Fire science glossary
Thermal runaway
Definition
Thermal runaway is a self-sustaining, accelerating exothermic reaction inside a lithium-ion battery cell, in which heat generated by internal chemical breakdown causes further breakdown — which generates more heat. Unlike most fires, a battery in thermal runaway provides its own oxidiser (oxygen released from cathode decomposition), making it resistant to conventional suppression methods. The end state is typically venting of flammable gases, fire, and in some cases explosion.
Progression
Six stages of thermal runaway
1. Trigger event
Overcharge, mechanical damage, external heat, or internal short circuit raises cell temperature.
2. Electrolyte decomposition
At ~80–100°C the electrolyte begins breaking down, releasing flammable gases and generating more heat.
3. SEI breakdown
The solid-electrolyte interphase layer decomposes (~130°C), accelerating heat generation.
4. Separator melt / collapse
At ~130–150°C the polymer separator fails, causing a direct internal short circuit.
5. Cathode decomposition
Cathode releases oxygen (~200°C+), feeding combustion internally — fire is now self-sustaining.
6. Venting and fire / explosion
Gases vent (often audibly), ignite, and the cell enters full thermal runaway. Propagation to adjacent cells begins.
Why water does not extinguish a battery fire
A lithium-ion battery in thermal runaway generates its own oxygen from cathode decomposition — cutting off the external air supply has limited effect. Water does not extinguish the reaction either, though very large volumes of water can cool adjacent cells and slow propagation in a pack. The fire services use flooding quantities of water (hundreds of litres) to cool electric vehicle battery packs — a technique not available to occupants. The correct response is always immediate evacuation. Once outside, maintain a safe distance and allow the fire service to manage the fire.
Reignition — the hidden danger
A battery that has undergone thermal runaway may appear extinguished but reignite hours or days later as remaining undamaged cells continue to self-heat. Fire services in the UK are advised to treat e-bike and e-vehicle fires as requiring extended monitoring. Do not return a device to use after any suspected thermal runaway event — even if it appears undamaged. Contact the manufacturer and, in the UK, report the incident to the Office for Product Safety and Standards.
Practical risk reduction
The three most effective risk-reduction actions are: (1) only use the manufacturer-supplied or approved charger; (2) never charge a device on soft furnishings or overnight in sleeping areas; (3) fit a smoke alarm in the room where you charge. For full charging guidance, see our e-bike battery fire risks guide.
Thermal runaway in home battery storage
Residential solar battery systems (lithium-ion home batteries) can also experience thermal runaway, particularly if installed incorrectly, exposed to high ambient temperatures, or using cells approaching end of life. Reputable manufacturers design battery management systems (BMS) that monitor cell voltage and temperature and disconnect the battery before dangerous conditions are reached. However, BMS failure, external damage, or substandard cells can bypass these protections. For solar and home battery fire risks, see our solar panel fire risks guide.
Thermal runaway FAQ
Questions about thermal runaway
The most common triggers are: overcharging (voltage exceeds the cell maximum, causing lithium plating and heat generation); over-discharging (copper dissolution damages the anode); mechanical damage (crush, puncture, or internal short circuit from debris or dendrites); external heat (exposure to temperatures above the cell's rated range); and manufacturing defects (contamination, poor separator integrity). Using a charger not rated for the specific battery type is one of the most common household triggers — see our e-bike battery fire risks guide for practical charging rules.
In most cases, no — once thermal runaway begins in a lithium-ion cell it is self-sustaining. The exothermic reactions release enough heat to continue without any external energy source. In a multi-cell pack, thermal runaway in one cell will almost always propagate to adjacent cells within seconds to minutes, depending on cell chemistry, pack design, and whether thermal management systems are functioning. This is why the fire safety response is always to evacuate immediately rather than attempt to intervene. Cooling with very large volumes of water can slow propagation in some cases, but this is a firefighter technique requiring specialist equipment.
Thermal runaway fires are self-sustaining at the chemical level — the battery provides both fuel and oxidiser (oxygen is released from cathode decomposition). This means the fire cannot be fully suppressed by cutting off external oxygen — CO2 extinguishers and fire blankets that work by smothering are largely ineffective. The fire produces toxic gases (hydrofluoric acid, carbon monoxide) and may reignite hours or days after appearing to be extinguished, as remaining cells continue to vent and self-heat. This is significantly different from ordinary Class A or B fires.
Key warning signs include: the battery becoming hot to the touch during or after charging; swelling or bloating of the battery or device casing; a hissing, crackling, or popping sound from the battery; a chemical smell (sweet or pungent) often described as burning plastic or nail varnish remover; and smoke or vapour venting from the battery vent holes. If any of these occur, stop using the device immediately, place it in a safe location away from combustibles, evacuate the area, and call emergency services. Do not attempt to handle a battery showing these signs.
No. Thermal runaway risk varies by cathode chemistry. LCO (lithium cobalt oxide) — used in consumer electronics — is higher risk. NMC (nickel manganese cobalt) — common in e-vehicles and e-bikes — moderate risk. LFP (lithium iron phosphate) — used in stationary storage and some EV applications — is significantly more stable and resistant to thermal runaway. However, all lithium chemistries carry some risk and all require safe charging and storage practices. The increasing use of LFP in home battery storage systems (solar) reflects this safety advantage. See our solar panel fire risks guide for home battery safety.