The batteries of electric and hybrid vehicles can potentially self-ignite after an accident, for example, if the protective mechanisms of the battery are impaired by an impact and the battery pack is deformed. This can generate a lot of heat, which ignites one or more battery cells. This can lead to a so-called "thermal runaway": one cell ignites the next. In this case, extinguishing with conventional methods is very difficult, because not only the fire itself has to be extinguished, but also the battery has to be cooled down to a temperature below the critical ignition temperature - and this over a longer period of time. Even if an e-car does not immediately start to burn after an accident, the vehicle must still be parked and observed for a certain time to ensure that chemical reactions in the battery do not ignite the fire.
Furthermore, it is critical to note that as a result of mechanical shock or other anomalies electric vehicles and HV batteries carry the risk of self-ignition even hours or days after the actual event. This is why technical quarantine areas (TQA) were established.