As an engineer (and SLF) it has been my experience that true "spontaneous combustion" events with batteries, of any topology, are very rare. Incendiary battery events almost always occur under high current conditions, regardless of the root cause of failure (mechanical damage, manufacturing defect, overload, etc.). Since the APU was running rather than being started when the battery fire occurred, I would suspect an issue with the battery charging circuitry as opposed to a problem with the battery itself. I'm not sure if in-flight APU operation is possible on a 787, but if so, I agree with Flexible that it should be prohibited until a complete investigation/inspection is conducted. Just my 2¢ worth.