Flight-recorder data from the Japan Airlines Boeing 787 which suffered a battery fire at Boston shows the auxiliary power unit's battery voltage started fluctuating just before the APU shut down.
But the voltage did not exceed its 32V level during the event, says the US National Transportation Safety Board.
NTSB investigators have released 547 pages of detail about the circumstances of the incident - including the recorder data, battery analysis process and the firefighting response - but have not drawn any conclusions over the cause.
The APU had been running for 17min as the arriving aircraft taxied to the gate on 7 January.
About 36s before the APU shut down, the battery voltage dropped by 1V from its designed 32V level. Data then indicated an increase in current flow, to 44-45A, into the battery before returning to 3A out.
Over the space of 15s the battery bus voltage dipped, then climbed, and an electronic indicator message stated that the APU battery had failed. If this battery fails, says the NTSB, the APU will shut down.
Within 30s of the fluctuations starting, the voltage fell to 28V, before flicking between zero and 28V three times as the APU shut down 7s later. The APU battery failure message was no longer indicated.
In its preliminary factual report the NTSB says the flight-data information "did not record any data indicating that the APU battery voltage had exceeded 32V".
Maintenance personnel and the turnaround co-ordinator started discussing the APU shutdown and, some 30s after it had occurred, the APU controller -which is powered by the APU battery - came back online. Two minutes later the co-ordinator reported smoke in the cabin.
Firefighters described a "white glow about the size of a softball" on thermal imaging equipment after entering the aft electronics bay, but none reported seeing flames. The firefighting captain said the battery was "hissing loudly" and that "liquid was flowing down the sides of the battery case".
Source: Air Transport Intelligence news