Both my thermal runaways were caused (post investigation) by a breakdown in the cell structure within the battery causing a massive discharge, this was exacerbated by the high OAT at the time.
The first went off like a hand grenade taking some of the lower cockpit area with it! The second contained itself within the confines of the battery compartment.
Interestingly, the helo I fly at the mo' (EC135) has its battery sitting right next to the output shaft from the MGB to the tail rotor (No1 thomas coupling!!!). And there is no battery containment department.
