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FlyingTinCans
11th May 2015, 20:33
On a 737NG in flight with 2 engine generators powering normally, with a single battery & battery charger what are the consequences of a battery charger failure?

The FCOM states that the battery provides power to the Switched Hot Battery Bus & Hot Battery Bus.

From the schematics it looks like it could be possible for the AC system to power these busses but the relays are set up in a way to purposely prevent anything except the Bat & Bat Charger powering them.

So with a single failure of a charger, you will eventually get a 'BAT DISCHARGE', and Boeing only commit to giving us 30 mins of battery power before we lose the ability to put out a fire, run the APU and IRS back ups?

Am I missing something here, if not why has Mr Boeing decided that a BAT DISCHARGE is not worthy of a 'land at nearest suitable' in the QRH?

vapilot2004
11th May 2015, 22:09
As long as you have the AC busses powered, the battery will last quite a bit longer than 30 minutes, in fact hours with a good battery. The 30 minute figure assumes loss of AC and with that, the battery is running a power hungry inverter (clearly with management). :p

With AC available the inverter is not powered and the discharge rate on the battery is fairly low in comparison to emergency electrical configuration. For confirmation, check the amps on the meter in the charger failed condition for the discharge rate. (not possible on NG)

The reason for the lockout arrangement on the TRUs is to be assured of at least some DC power in the event of a bus failure and to satisfy autoland configuration.

Mikehotel152
13th May 2015, 08:47
It's worthwhile reading this discussion from last year:-

http://www.pprune.org/tech-log/520676-b737ng-bat-discharge.html