Thermal runaway (lithium-ion)

Overcharge, internal shorts from manufacturing defects or damage, crushing, overheating or charging in the cold can push a cell past the point where its own chemical reactions generate heat faster than it can shed it. Temperature climbs rapidly, the cell vents, and fire or explosion can follow. In a pack, one failing cell can heat its neighbors and cascade (propagation).

Prevention is layered: quality cells from documented suppliers, a BMS with redundant protection, mechanical protection against crush and puncture, thermal design that keeps cells cool, and pack layouts that limit propagation. Safety and transport standards such as UN 38.3, IEC 62133 and IEC 62368-1 exist largely because of this hazard.