B737NG BAT DISCHARGE
 

Hi there.
Scenario: You are one hour into a three hour flight and the bat discharge light illuminates. A quick check shows -7 amps. What do you do?
My thoughts are that there is most likely a problem with the Battery Charger or with the TR 3 unit. I would be particularly interested in the state of the standby busses to see if they were showing a normal voltage.
Assuming IFR weather everywhere I would most likely return to destination or divert to nearby base. My thinking there is that we could possibly lose the standby system or worse it could be the first indications of a thermal runaway.
My understanding of the system is basic so I would appreciate any input from those more familiar with NG electrics.

Deficit of Straight Amps.
 

I suppose it would depend on whether you are a 1 or 2 battery equipped aircraft?

The QRH is not particularly helpful, giving no actions to undertake.

If the Ni-Cads are into thermal runaway, then it belongs on the ground ASAP, but there again we have no information as to diagnosing such a runaway til the smoke starts or the bits begin to burn up?

Reading the UPS 747F Dubai report will chill the bones and may influence our subsequent actions, better safe than sorry. Maybe some research into Ni-Cad runaways would be useful.

Otherwise keeping an eye on the systems, the weather, your location and maybe continuing to destination or suitable commercially desirable diversion where the defect can be rectified, if the craft continues to function normally, would be the other option.

I am presuming the 60 minutes endurance on the "fully charged battery" applies in the event of a further failure involving loss of generating ability, so we're looking at a double failure case.

If Mr Boeing regarded this as a LAND ASAP option, I presume he would have stated thus in the QRH?

It will be an interesting tea (no biscuits) discussion with Fleet Management if one diverted because of a Batt Discharge light with no other adverse symptoms.

Thanks for the post, it will stimulate me into ferretting around looking for further guidance! :)

The batt discharge light does not come on if AC power is used. (Tranfer bus 1 powered). So in normal ops you would not notice. So why divert?

Thanks for the replies.
I am operating single battery Barkingmad.
Latetonite, the discharge light comes on if there is a discharge. If you have fault with the battery charger it can come on even if the No2 transfer bus is supplying AC to it. The reason I am considering divert or return in this situation is that I wonder if eventually I'll lose the standby system ( or part of it).
Cheers

If you see the battery discharge light, then (single battery) it has a min of 30 mins remaining.

As I understand it, the battery bus and dc standby bus will remain connected to the main system with the STANDBY POWER switch in AUTO and TR3 functioning, so those buses will remain powered. However, the hot battery buses will not be powered once the battery is flat, APU and engine fire extinguishing will not be available.

For that reason I think popping in to visit the nearest suitable (in time) a rational and safe choice.

Thanks Kefuddle,
I think the cargo fire suppression and also the parking brake is on the hot battery bus as well. So I guess you could consider the rate of discharge when making the decision about where to go.I gave a random figure of -7amps, does anyone know how long the battery would last with a low discharge like that?
In reality I think I would just go somewhere ' suitable'.

IIRC (and it's been 10 years since NG avionics school) The ADIRU's or at least #1 are on the hot battery buss as well...

"If Mr Boeing regarded this as a LAND ASAP option, I presume he would have stated thus in the QRH?"

"It will be an interesting tea (no biscuits) discussion with Fleet Management if one diverted because of a Batt Discharge light with no other adverse symptoms."

It would not be the first time Mr. Boeing had not thought of all the unimaginable possibilities at publishing stage of the QRH.

The old adage of "if there's doubt there is no doubt", & "I'd rather be on the ground wishing to be in the air, than in the air wishing......." That will be save you bacon and I'd enquire where are my biscuits.
Since the FE disappeared you do not have the information nor the knowledge to make accurate decisions. You are not on your own up there and you have a duty of care to many.

Thanks a lot for the input folks.
I think from now on I will consider the BAT DISCHARGE light as a THE BATT CHARGER MIGHT BE DICKY light.
Cheers

As RAT 5 says follow the QRH...no LAND ASAP note at the end = continue (assuming not part of a multiple failure).

Why would a bat disch light come on if you are charging?

What about this from the QRH checklist instructions?

In some circumstances I would put this into the Catagory of potential significant adverse effect on safety if the flight is continued.
An ethos of " no Land ASAP= continue " is a bit ridiculous. If it was that simple there would be no need for you sitting up the front. Sometimes you have to make a decision on your own as Boeing have pointed out in their QRH.

Latetonite,
In normal circumstances the charger doubles as a fourth TR and supplies DC to the HBB and the SHBB and that is why you don't normally see a discharge on the battery. If the charger fails to do that then the battery will be supplying the power for those two, hence the discharge.
That is my understanding of the schematic I have in the FCOM. If I have misinterpreted it I look forward to being corrected.
Cheers.

"As RAT 5 says follow the QRH...no LAND ASAP note at the end = continue (assuming not part of a multiple failure)".

Not so, quite the opposite.

Framer,
Looking at the MRG it seems that there isn't much on the hot buses to consume power. I suspect that the battery drain rate would be merely slight compared to also having to power the battery, dc and ac standby buses.

LASJayhawk,
In this scenario, the IRS's should still be powered by the AC transfer buses. If and when the battery does run out of juice the IRS DC FAIL lights will show, but both IRS' will still function normally on AC.

Assuming the Battery charger has failed as well, causing the BAT discharge - Any equipment that is powered by the switch/hot battery bus will fail.

What nobody has mentioned is that the fire protection is powered by it.

You will get the detection as this is powered by the battery bus but zero protection due to no hot batt bus.

The battery is your get out of jail card if your having a bad day, depending on where you are, I think a divert would be a good idea.

Batt disc light not working is a C item in MEL.
So you can fly 10 days without fire protection?
As long as you have transfer busses, and TRU's, you do not need batteries for flight..

Latetonite:

Its referring to the batt discharge light, which is only a light. The classics dont even have a warning light.

If the battery is discharging, then there is more underlying issue.
If your willing to depart without a battery then.....

I am not departing without a battery. I am flying and now suspectively losing my battery. So what? I do not have a 60 min backup for my instruments if I loose all three generators? You think the safety of the aircraft is jeopardized if this single battery dies enroute? You do not think that Boeing would not install a 2nd one as standard equipment, if the life of 180 people would depend on it at all times?
Looks like common sense left the cockpit again.

It was never said that the safety of the flight was jeopardised in any way as long as you have the other means of power.

In the scenario given its a normal flight, but with a bat discharging.

Fact of the matter is the Hot battery bus and switched hot battery bus will eventually be zero after 60 mins (dual battery), and the fire protection uses it.

And if my understanding is correct of the electrical schematic, nothing else powers it except the battery.

With that in mind, and also the fact it will be a no go once landed, i will be on to maintrol to discuss what they're thoughts on it seeing as they are engineers and know more than me.

Your engineers will tell you that current can flow from TR's to the DC busses to the switched and hot batt bus...

I refer the FCOM 2
" The hot battery bus is always connected to the battery. There is no switch in this circuit. The battery must be above minimum voltage to operate units supplied by this bus"

From my understanding, Mr Boeing wouldn't put this statement if it could be powered by another source.

Look at the electrical diagram. Then quote half statements..

latetonite funnily enough this topic has already been spoken about.

I also reference Mr Chris Brady. 737 Electrics

Just to clarify I am talking about a scenario if BOTH the charger and Battery has failed.

As others have pointed out, BAT DISCHARGE means that the charger (or its supply) or the battery has failed. For the first two cases, the battery will now slowly discharge while supplying the hot battery bus and switched hot battery bus loads. Minus seven amps is not full standby load, indicating that AC and DC standby are still being powered by their primary sources. Given that discharge rate and a 36 Amp-hour battery, you will have approximately 5 hours until the battery reaches minimum voltage. If nothing else happens, you will be cutting into your standby capacity. That 30 minutes is what you have in the event all main power is lost and standby switches to the battery. Now that you are nibbling into that capacity, allowances may have to be made in the event an alternate airfield is needed.

Good move checking the battery current and bus voltages. But there is a limited amount you can diagnose beyond making sure things stay relatively stable. The reason for the charger failure could be of no consequence. Or it could be serious.

Thanks Eengnr, good info.
Latetonite you seem to be the only one posting here who hasn't understood the scenario we are talking about.( either that or we all misunderstand the system). I'll spell it out once more for clarity:
One hour into a three hour flight.
Single battery aircraft.
BAT DISCHARGE light illuminates.
That's it. That's the only indication you have that something is out of the ordinary with your aircraft.
In this circumstance something is amiss with the supply of power to the SHBB and the HBB and the battery has taken over supplying the power.
If the problem is not the No2 Transfer Bus then it is downstream of there and I don't see how you can be confident that the draw on the battery won't continue or increase as there is obviously a fault in the system.
When your battery gets low you are going to lose some critical systems from the SHBB and the HBB and you can't calculate with any certainty when that will be.

You seem to think that even though the battery is discharging, the TR units are supplying power to these two busses.
If this is true it is important information and we really need a source other than " your engineers will tell you". Where did you get that knowledge from?
I doubt it is true and think you may need to have an open mind towards changing your understanding of how the system works ( for reasons of flight safety).

Ok, if the battery fails, together with the batt charger, the hot battery busses are powered by the standby DC bus or stby power control, both fed by the dc busses, through the TR 's. this is clearly readable in the electric power diagram.
If the bus fails altogether, due to short, you have another ballgame.:)

Not so readable for me, please show me?
http://img6.imageshack.us/img6/4043/i59i.jpg

MULTIPLE FAILURES.
 

"I am presuming the 60 minutes endurance on the "fully charged battery" applies in the event of a further failure involving loss of generating ability, so we're looking at a double failure case." My post # 2.

I don't recall the OP mentioning further failures, which puts us in a whole new ballgame.

However, as the battery or batteries are there as a backup to loss of generator capability, then it's going to be an anxious time for all.

Maybe we won't get a hard and fast answer to the question, shame as the doubt has now been raised.

Correct. But the reason for the charger failure has not been determined with certainty. If it is a benign failure, then there should be no problem continuing on. If there is something smoldering in the panels, that's another matter. From the cockpit, probably the only diagnostics are the bus voltages and currents. A stable battery voltage (around 24 V) means all is probably well. Fluctuating voltage readings indicate the possibility of more serious problems.

By the way, the post #25 diagram does not make the source switching for DC Standby and Battery Bus clear. Treat the arrows feeding the buses as diodes (although that may not be the actual implementation). Current does not flow through the battery bus from the TR unit to back feed the hot battery bus.

Can we all agree that the primary source of power for the SHBB and the HBB in normal flight is the battery charger ( operating as a TR) and the secondary source of power for these two busses is the battery, and there is no third source?
If that's the case you don't have the ability to determine how long you have before losing your fire extinguishing capability in the described scenario.

Sorry Framer, cannot agree. I wanted to upload an electrical diagram, more detailed than the one posted before, but have problems doing so. But there are available on the net.
Do not confuse a battery failure with a bus failure.
In your example, when the battery fails, plus the charger, the hot bat bus is fed via TR3. Then there is an auxiliary batt charger on the airplanes where there is a dual battery installed, also feeding the hot batt bus.
Yes, in case the hot battery and the switched batt bus would fail, you have another can of worms.

Latetonite :

We are NOT talking about a dual battery system. If the diagram is on the internet can you please give a link to it.

TR3 powers the battery bus but does not back feed through to SHBB and HBB like previous posters have said.

If you can show that it does then I shall walk with my tail between my legs.

http://www.b737mrg.net/downloads/b737mrg_electrical.pdf

Start walking.:)

TR3 does not power the hot buses under any circumstances. This is shown, but not obvious in the diagram you posted. However, it is clear in FCOM2.

Kefuddle,

Looking at your FCOM 6.20.3, TR3 does feed the DC stby bus, no?

And in FCOM 6.20.15, DC power distribution, it is clear that TR3 feeds the battery bus via stby pwr PCU.

As in this post by EEng you need to treat the arrows as diodes.

You can only lead a horse to the water. You cannot make it drink.

The diagram in post nr. 29, not 25, is extremely simplyfied.

The link I provided to the B737MRG in readable by any 2nd year technical student.

For anybody to think to find simple flaws in studied and proven engineering, sounds just ridiculous.

Trust your commandership does not reflect the logical analysis exposed in this tread.

From the FCOM:
Normal operation.
So in normal operation the SHBB and the HBB are powered from the battery charger ( which in our scenario is faulty)
Then from the FCOM:
So in Alternate operation ( with the charger failed as per scenario) the SHBB and the HBB are powered from the battery. There is no mention of your mythical TR3 feed to the HBB as you say here .