Thermal runaway
Thermal runaway is a dangerous self-heating failure process in which heat generation exceeds heat removal, potentially causing venting, smoke, fire, or explosion.
What thermal runaway is
Thermal runaway is a feedback process: rising temperature causes reactions or failures that release more heat, and that extra heat raises the temperature further. The term can apply in several technologies, but it is especially important in lithium-ion batteries because cells store a large amount of energy in a small volume.
How it starts in batteries
A lithium-ion cell can enter thermal runaway after internal failure or external abuse. Triggers can include an internal short circuit, overcharge, repeated overdischarge followed by charging, external short circuit, mechanical damage, manufacturing defects, overheating, or operation outside the cell's intended temperature range.
What happens inside a cell
As temperature rises, protective layers and cell materials can break down. Exothermic reactions can release more heat and gas. Pressure can build until a cell vents, ruptures, ejects material, or ignites vented gases. The exact sequence depends on chemistry, geometry, state of charge, construction, and the trigger.
Propagation
Thermal runaway is most hazardous when it spreads. Heat, flame, hot particles, or vented gases from one cell can push nearby cells beyond safe limits. Battery pack design therefore focuses not only on preventing the first failure, but also on slowing, isolating, detecting, or containing propagation.
Detection and controls
Battery management systems monitor conditions such as voltage, current, and temperature. Pack design may add fuses, current interrupt devices, thermal barriers, cooling, spacing, vent paths, fire-resistant materials, and controls that stop charging or discharging when conditions move outside limits.
Testing and modeling
Researchers use abuse tests, calorimetry, high-speed imaging, radiography, gas analysis, and computer models to study how cells fail. Test results can vary by cell model, size, trigger method, state of charge, and enclosure, so safety decisions usually rely on repeated tests and system-level evaluation.
Beyond small devices
Thermal runaway matters in phones, laptops, e-bikes, electric vehicles, grid batteries, aircraft, spacecraft, warehouses, recycling facilities, and repair shops. The same basic feedback process can appear at different scales, but response tactics and prevention measures differ by product, enclosure, and setting.
Why it matters
Batteries are central to portable electronics, electrified transport, backup power, and renewable energy storage. Understanding thermal runaway helps engineers design safer cells, packs, chargers, buildings, and emergency response plans while still using high-energy batteries where they make sense.