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?


2 Answers 2


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)

  • 3
    $\begingroup$ I'm not a pilot and don't know a thing about airplanes, but... in general you should always measure a battery with a load attached. Measuring without a load will give false readings. Care to explain why this is different for airplanes? (except for the charging on the ground part. That makes sense) $\endgroup$
    – Opifex
    Nov 27, 2020 at 9:04
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    $\begingroup$ @Opifex I usually check batteries with and without load. Either reading won't give you the complete picture, both will. For example, in case of a (partial) short, the battery looks damaged under load while it's not the batteries fault. $\endgroup$
    – Mast
    Nov 27, 2020 at 10:20
  • $\begingroup$ @Opifex, measuring without load (well, with little load; even the voltmeter generates some load) gives you different readings than with load, but they are not false—you just have to be aware of the different voltage-charge curve, which is why it requires 25.5 V on what would be called a 24 V battery. On the other hand measuring with load risks measuring the ground power instead by mistake, which would be a problem. $\endgroup$
    – Jan Hudec
    Nov 30, 2020 at 6:30
  • $\begingroup$ @JanHudec I doubt that a user of a device (a pilot using an airplane in this case) would know the voltage-charge curve for the particular battery that happened to be installed in this plane at that particular moment. Especially because the curve would also be dependent on temperature and age of the cell. Measuring without load will give you a significantly higher reading, even if the battery is already broken. For example, a broken car battery can easily measure 12-13V, but once a load is applied it can dip to 6V. $\endgroup$
    – Opifex
    Nov 30, 2020 at 8:37
  • $\begingroup$ @Opifex, the user does not need to know the voltage-charge curve, the manufacturer has to. And the manufacturer does, and provided an exact value to check for. Yes, a broken car battery can easily measure 12V unloaded, but it will still be lower than a good one. By 0.1. Well, the criterion is 0.1 here. 25.5V, good enough, 25.4V, must charge for 20 minutes, then check again, if not 25.5V, take out for maintenance. For a battery with 24V nominal voltage. $\endgroup$
    – Jan Hudec
    Nov 30, 2020 at 23:01

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.

  • $\begingroup$ Thx dude.This is very practical precedure and exactly in SOP. $\endgroup$
    – Leo
    Nov 28, 2020 at 7:34

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