just thinking, when standby power is activated is the only time the main and standby batteries are connected electrically!
so, will activating standby power help in this situation by increasing the capacity, and therefore the voltage, of the power source to start the APU?
I'm not sure of the reason behind it, but when the APU is started, the APU ECU sends a signal to the Standby Power Control Unit which activates a Remote Control Circuit Breaker ("C1212") which stops the two batteries (Main and Aux) operating in parallel.
Ref: AMM D&O 24-31-00
Rgds.
NSEU