Checked the FCOM. Seems like all fuel pumps (even the APU startup pump that runs through inverter when aircraft is on battery power) should be running from AC busses, only the contactors are meant to be powered from the DC bus. Seems like each pump consumes couple of amps from the TRUs, which is way too much for just a contactor (that should consume less than a single ampere).
Another issue is with batteries. Even if I switch some batteries OFF they are still getting charged or discharged.
Steps to reproduce the issue (to verify the battery disconnection bug):
-Start aircraft in cold and dark state
-Turn on only battery 2 and 3, do not turn on battery 1, also remember to do not turn on battery override switch
-Switch the DC ammeter and voltmeter selector switch to battery 1 (to monitor battery 1 voltage and current)
-Try starting the APU
-When starter motor runs notice that battery 1 ammeter shows discharge current, even when battery 1 is isolated from the bus (OFF light on the button).