@Hetfield,
Securaplane said it makes two battery charging units used on the 787, one for the APU battery in an aft bay, and one for the main ship battery used in a forward bay, which provides backup power for flight critical controls.
Very interesting, would have thought both are identical. Guess these are customizations of the same basic design.
From page 15 of
www.oalj.dol.gov/Decisions/ALJ/AIR/2008/LEON_MICHAEL_v_SECURAPLANE_TECHNOLO_2008AIR00012_%28JUL_15_2 011%29_172333_CADEC_SD.PDF
The root cause study also showed failure to use the signal harness was one of the possible causes of the fire.Various types of signals transmit between the battery and BCU, and they operate almost together. As Leon used the battery in the test, he didn't have the signal harness connected; thus, he couldn't monitor the internal workings of the battery.
Improper to work with the battery "open loop".
Good to look again into the charger history. Also the history of the batterie's installation, was the main battery ever used as an APU battery? Can a bus "charge/reverse flow" the battery without the chargers involvement?
The charger is only part of the story. The installation should have prevented the mess by protecting the battery from any charge.
Other than electrical management failure, thermally induced thermal runaway of many cells without electrical involvement after one cell failed (for any cause) seems less likely but is not impossible.
What makes cells fail? Design/manufacture, storage, overload or deep discharge or overcharge or the bus reverse flowing. The investigation will determine which.
Overcharge of individual cells seems a more likely scenario to me than the cells itself. But it does not have to be the charger, it could also be the bus.