Aug 3, 2020

For the B738: In case of BATTERY DISCHARGE light illuminates, then loss of battery power, will the HOT BAT BUS be powered? (Fire extingui)

12 answers2 replies

Thank you very much for your good answer. I guess both for the CL and the NG, it is better to land quickly if the battery is showing discharge. We would not like to have a fire that we could not extinguish..

Have a good day. Thank you.


In case of total battery discharge and no ac power source available, the hot battery bus won’t be powered either.


Hi Soren!

If both the MAIN BAT and AUX BAT (in two battery setup) are depleted, HOT BAT BUS is not powered.

The only system which has an extra battery supply is the Fuel Spar Valve which has a dedicated battery to make sure that it always has power to close, and ISFD.

I hope this helps,



I meant, if the AC power is normal and the BAT DISCHARGE light is illuminating, then the HOT BAT BUS will not be powered, correct? in this case there will no fire extinguishing available?


Hi, Soren! I think this case it (Hot Bat Bus) will be powered by Bat Chargers


But if the battery chargers are working, would the BAT DISCHARGE light come in the first place?


The BAT DISCHARGE light indicates that current draw is more than the pre-set threshold. The BAT DISCHARGE would indeed indicate that the battery charger is not charging the battery (nor supplying DC power to the standby system) and once the main battery is depleted the HOT BAT BUS will not be powered.

In this setup (AC power sources available and supplying power), the RCCB will be open, you can force it to close by selecting the Standby Power Switch to BAT. The RCCB will close and you will get DC power supply from the AUX Battery Charger and the AUX Battery (provided that at least one works, otherwise it is not your day).

You can play around with this in the Interactive Section of The 737 Handbook.


If this light illuminates, do I need to divert quickly? - because after the batteries are dead, I would have no fire extinguishing. Or can I continue since another bus may power the HOT BAT BUS?

QRH does not give much guidance here. It is an interesting scenario since it will affect a lot of essential systems, not only fire extinguishing. If the BAT DISCHARGE stays illuminated, I would divert.


Thank you for your answers.

Petr, so this BAT DISCHARGE light only monitors the normal battery? Can the AUX battery still be working good with the BAT DISCHARGE light on?

Could you please tell about this RCCB. Is it a circuit breaker that I need to pull if the BAT DISCHARGE light illuminates - and then select BAT position... Will I then “bypass” the normal battery and switch to the AUX battery?

Thank you for your help.

Problem is that the BAT DISCHARGE can illuminate for either AUX BAT or/and MAIN BAT. You don't know which one it is until you land and ELEC light comes up and the engineers test the system to find out. But since the AUX BAT normally does not provide any power with the RCCB open, it will most probably be the MAIN BAT in this scenario.

RCCB (Remote Control Circuit Breaker) works automatically, it is normally open. When closed, it puts the MAIN BAT and AUX BAT in parallel so that they share the load. It closes when:

  • TR3 Fails and Standby Power switch is in AUTO

  • AC Transfer Bus 1 or DC Bus 1 loses power

  • Standby Power switch is in BAT

Now closing the RCCB by placing the standby power switch in BAT would in theory supply HOT BAT BUS and SW HOT BAT BUS and charge the MAIN BAT back. But I do not recommend doing this as QRH does NOT instruct you to do that. These steps above are considered troubleshooting, on one hand, it could improve your situation, on the other, it could fry the other charger if there is an issue in the elec system. I would work with that 60 minutes QRH states and find a suitable airport and land within that time frame.

With one battery the standby power is provided for 30 minutes, however, here the BAT would only provide power to two buses.

You can play around with the interactive electrical schematic in The 737 Handbook where you will also find more info about the system.



I suppose the B737 CL with only one battery cannot do this trick. Just divert to nearest airport if the BAT DISCHARGE light illuminates, correct?

Aug 20, 2020

Hi all, following this interesting thread about Bat Discharge alert:

with no action from the crew, the RCBB won’t probably close automatically and we´ll probably get only 30min ? But maybe we are taking the problem the wrong way: the large current draw that triggers the Bat Discharge light may be due to-for example- a previous activation of stby pwr, which closes the RCBB and gives the 60min..?

Dec 11, 2020Edited: Dec 11, 2020

In case of bat discharge is high in flight ....QRH says bat life is 30 min or 60 min ( 2 bat) only .... 28 volt DC power is supplied by three TR units, which are energized from the AC transfer busses.. My small brain asks why QRH didnt suggest to LAND AT SUITABLE AIRPORT.... Why some of us are thinking to land ,some trying to continue!!!!