Boeing 737 - Dead battery implications.

[Scottie]

Just doing a bit of revision regarding the 737.

If on the IRS panel both "DC FAIL" lights were to illuminate it would indicate a dead battery (we're talking inflight, not at alignment). So no back up available if the generators are both lost.

However someone pointed out that a dead battery also limits your fire protection. Assuming you had both generators online, but the "DC FAIL" lights on the IRS panel came on it would indicate a dead battery. Detection for Fire is powered off the battery bus which would still be powered but the extinguishers are powered off the hot battery bus which would no longer be powered. So you would see you had a fire but you'd be unable to put it out!

have I got this right? Dangerous to ask on a anonymous forum but all thoughts welcome!

[Jetavia]

How do you assume you should loose battery power? Since Bat. power is primarily our backup system in flight you would first have to loose your engine driven generators, natural action would then be to turn on the apu and get it on gen bus 2 (power to transfer bus 1+2, main bus 2, AC stdby. bus, TR1+2+3..), if it also fails you would have to rely on bat power thru the standby power system and get the bird on the ground within 30 minutes (fully charged bat.).

I do not think loss of all electrical AC/DC power is a failure that has been taken into account by Boeing. Can't see it happen unless you lose both generators, apu generator and deplete the battery... talk about having a very bad day..

[alexban]

Let's assume that the battery will become dead by whatever reason....selfdestroyed,whatever.
This doesn't mean that the hot battery bus is not powered.The hot battery bus it's not inside the battery,it's after it in the circuit.The battery bus will be powered by the charger or the TR3. Check the schematics at 6.20.2 (fcom2 dec7/2000)
So,no problem...
Brgds
Alex

[Scottie]

Jetavia,

My thoughts were along the lines of Alexban with the thought that the battery may short, die on you, whatever. Boeing says it's impossible. But nothing is impossible.

Alexban,

My initial thoughts were similar to yours. Looking at the circuit diagram I'd have thought the battery charger would power the Hot Battery bus, after all once it's done charging it reverts to a TR mode.

However a TRE in my company has come up with this one and I'm not sure he's right. My Boeing Vol 2 diagram suggests that the battery charger would power the hot battery bus whilst in TR mode. But then say the battery had shorted would the battery drag the charger down with it therebye rendering the hot battery bus unpowered? Just trying to figure out where he's coming from!

Multiple multiple failures! But interesting nonetheless!

[xetroV]

Loss of battery power in flight CAN happen! I have witnessed this failure myself in an MD-11, and AFAIK it was caused by corroded or otherwise faulty connectors. An intermittent "BATTERY LOW" indication was the first sign that the system didn't work right; the battery voltage indication literally swapping between 28 and 0 Volts (no intermediate low voltage indications, so the battery charger was functioning fine). At first an indication problem was suspected, but when the captain's clock also failed it soon became clear that battery power indeed was intermittently (and later permanently) lost.

Now, I have taken a look in the MD-11 electrical schematics, and its DC arrangement seems to be quite similar to the B737 in that a 'Battery Direct Bus' (comparable to the 'Hot Battery Bus' of the Boeing) is powered either by the battery charger or by the battery itself. But since this diagram is even more confusing than the drawings in the 737 FCOM, I can't really figure out whether this 'Battery Direct Bus' indeed powers the captain's clock or if there is perhaps some even more direct connection not shown in the diagram, and I am also not clear where exactly the battery voltage is measured.

Anyway, the consequence of a "BATTERY LOW" warning is to land at the nearest suitable airport, which is due to the fact that this situation unpowers not only the captain's clock but also the fire bottle squibs. Due to the similarities in design I wouldn't rule out the possibility that this problem can occur in a 737 also.

[matkat]

Also seen battery failure on a B742,the A/C was just coming into Madrid and the battery C/B tripped,the battery had suffered "thermal runaway" and a good decision by the crew not to reset it averted a serious problem.

[80/20]

Jetavia…How do you assume you should loose battery power?
yes it can ..and has happened...
...maintenance had not connected the charger cable properly. It disconnected during takeoff and the battery was depleted on a long flight and ended up with this scenario

[Jetavia]

OK... but ever heard of a 737 or other type losing all generators, AC, DC, standby systems and battery? As far as I know we do not have a NNC for total electrical failure on the 737.

What kind of scenario can bring us into such a situation?

[Scottie]

OK... but ever heard of a 737 or other type losing all generators, AC, DC, standby systems and battery? As far as I know we do not have a NNC for total electrical failure on the 737.

Thats not quite what I was getting at.

If both "DC FAIL" lights illuminate on the IRS panel it means the battery is dead.

Yes the IRS will still work with two generators working but say you then had a lightning strike and lost both generators (a bad day) it would turn into a very bad day as you'd have no battery to fall back on!

But my question was about losing the battery and the implications for the fire extinguishers as they are fed from the Hot Battery Bus. Would the HBB still be powered or not? The Boeing Vol 2 is not clear about this.

[LEM]

If both "DC FAIL" lights illuminate on the IRS panel it means the battery is dead.
Wrong assumption, Gents.
And that's why you're confused.

Both DC FAIL lights illuminated would mean Battery dead and Battery charger dead also, which makes the scenario of your TRE wrong.

Even if you loose your battery, for a thermal runaway for example, the Switched Hot Battery Bus will still be powered by the Battery Charger.

The whole DC circuit will still be powered, so actually you won't loose anything, nor the IRS DC backup, nor fire protection, nor clock.

:ouch: ?

[Blacksheep]

Now assume you lose your battery charger and the battery runs down. Not an impossible scenario. Any consequences?

[LEM]

Hi Blacksheep, quite unlikely scenario...

Btw, assuming all the rest is working fine, the consequences would be:

Losing the inverter. No big deal, as the AC Stby bus would be powered by the Transfer bus 1.

Losing the Switched hot battery bus. In my book, that means losing the APU generator control, Autopilot A and B warning, DAA 1, Fuel SOV indication, Fueling valves, IRS 1 and 2 (DC), Thrust Reverse control.

Losing the Hot Battery bus. The list gets longer... Relevant items are: APU starter, all Fire bottles, 1 and 2 Eng Fuel Shutoff valves.

Losing the Battery Bus. Very long list.... Many relevant items lost, including Fire Detection.

In actual life, this scenario would come up only after losing all generators and wasting too much time in the air....

Cheers, LEM