poorjohn:
Per the NTSB report, after the APU starts, the APU battery must continue to supply power to the APU control electronics. When the JAL battery failed, the APU shut down.
And when the APU shut down, the ventilation to remove the smoke from the burning battery also shut down. In flight this might not have been as serious if the ventilation was also powered by the other generators and pressure difference to the outside. It does argue strongly that Boeing's proposed sealed case and pressure vent is a step in the right direction.