Preliminary cockpit preparation section requires BAT check for 25.5V with BAT off (The beginning step). Since BAT indicator always show respective voltage, why they must be switched off?
The voltage varies with what is connected to the bus.
- Adding load will, for every electric source, reduce the voltage. And you wouldn't know how much is still OK without also knowing what is powered.
- When the battery is charging (e.g. from ground power), you'll see the charging voltage even if the battery is empty.
With battery off you remove this variability, so they can give you exact number that signifies the battery is sufficiently charged.
(IIRC there are few things on the hot bus that are always connected, but their power consumption is known, so you check the battery under well defined conditions)
This check is done if the aircraft has been parked without ground power for more than 6 hours. The thing is that the LCD voltmeters for the batteries in the overhead panel is connected to the battery HOT BUS and they tend to draw power even when the aircraft is cold and dark (i.e not powered).
If the aircraft was parked like as mentioned, you get in the aircraft and check the battery voltmeters to see if the battery voltage is above 25.5 V. If they are below that, you need to get them charged before commencing the flight. To do so, you turn on the batteries (set them to AUTO) and then connect the aircraft to a Ground power unit (GPU). This will begin a charging cycle and it takes about 20 minutes to charge the batteries according to the Airbus FCOM. During the charging, if you look at the reading on the voltmeters you will simply see the voltage supplied by the GPU and not the batteries. When the batteries are charging, the BCL (Battery charging limiter) connects the two batteries to the DC BAT BUS. This bus is powered by the GPU when the aircraft is powered by it and when the batteries are charging.
When charging the two battery contactors are closed and the batteries are connected to the DC BAT BUS.
Once the 20 minutes is over, you turn off the batteries to make sure that the batteries are isolated from the DC BAT BUS. This will ensure that the reading on the battery voltmeters come from the Battery HOT BUS and gives you the actual voltage of the batteries and not the voltage from any other source. When the batteries are fully charged the BLCs will automatically isolate the batteries from the DC BAT BUS. In flight for instance, the voltage you see in the voltmeters is the actual volatage of the batteries because more often than not they are fully charged when in the air.