waste facilities cause a very large number of fires. Use a battery collection point.
Check airline rules; spare cells must travel in the cabin.