Hi,
TURIN,
JAL APU at BOS was started by AC or from battery?
No info on that sorry.
In normal ops the APU would have been started on taxi-in off the normal AC supply. However, in my experience it is not uncommon for the ground electrical supply to drop off line (after engine and APU have been shutdown) requiring the APU to be started off the battery.
Happens a lot where I work as the airport infrastructure is diabolical in regard to suitable multiple 90KvA supplies.