When accidentally leaving the battery switch or something powered by the hot battery bus on over night, an aircraft's battery could completely discharge. Normal startup, where the battery is switched on first and then external power is connected, would no longer be possible.

Is it possible to somehow connect an external power unit to the aircraft such that the drained battery is being recharged? Or would maintenance have to swap out the battery or charge it in some other way?

Details for the Boeing 737 are below, but answers for other airliners are welcome:

On the Boeing 737 NG, the batteries are charged via their own transformer rectifier units:

Battery Charger Transformer/Rectifier

The purpose of the battery chargers is to restore and maintain the batteries at full electrical power. The main battery charger is powered through AC ground service bus 2. The auxiliary battery charger is powered through AC ground service bus 1.

Boeing 737 DC Power Distribution

(Boeing 737 NG FCOMv2 6.20.12 - Electrical - System Description - DC Power System)

These ground service buses can be turned on independently of the main aircraft AC buses:

Ground Service

For ground servicing, a ground service switch is on the forward attendant’s panel. The switch provides ground power directly to the AC ground service busses for utility outlets, cabin lighting and the battery charger without powering all airplane electrical busses. The ground service switch is a momentary push button and is overridden when both AC transfer busses are powered.

(Boeing 737 NG FCOMv2 6.20.2 - Electric - System Description - Electrical Power Generation)

This sounds like the battery chargers could be powered via ground power in ground service mode. It is however not clear to me, if this push button would even work without any battery charge left.

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    $\begingroup$ Wet cell NiCads that most airliners use can suffer from "thermal runaway" when charging, for a range of reasons like overcharging, bad connections, low electrolyte, excessive ambient temperature, cell polarity reversal, etc.A high charge rate and an ambient temperature over 100F during charging can trigger a thermal runaway. If a battery is completely depleted, it needs to be carefully monitored during recharging to control temperature and cell condition. Much safer to remove it and take it to the special battery room to be recharged, so if it blows up it just makes a mess in the battery room. $\endgroup$
    – John K
    Commented Aug 27, 2020 at 12:14
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    $\begingroup$ Surprised this isn't from "Sean" :) $\endgroup$ Commented Aug 28, 2020 at 4:06
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    $\begingroup$ @Harper-ReinstateMonica Sorry, no footnotes :D $\endgroup$
    – Bianfable
    Commented Aug 28, 2020 at 7:12
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    $\begingroup$ I now have this image stuck in my head of two 737's, parked nose to nose, with two huge jumper cables between them :) $\endgroup$
    – MSalters
    Commented Aug 28, 2020 at 12:41
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    $\begingroup$ @Mast No, the APU can for sure no longer be started with a drained battery (see the "TO APU STARTER" in the diagram above, or Ralph's story). I am asking about a GPU (Ground Power Unit), also called external power. $\endgroup$
    – Bianfable
    Commented Aug 29, 2020 at 19:06

3 Answers 3


My one experience with this was in a 737-300 that had the battery charger die, and the battery depleted while in flight. The Battery Bus was still powered by TR3, but the Hot Battery Bus died (interestingly, not a scenario I'd ever seen in the simulator). We got to the gate & couldn't start the APU and couldn't connect ground power, so when we shut down both engines, we immediately had "one blue light" -- i.e. ground power is plugged in & available, but NOTHING else -- dark airplane.

I don't recall how longer after that it was, but we eventually were able to connect ground power, probably to the ground service bus first. Then, with the ground service bus powered, the battery would recharge. I don't recall if we were able to get anything beyond the ground service bus powered after that. (Presumably, if the battery recharged enough, we could have gone back to normal routines.)

As a practical matter, though, our maintenance does exactly as John K mentioned: if the battery has been depleted, it gets replaced. How much of that is a matter of what happens during charging, and how much of that is concern about damage to the battery itself, I don't know, but in practice they won't use the aircraft's battery charger in the manner suggested in the OP. (The battery charger brings it back up to full charge after things like starting the APU or running on battery power for the time between initial power-up and when APU or ground power are connected -- but that's a long way from being fully depleted.)

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    $\begingroup$ I worked on NICAD aircraft batteries in the USAF. Batteries are compose of multiple cells. as the battery discharges and gets near 0 volts, one or more of the cells may actually reverse its potential. One it has reversed it cannot be easily recharged. We would apply a load to drain the cells monitoring each and when it reached near zero volts, it get shorted out. Once all cells are shorted out, and have cooled down, the shorts were removed and the battery charged, carefully monitoring the voltage of each cell. Some time a depleted cell would not recharge and it was replace. $\endgroup$
    – Jim
    Commented Aug 25, 2022 at 20:44

You ask "would it be possible".
The answer is, with certainty, yes.
"Is it done?" is another matter.

John DeVries notes that " ... the external power contactor requires battery power to close. Dead battery, no ground power."
I note below that this is by conscious design choice - ie the choice that has been made is that the aircraft cannot be powered up from an external source if the battery is 'flat'. Technically it's possible.

I can answer this from a "how it is almost certainly designed to be" basis.
Note that this applies to the situation that you described - with the aircraft on the ground, with the engines (presumably) not operating, and the battery not being charged from external sources. As Harper ... notes, if this situation occurred in flight the decision may be to risk damaging or destroying the battery if this increased the prospect of not destroying the aircraft.

The block labelled "BAT" on your diagram is not just a battery but a battery subsystem with its own protection and management capabilities.

I'm an electronic designer.
I would expect with near certainty that Boeing (and any airline) would do what any competent designer would do, and

  • Be utterly certain to use a design that would automatically disconnect the battery when a minimum safe voltage was reached so that the battery would not be in any way damaged by 'running it flat'. Such a system would be wholly unable to be bypassed by any manual or intended automated action. Only a fault condition within the battery subsystem (labelled "BAT" on your diagram) would allow otherwise. Such a fault is of such seriousness that it would be protected against separately and any condition which sought to cause it would be both prevented and reported on.

  • Arrange the battery to be in a condition such that if it was ever able to be charged in place by external power then it would be able to be when flat.

  • Make design decisions that suited their operating procedures.

  • Recharge from flat would definitely be technically possible, but it may not allow recharge from flat because any event that allowed this state to be reached indicates a systems failure (whether due to human factors or other) that would make it deemed-unsafe to trust the battery state (even though the battery would almost certainly be undamaged).

Powering up in a battery-less state from an APU or other source is "just a matter of design. It can be done if desired. Whether it is able to be done depends on high level system design considerations, and not technical ones.

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    $\begingroup$ I wouldn't necessarily presume Boeing would protect the battery at the expense of aircraft function. A similar case is a diesel emergency fire pump; they don't have low-oil or low-water trips because the engine tripping out in the middle of a fire. The engine is expendable if it gets the fire out. $\endgroup$ Commented Aug 28, 2020 at 4:11
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    $\begingroup$ @Harper-ReinstateMonica I agree with your point - but/and note that it lies outside the scope of what the OP intended. Low battery in flight due to loss of in flight charging would place it in your scenario. Hopefully there would be a detected and staged reduction in battery use if such a scenario eventuated. $\endgroup$ Commented Aug 28, 2020 at 6:38
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    $\begingroup$ You probably assume there is a micro controller present. But 737 first flew in 1967. In some old aircraft this battery manager may not be so capable of of. $\endgroup$
    – h22
    Commented Aug 28, 2020 at 7:34
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    $\begingroup$ @h22 Good point, but the same rules apply when it comes to battery design in mission critical applications (and anything serious). I'd been 'playing' with electronics for about 5 years by then :-). 2years & 3 months and a few weeks before the 1st moon landing. I imagine that the battery system may have been somewhat upgraded in the intervening 53 [!!!] years. || FWIW - congratulations on making a good point politely rather than making the recipient feel put upon :-) - often a bit of a lost art on the 'net these days. $\endgroup$ Commented Aug 28, 2020 at 9:27

It is correct that the battery charger will work on the ground service bus, but the external power contactor requires battery power to close. Dead battery, no ground power

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    $\begingroup$ So there is no way to close this contactor manually? $\endgroup$
    – Manu H
    Commented Aug 28, 2020 at 7:09
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    $\begingroup$ Nice to know. A design choice. $\endgroup$ Commented Aug 28, 2020 at 9:28

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