Sakkyndigrapport etter inspeksjon av et nikkel metallhydridbatteri som hadde vært årsak til en brann

Norwegian Defence Research Establishment (FFI) has been working on battery safety for many decades. Reidar Skrunes from IF Insurance asked if FFI could assist on a fire investigation where a battery in an Apollo DPV (underwater scooter) was suspected to be the cause of the fire. (DPV = Diver Propulsion Vehicle). The battery (24 V/20 Ah) was composed of 20 DD nickel metal hydride cells. The battery was sent to FFI and inspected in presence of the importer of the DPV, DACON AS and IF insurance. End of charge of nickel metal hydride batteries is commonly determined by an increase in cell temperature. The temperature increase is used to turn off the charger. In this case, the charging was not terminated; either because the temperature sensor did not work or the circuit in the charger measuring the temperature did not work. FFI did not analyze the charger. Normally, overcharge of NiMH only results in destruction of the battery cells from venting of gas, drying out of the electrolyte and internal short. In this case however, one cell vented violently. The cell can was found in the garage door and was empty. According to Energizer which makes similar cells, charged metal hydride burns spontaneously if brought into contact with air.