I have had a failure mode in an HS125 whereby the batteries went into series (which they should only do for engine start) thus putting 56v through everything that was expecting 28v. It took out nearly everything that was switched on at the time (including all three inverters, nearly all the avionics, and much else besides.)
I have no idea if such a thing could happen in a 747, but it certainly gave pause for thought on our flight deck.
Compasses and mechanical stopwatches are a good idea.
W