From what I could find quickly, there's no overheat or fire warning for batteries; the only EICAS warning I could find was smoke detection for the equipment cooling systems. There are three: forward (including flight deck), aft (where the APU battery is) and "misc" for all other stuff spread over the aircraft.
In normal operation fans pump air through the forward E/E equipment and flight deck equipment and dump it overboard or into the cargo hold (for heating) by means of more fans.
Setting the forward equipment cooling system to "override" mode can be used to help clear smoke from the flight deck; in this mode, suction from an overboard venturi pulls air from the flight deck through the equipment in reverse flow, thus sucking air/smoke from the flight deck as well. I cannot tell from the pictures if in normal operation, flight deck cooling exhaust air goes into the flight deck.
Will be interesting to hear what the actual warning was that they got.
And so far it is not at all clear this one had a battery fire.
EDIT:
ANA said the battery in the forward cargo hold was the same type as the one involved in a fire on another Dreamliner at a US airport last week.
They probably mean the forward E/E bay. I wasn't aware that the main battery located there was also Li-Ion; I thought only the APU battery (in the aft E/E bay) was.