Wires trapped under the ELT's battery compartment cover-plate seem to have chafed and caused a short-circuit which caused thermal runaway failure of a cell in the Li-MnO2 battery. There were cell-level and battery-level safety features but they didn't prevent this single-cell failure, which propagated to adjacent cells, resulting in a cascading thermal runaway, rupture of the cells and consequent release of smoke, fire and flammable electrolyte. And off it went.
Plus, as usual, other factors: design of the ELT, its location in the aircraft, safety assessment for the ELT certification didn't identify battery failure modes, etc.
I think we know quite a lot more about lithium batteries generally now, and every competent manufacturer and operator is being a lot more careful.
Last edited by OldLurker; 19th Aug 2015 at 10:19.
Reason: small typo