The charge rate indicates that for whatever reason (probably the routine APU startup 15 minutes earlier) the battery was still substantially discharged at the time of the event. It was also being rapidly recharged after the substantial load of the APU start.
And...
Quote from Lyman:
"Didn't APU start off one of the donks?"
Yes and (maybe) no. I'm not sure our experts in PPRuNe know enough about the system to establish what proportion of the APU starter-generator load is taken by the main electrical system (when available, as in this case), and how much (if any) is taken by the APU battery. On my last a/c type, the battery is used to supplement the main-system TRs in supplying the 700-1000 amps initial load. However, that system architecture may not be necessary or applicable on the B787.
If the APU battery took no significant part in the APU start, it seems an odd coincidence that the failure should have taken place so soon after APU start, at the end of a 12-hour flight.
My understanding of the APU start system is that one Starter/Gen is powered from the main electrical busses and one from the APU battery bus.
It is an either/or scenario. Not both.
If main busses are powered then that's what is used, if not, it's over to the battery.
This concept of the APUC being powered by the APU battery however, regardless of main bus power does seem odd.
I'm not sure if this is the case on other a/c types. Research required methinks.