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On the electric panel of the Airbus A340 there are three battery buttons (BAT 1, BAT 2 and APU BAT, indicated by (3) and (4) in the diagram) controlling battery charge limiters. When the aircraft is on the ground, below 50kts and batteries are the only power sources (as FCOM states) batteries 1 and 2 should be connected to the DC-Battery bus (DC_BAT_BUS) and the DC-Essential bus (DC_ESS_BUS).

A340 overhead electrical panel
source: Airbus A340 FCOM 1.24.20 P1 REV08

Should both of battery buttons 1 and 2 be pressed to Auto mode to have these connections? Or it is possible to connect battery 1 separately from 2 to DC_BAT_BUS? Seems that both buttons should be pressed to have such connection to DC_ESS_BUS, but for DC_BAT_BUS it is not so obvious.

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From A340 FCOM 1.24.20 P2 (REV 8):

BAT 1 (2) pb sw

Controls the operation of the corresponding Battery Charge Limiter

AUTO:

The Battery Charge Limiter controls automatically the connection and the disconnection of the corresponding battery tot the DC BAT BUS by closing and opening of the battery line contactor.

  • The batteries are connected to the DC BAT BUS in the following cases:
    • Battery voltage below 26.5 Volts (battery charge). The charging cycle ends when battery charge current goes below 4 Amperes (for 10 seconds on ground 30 minutes is flight)
    • On the ground (with speed below 50 knots) when batteries only are supplying the aircraft.

My understanding is that for the DC BAT BUS this means when a button is set to AUTO the corresponding battery is connected when that battery needs to be charged or:

  1. The aircraft has a speed below 50 knots AND
  2. The only power source is batteries (i.e. no generators, APU or external power supply are connected to the electrical network)

A note further down mentions that the batteries are disconnected when the voltage drops below 23 Volts for more than 16 seconds, in order to protect the battery from discharging completely.

For the DC ESS BUS it states:

  • The batteries are connected to the DC ESS BUS when batteries only are supplying
    • in flight
    • on the ground (speed below 50 knots) provided they are both selected auto

That clearly states that both push button switches need to be set to AUTO in order for the DC ESS BUS to be powered from the batteries on the ground.

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  • $\begingroup$ (Sorry for mess, have problems with formatting message, help needed here) FCOM 1.24.10 P19 (REV10) states (regarding DC busses): ON GROUND BATTERIES ONLY Provided they are both selected AUTO, the batteries supply: - the DC ESS BUS, - the DC BAT BUS, - the DC LAND RECOVERY So maybe it is like this: when battery is charging it could be connected to DC BAT BUS independently from the other one, but when they are supplying this bus, both push buttons need to set to Auto? $\endgroup$ – Robert Oct 15 '15 at 19:51
  • $\begingroup$ @Robert I think they can power the DC BAT BUS independently as well. Do you know any systems that are powered from the DC BAT BUS? $\endgroup$ – DeltaLima Oct 15 '15 at 21:23
  • $\begingroup$ Maybe you are right, i could consider something like that for my purposes. There seems to be some systems powered from DC BAT BUS, probe heaters, fire and smoke detectors and some engine control stuff as well. $\endgroup$ – Robert Oct 15 '15 at 22:24

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