GM Electronic Service Information said:
Battery Maintenance Mode is an automatic function that prevents deep discharge of the 12 V battery in specific circumstances. It is enabled when the drive motor battery charger cable is plugged into the hybrid/EV battery charger receptacle and the hybrid/EV battery pack is not being charged. Operation differs depending on whether the power mode is Vehicle Off or Vehicle in Service Mode.
While Vehicle Off, the 12 V battery is monitored at periodic intervals up to a maximum of 30 days. While Vehicle in Service Mode, the 12 V battery is monitored at periodic intervals up to a maximum of 60 hours. If voltage is low, the 12 V battery is charged by the battery charger using external power for up to four hours. This function is only intended to prevent a dead 12 V battery and does not provide a full charge. This is more efficient than continuously trickle charging, thus minimizing the usage of external power.
Under normal circumstances, with a battery that is charged when the vehicle is parked, Battery Maintenance Mode will simply monitor the voltage level, and no charging will be necessary. With Vehicle Off, Battery Maintenance Mode will maintain the 12 V battery for up to 30 days. For longer periods, refer to Owner Manual for instructions.