Thermal runaway is an uncontrolled exothermic chain reaction in lithium-ion cells, in which temperature and pressure rise exponentially. The process typically begins at approximately 130°C for NMC cells and 270°C for LFP cells.
The process follows a characteristic cascade: First, the SEI layer (Solid Electrolyte Interphase) decomposes at approximately 90°C. At 130°C the separator melts, causing an internal short circuit. This is followed by electrolyte decomposition releasing flammable gases (methane, ethylene, hydrogen). Temperatures can rise to over 700°C within seconds.
The most critical scenario in large-scale battery storage is propagation — the spread of thermal runaway from one cell to adjacent cells and modules. Without appropriate countermeasures, a single cell event can lead to fire in the entire BESS container. Evaluating propagation resistance is a central component of every BESS safety assessment.
| Standard | Scope |
|---|---|
| VDE-AR-E 2510-50 | German safety requirements for stationary Li-ion batteries |
| NFPA 855 | US standard for stationary energy storage systems |
| IEC 62619 | International safety requirements for industrial Li-ion batteries |
| UL 9540A | Test method for thermal runaway propagation |
In NMC cells, SEI decomposition begins at approximately 90°C, with the actual thermal runaway starting at around 130°C. LFP cells are thermally more stable — the process only begins at approximately 270°C. This is one of the reasons for the increasing adoption of LFP in the stationary storage market.
Thermal runaway can be minimised through multiple layers of protection: BMS monitoring with cell voltage and temperature monitoring, active cooling, thermal barriers between modules, gas detection, fire suppression systems, and structural fire protection measures. A comprehensive safety concept per VDE-AR-E 2510-50 is mandatory.
Last updated: 2026-06-16