Maybe you could improve the replies' quality, if you give a little more info how you ended up in these very hypothetical scenario...
Technically speaking:
What you describe as being 'only on battery power' is called LOSS OF ALL GENERATORS on Ground. I assume the A/C is stopped (i.e. speed < 50 Knots) otherwise you woulnd't be doing fire procedures.
In this case only the following busses are powered: HOT BUS 1/2, DC BAT BUS, DC ESS BUS.
FWC1 ist powered by the AC ESS BUS. Unfortunately the AC ESS BUS is not powered in the above scenario, so without any FWC working, I doubt there will be any fire indication in the Fire P/Bs. If I remeber correctly from the Box, the Eng Fire Lights in the Centre Pedestal would be working.
An Engine Fire with all Generators is a bit remote, but why are you asking about this scenario with batteries switched off?
@ spannersatcx: Any mention on the power source for the fire P/Bs in the FCOM? Couldn't find one...
Regards, MAX