757 Electical technical question
 

Hello all.

Your flying a 757 ER plane. The EICAS illuminates a MAIN BAT CHARGER message.

Nothing listed in the QRH.

You fly another hour or so land and alert maintenance. They come out change the charger and the battery. You proceed on your way.


From our systems book it states that the main battery charger charges the main battery and then the HOT BAT BUS.

The HOT BAT BUS has the following items on it.

• Wing spar fuel shutoff valves

Its obvious that none of the items on the HOT BAT BUS are being used continuously until you operate a system, except for the clocks.

Can anyone tell me if you were on your way across the atlantic, would you want to divert?

How long until the main battery completely fails.
Would it fail?
If the battery fails you lose the HOT BAT BUS also.

What are the implications of this such as the American Airlines Flight dealt with going into Chicago a few years ago. (They "inadvertently" discharged their battery with a rather unpleasant outcome).

Any information on this would be helpful

If the "QRH says nothing", then clearly it doesn't say to divert. Also, was the message "MAIN BATT CHARGER" or "MAIN BATT DISCH"?

In theory, inflight you should not lose the Battery if the charger fails. As far as I can tell from our manuals, the HOT BAT BUS can be powered by the Battery, the Battery Charger from the R MAIN AC BUS (normal) or via the BAT SWITCH from the L DC BUS if the charger is not doing it´s thing. So you should not lose the HOT BAT BUS.

The Boeing QRH often does not tell you when you need to divert, f.ex. the STANDBY BUS OFF checklist ends up telling you you have got 30-90 minutes of standby bus power, but it does not tell you to divert (or land asap).

I have had an issue with "MAIN BAT CHARGER" STATUS msg on 757s at least twice. In both cases I can recall it was on the ground pre-departure.
In the first case, Mech´s tried swapping APU bat charger (identical) but no change. Turned out MAIN BATTERY was gone bad.
Next time, instead of waiting for Maint to do any swapping I had them look at the battery, same thing. Replace battery. So the status msg may indicate that the charger is unable to charge the battery.

- have you got a reference for that? My understanding is that the HBB is always powered through the battery alone EXCEPT when the Hyd generator is operating.

Hi BOAC,
That would seem to be a very odd design. Why can't all the DC bus bars be fed by the main TRUs?
If the Engine & APU generators fail, then the main TRUs can't work, and then you'll depend on the batteries until the RAT comes on line.

My only real Boeing experience is the 737 and I am working from the Smart Cockpit ELEC pdf for the 757 BUT the HBB is, in my experience, powered only by the battery via diodes - and we have had a thread on this not too long ago.

The 757 may be different, but I have not seen an HBB powered from a main DC Bus except for the Hyd gen on the 757.

Over to 757 experts?

Glennbb, why are you disguising yourself?

Hi BOAC,
Thanks for the reference; From http://www.smartcockpit.com/download...-Electrical...

You are correct.
"Hot Battery Bus.
Prior to establishing electrical power, the main battery supplies the hot battery bus. After establishing electrical power, the main battery charger powers the hot battery bus."

I was confusing the battery bus which can be powered by the Left DC system via the TRUs.

The HBB is connected to the BB with the battery switch on so any time the battery switch is on they are one circuit.

Wrong - there is no switch! That's why it is called the 'hot' bus.

I merely said the HBB which is the bus connected to the battery becomes one with the BB with the battery switch on.

The Hot Battery Bus (HBB) is always hot. The 'BAT' switch controls the Battery Bus connection to the HBB and therefore its state (energized or not).

To answer genbb's question, upon failure of the battery charger, the BB and HBB loads will be powered from the battery. This will slowly discharge. Not as rapidly as it would when powering the standby systems (30 minute design capacity, more with the hydraulic generator) but over the course of several hours. This assumes that main generator power remains to support all other electrical loads. The rate at which the battery will be depleted depends on the BB/HBB loads. I don't know off the top of my head, but assume 4 Amps for the purpose of discussion. This would deplete the battery in approximately 10 hours.

Absent a QRH procedure or company policy, its a juudgement call whether to divert or not.
:8

Oh, by the way. Add to glenbb's list of loads all of the Battery Bus loads that will be lost when the battery goes dead. There is some important stuff on that bus as well.

The systems manual seems to disagree with you. The Battery Bus (not the Hot Battery Bus) will be powered by the Left Main DC Bus which is powered by a TRU, so items on the Battery Bus will not be lost.

EEngr - I have never seen a 'suggested' HBB load before and that is useful. It now begs the question of what happens when you say 'goodnight' and park up for 24 hrs? Does something kill the HBB after a while or will it eventually flatten the battery?

If the "eventually" is long enough, the battery will go flat with or without the HBB.

BOAC the HBB loads are those that are always on. So this becomes a case similar to parking an airplane in storage. How long can the battery endure this without going flat? Hours, days? I don't have the design requirements here, but I think there is some sort of number aattached to that condition.

flyboyike Right. I stand corrected. This is a case of all buses normal except for the battery charger. Only the HBB loads will draw down the battery.

Perhaps to summarise, then, looking at glenbb's list of HBB services it appears that unless some of the 'valves' etc are electrically powered in their normal 'operating' position the actual current draw will be minimal - clock time system only? If so, I would guess that even EEng's "4 amps" draw would be pessimistic, so there should be no need for an 'automatic' divert.

I believe the clock load on a 757 is similar to the drain on your car battery. The clock draws miliamps so a good battery should still be good for 6 months to a year.

I pulled that figure from memory regarding a similar scenario posted about a 737NG. But that model has a switched hot battery bus and therefore more load connected to the main battery/charger source. So the battery load on a 757 would be much less.