As long as you have the AC busses powered, the battery will last quite a bit longer than 30 minutes, in fact hours with a good battery. The 30 minute figure assumes loss of AC and with that, the battery is running a power hungry inverter (clearly with management).
With AC available the inverter is not powered and the discharge rate on the battery is fairly low in comparison to emergency electrical configuration. For confirmation, check the amps on the meter in the charger failed condition for the discharge rate. (not possible on NG)
The reason for the lockout arrangement on the TRUs is to be assured of at least some DC power in the event of a bus failure and to satisfy autoland configuration.