Detecting thermal runaway before flame or a full venting event gives a system time to warn occupants, isolate the pack, or trigger mitigation, which is why detection strategy is treated as a distinct design layer, separate from prevention and containment.
Common approaches include off-gas / gas composition sensing (detecting electrolyte vapour or specific gas species released before smoke is visible, see gas chromatography-mass spectrometry and hydrogen gas), rate-of-temperature-rise sensing rather than absolute temperature alone, and internal pressure sensing (see cell gas pressure).
A recurring theme in the literature is passive detection: sensing methods that do not depend on the battery management system remaining powered, since a BMS can itself be compromised during a severe event. This is an active area of development rather than a fully standardised one, and detection strategy is usually paired with a specific response: an early warning to occupants, a contactor-open command, or activation of a mitigation system.
Related: Triggers of Thermal Runaway, Contactor Control, Fire Suppression, Propagation Resistance Testing
See also: Electrolyte Detection