OK... but ever heard of a 737 or other type losing all generators, AC, DC, standby systems and battery? As far as I know we do not have a NNC for total electrical failure on the 737.
Thats not quite what I was getting at.
If both "DC FAIL" lights illuminate on the IRS panel it means the battery is dead.
Yes the IRS will still work with two generators working but say you then had a lightning strike and lost both generators (a bad day) it would turn into a very bad day as you'd have no battery to fall back on!
But my question was about losing the battery and the implications for the fire extinguishers as they are fed from the Hot Battery Bus. Would the HBB still be powered or not? The Boeing Vol 2 is not clear about this.