I'm a tad confused, why are Chris Yates ("aviation expert") & Mary Schiavo (ex NTSB) constantly referring to the batteries when they aren't located where the fire was? Yes, I know fire travels, but that far? Is it just media speculation/misinformation?
Well, probably because the initial cause of the fire could well be a battery problem even if the batteries themself are unarmed. For example, if they produced to much power it's possible that the wiring became overheated and at a specific point (a weak point where there's for example flamable material arround the wiring) caused an actual fire. Before such a fire happens it can easily take a while even hours. While there may not be any damage to the batteries they may be the root cause.
So in the end, they cause of the fire could well be the batteries. Which I don't say is the actual cause, this is something the investigation will tell us eventually but because the fire happened somewhere (far) away from the batteries it's not something you can easily rule out.
At this point an electrical cause seems to be the most likely since it can take a while before it becomes a real fire (there needs to be enough heat transfer without an open flame). If an open flame was the cause, like for example a cigarette (yes, even though smoking is prohibited you can never rule this out, maybe someone from the cleaning crew had a smoke for example), it think we would have seen something completely different.
Sad for Boeing that they have such problems with the 787...