Hi,
Kiskaloo:
In the case of JA804A (NH), electrolyte that reached the bottom of the EE bay was vented out of the plane (via either the outflow valve or a drain valve - I am not sure which). This is the brown streak visible on the underside of the fuselage. The airflow cooled the material before it could do any real damage to the CFRP of the fuselage. This would also be the case with a controlled vent directly from the battery container.
no-hoper:
All main,apu or emergency batteries (NiCad) on Airbus and ATR aircraft are vented outboard due to gaseous fumes produced by the battery during charging.
A battery "breaths" and venting is normally used. The case of the 787 battery was designed, i suspect, with some assumptions like:
Cell will not heat
Venting is not required
And also there is a VERY STRANGE POINT
(to be confirmed):
Each cell has it´s own valve (hole near plus terminal as posted earlier).
Where is the valve of the battery case?
(*) By valve i mean a hole where breathing of cells can pass through. A pressure gas passage.