Just to throw my 2cents in.
The APU cannot be started on batteries above 25,000ft. (According to my manuals anyway, this is the LIMITATIONS section, APU)
As was rightly stated, in EMER ELEC Config, the DC BAT BUS is NOT supplied, so you are draining a significant portion of BAT power.
The only question I have is, in flight in EMER ELEC Config, what is connected to the BAT HOT BUS(s)'? Because, after some time you are going to lose the batteries anyway. is it worth trying to get the APU online at the risk of losing the batteries?
What happened during the Air Transat flight that lost both engines? If I recall correctly they lost the batteries while flying and after touch down when the RAT stopped spinning fast enough, they lost all electrical power.
And on another point with no electrical power whatsoever is brake pressure via the accumulator, and nose wheel steering still available? I'm new to this aircraft and as far as I'm concerned it seems that there's a computer running absolutely everything so with no power I feel that nothing will work except for the handles that open the windows, and even then I have my doubts about those as well
