Thermal runaway is the rapid and uncontrolled release of heat from a battery cell. This occurs when heat is generated faster than it can be dissipated.
Overcharging and/or overheating are key factors in generating the conditions necessary for battery failure and thermal runaway. Overcharging creates a chemical reaction that vaporizes electrolyte, whereas overheating vaporizes it directly. Either way, vaporization allows a cocktail of toxic and flammable gases to form.
Subsequently, an exothermic reaction causes the separator to fail. When the separator is compromised, the battery short-circuits. This often provides an internal source of ignition.
Eventually the vapor creates pressure and is vented through pressure relief ports. Vapor release is a sure sign that battery failure and thermal runaway are well underway.
Once a battery fails and thermal runaway occurs, the event can propagate, or spread, to adjacent batteries within a bank or cells within a battery enclosure.