The battery that caught fire on the Japan Airlines 787 in Boston was the second main battery. This unit’s primary purpose is to electrically start the APU when neither of the engines is running and the aircraft is not connected to external ground power. In this case, the battery energizes the right hand of the two starter/generators connected to the APU. The aft battery also provides another minor role, namely power to the navigation lights during battery-only towing operations. The unit in the second incident, which forced an ANA 787 to make an emergency landing in Japan on Jan. 16, involved the main battery in the forward E/E bay. In this case, there was less damage, though spilled electrolytes, fumes and minor thermal damage indicated signs of overheating.
(Aviation Week)
So the Main battery as well as the APU battery catched fire..., hhmmm.
Here a small example what to expect....