Possible mechanical damage at assy.
Looking at RR_NDB's link to the Japanese report and the image on page 7, the terminal post on top of battery no. 3 is gone and burned away as is part of the connecting strap. My question is, 1. Could mechanical damage to the battery have been caused at assembly by over torquing the connector strap nut? 2. As the nuts on the battery posts do not appear to have any locking or self locking properties, could it have come loose and thereby caused arcing.