The battery automatic cut off only works when the batteries are left on
AUTO. When the battery voltage is low, the battery contactors automatically open, which cuts off power from battery bus and the DC essential bus which preserves the battery charge. It is there for a situation where someone forgets to turn off the batteries during an aircraft securing.
However, the hot buses remain powered. So, the logic does not entirely prevent the batteries from discharging. The two voltmeters for the batteries on the overhead panel are actually connected to the hot bus and they keep showing the battery voltage even with the batteries off. And this can slowly discharge the batteries. That is why, it is asked in the FCOM to check if the batteries have a minimum Voltage of 25.5 V, if the aircraft was parked unpowered for more than 6 hours. And if the Voltage is below the limit you need to connect the external and charge them for 20 minutes.
When the aircraft has to be parked for a long period of time, the batteries have to be physically disconnected from the aircraft. According to our engineers, they take the batteries off, if the aircraft has to be parked for more than two days.