BAT DISCHARGE 737-800

[framer]

Howdy folks,
I have a sim session coming up and we are going to look at some electrical failures. I am studying the systems and am making some slow progress but the electrical system has always been a weak point for me on this aircraft. I have been contemplating what the BAT DISCHARGE light means in a practical sense and would appreciate any help or advice with it. We have single battery systems.
If the BAT DISCHARGE light illuminates during flight and I go to the QRH it tells me that the discharge rate is too high and that the battery will last for a minimum of 30 minutes. I understand that, but I can't really picture what has happened in the electrical system.
Looking at Vol 2, the only way I see this light illuminating with the normal configuration of Bat switch in ON and Standby Power switch in AUTO, is
A) if both transfer busses are unpowered ( in which case there would be other problems to deal with first) or
B) there is a draw on the battery from the hot battery bus or the switched hot battery bus and the battery charger is not working.
In scenario B you still have both transfer busses working so I imagine that although the battery could discharge, it wouldn't matter because you have all
The electrical power you want from your generators.
So....my conclusion is that if you are in flight and the BAT DISCHARGE light illuminates all on it's lonesome, it is not going to have a detrimental effect on the flight. I would appreciate your opinion on that conclusion. Also, I would like to hear from anyone who has had inflight problems with the system so that I can gain an understanding of what things actually do go wrong on the line.
Bit of a dry subject I know, any input appreciated.
Framer.

[de facto]

So....my conclusion is that if you are in flight and the BAT DISCHARGE light illuminates all on it's lonesome, it is not going to have a detrimental effect on the flight. I would appreciate your opinion on that conclusion

Thats right,and the checklist says to continue on your way..
The APU if needed can be started as long as xfer bus 1 is powered and if not, by using DC from the battery if its voltage is sufficient (i think 20 dc in air and 18 on ground).
Your A and B are correct.

Never had a this problem so far so no first hand experience here.

[JeroenC]

Hi,

Is it not correct that in situation B the 28V DC BAT BUS and 28V DC SW HOT BAT BUS reain unpowered?

Or does the SBY system recognose the failure and switch the SBY NORM relay (K5) and STBY DC ALT relay (K3), powering these busses from TR1?

[Tapshi]

The BAT DISCHARGE light illuminates if the discharge is more than 5 amps for 95 seconds, 10 amps for 25 seconds or 100 amps for 1.2 seconds.
Both the main and the aux battery are monitored.
either the battery charger is to be suspected inop or its bus is not powered.
On ground, ELEC light will also illuminate with this and additional information will be available on the LAD!

[SIRANI]

After the Bat discharges and if the charger is not working which acts as a TR and powers the Hot Bat Bus and Switched Hot Bat bus who will power these busses ones the bat discharges and the Bat charger isn't working???

[RAT 5]

What does the MEL say? The reason I ask is I'm always very wary of the QRH when it says continue, but then before the next departure you find you're grounded. Saying continue makes you think there's no big deal, but I suspect there's something more sinister going on and another flight will not be allowed.

[Swedish Steve]

An AA B757 that had battery discharge, and ran out of battery.
Untitled Page (http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20081007X03940&ntsbno=CHI08IA292&akey=1)

They continued on battery power until the battery died, then wished they hadn't.

[Kefuddle]

In scenario B you still have both transfer busses working so I imagine that although the battery could discharge, it wouldn't matter because you have all
The electrical power you want from your generators.

Not from the generators. With the transfer busses powered, the only source of power for the battery bus, switched hot bus and hot bus is the battery. The combination of these busses would be the draw on the battery cauing the discharge light to illuminate. However, I don't imagine the draw would be very much, probably good for a few hours yet.

[framer]

I can see that now kefuddle, thanks.
Great link Sweedish Steve.