re from captain87
As Coffee Black None says,
it's impossible to start the APU on BAT only in flight;
this to preserve the batteries in case of an APU start fault and a RAT stalling at the same time. In this case we will have no generators supplying all the buses. Once the RAT ceases to supply the ESS AC and DC below 100 kts, the battery power takes over until reaching 50 knots ...
Furthermore the APU can be started on batteries only on the ground, since the complete lost of electrical power would be less serious.
The minum battery voltage is 25.5 but the batteries cannot discharge below 22.5 to allow an eventual APU starting.
This has been programmed by Airbus as system logic ...
Kind regards !