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CaptainSandL
14th Nov 2004, 18:15
I was discussing the flat/failed battery in-flight with someone recently that seemed to raise more questions than we could answer. These were our conclusions:

Identification:
A flat battery will probably be noticed by an IRS DC FAIL light on the classics, since there is no failed battery light. There is a BAT DISCHARGE on the NG’s but the QRH does not give much help other than to say that you have 60 mins of standby power, which we knew anyway. Clocks will stop.

Consequences:
The hot battery bus would become unpowered so you would lose all fire protection, hence the APU if running would auto shutdown!

The QRH does not have a failed battery checklist (although BAT DISCHARGE is close). Surely if the above consequences are correct it should be in the QRH, perhaps with a recommendation to land at the nearest suitable airport.

Other things to throw into the melting pot include the clocks, they would also lose power so would you lose time data in the FMC? Will this be the case for both classics & NG’s since the NG FMC gets its time data from the GPS? What will losing time data do to VNAV & LNAV?

Why doesn’t the battery charger (NG) hold up the voltage to all the various battery busses when the battery has failed and if the Main Battery fails will the Aux Battery continue to power the batt busses as normal?

Any answers or experiences would be most enlightening.

S & L

Flight Detent
15th Nov 2004, 03:19
Hi Captain,

My understanding is a little different from yours.

Firstly, the main battery charger reverts to a "TR" mode following completion of the primary charge cycle, (reverts to a constant voltage mode), and powers all the loads connected to the battery and hot battery busses.
So all loads would be supported at this stage.

As I understand it, the BAT DISCHARGE light only comes on when there is a high discharge from the primary battery, (like starting the APU), so this one won't come on if the battery internally fails.

The standby power only comes on automatically if either AC transfer bus 1 or the source of power to DC bus 1 is lost. AC bus 1 is normally powered by IDG 1, and subsequently powers, via TRU 1, DC bus 1. So the standby bus will not automatically initiate.

By the way, the ONLY time the auxiliary battery is connected to the aircraft system is when standby power is in use, it is not used for any other purpose.

All those things, clocks etc, I would think would all remain powered.

The main problem, as I see it, would be the reduction of standby power capability from the normal 60 minutes, to something around half that, depending on what effect the failed battery would have on the auxiliary battery, since it would be still connected in the system (assuming the BAT switch is ON).

Thats about all I can think of off the top of the head, please if anyone can 'shoot me down' (bad choice of words) regarding these thoughts, please go ahead, I'm sure I too will learn something here!

Cheers

:eek:

Loose rivets
15th Nov 2004, 05:40
In a generic aircraft system, it would be difficult to have a ‘flat' battery. I am assuming that by flat, you mean that if it were removed from the system and tested there would be little or no potential difference (voltage ) across its terminals.

In the first place, thankfully this is fairly rare and would probably indicate a mechanical breakage or fusing of its internal conductors. Without sophisticated detection, such an occurrence would probably go unnoticed in flight. The detection would have to know the difference between a typical trickle-charge and, for instance, a sudden zero charge as the battery fails by going open circuit. A thermal runaway or a short to a hot bus bringing the voltage to zero, would be horribly apparent.

When the system, (see TR mode above ) ceases to cause a stabilized voltage to which the battery would be held, and a demand is made on the faulty battery, the fault would become apparent. In the real world the first you would probably know about a sick battery would be when you most needed it.

If circumstances allowed, I would make a point of checking the batteries twice. Once with no load, and then again with a few items to load it. It wasn't standard procedure then, but I got used to seeing a certain voltage for each condition and it told me a lot about the battery condition.

LEM
15th Nov 2004, 11:41
The hot battery bus would become unpowered so you would lose all fire protection
We had the same concern in the same discussion some time ago.

Probably Flight Detent is right, and this is reassuring! :ok: