@Romasik:
Well, it's not a battery issue, but definitely electrical issue related to too much electricity on this airplane.
Surely, this was meant as a joke!
The timing of the fire, and location of the visible damage, might be consistent with an unextinguished cigarette having smoldered among other inflammable materials (for example, linens in the crew rest compartment).
When a 787 has all its turbomachinery stopped, and is not hooked up to a ground cart, how "much electricity" does it have?