Why do A330 and other types have an APU battery?

[9 points]

For the test pilots and engineers out there. Why have an APU battery?

My guess:
1. Located closer to the APU than the ships batteries might help with starting.
2. Located in an easier position and perhaps lighter than ships batteries to allow for easier removal when securing for cold soak.

Any ideas?

Thanks

:)

[HEALY]

Possibly redundancy for ETOPS requirements

[9 points]

Good thought, but on the 330 you would still have to divert for ETOPS with all generator failure even if all 3 batteries were still working.

[TURIN]

Redundancy. Allows dispatch with other systems inop.

Someone with an MEL handy will no doubt post an example.

[barrel]

Hi,

I'm just an aircraft mechanic so I'm not an expert.
APUs, in general, have a DC starter which demands a lot of energy in the APU start, this is why APUs have their own batteries and they are normally bigger and heavier than others (more cells).
Is it an answer to your question?
Sorry my english is not very good.

[TURIN]

Same battery as the main batteries on many a/c. Which is handy if one of the main batteries fails down the line without a local spare available.


As the OP states, not all a/c have a seperate APU battery.

[mustafagander]

Ignoring the B744 case where the APU battery powers a stand-by DC bus, AFAIK one of the primary drivers of a separate APU battery is so that a cold soaked, hard starting APU won't almost stuff your ships battery. Think about it - would you really want to launch at night in dodgy weather with a ships battery(s) strongly charging after being flogged by an APU start? Think also of the air start case with, say, B767 - what about an aborted start off the ships battery, now badly flogged? 30 minutes anyone?

It's just too easy and simple to have a dedicated APU battery and ease the regulatory burden by leaving the ships battery alone in case of major electrical failure necessitating APU start in the air. Don't tell the bean counters, but it's also good engineering practice to segregate systems whenever possible.

[nitpicker330]

Extra redundancy. Simple.

[9 points]

I had a look at the 330 MEL and you can't dispatch with a main Batt inop.
I guess you might be able to swap with the APU Batt if they are the same but I don't know if they are and I doubt Airbus would have installed an APU batt just for spare parts.

So considering the possibility that it is there to help the main batts to be charged for take off then how does the aircraft prioritise drawing current for APU start from the mains versus APU batteries? They are all switched on for starts. I've not read anything about this in FCOM1 or elsewhere.

(thanks to all, keep it coming)

[barrel]

A330 FCOM 1.24.10 p4:

"Two main batteries, each with a normal capacity of 37 ampere-hours, are permanently connected to the two hot buses.
A third battery (37Ah) is dedicated to APU start."

I'm not sure if this an answer to your question, sequence:

APU batt => APU start => APU AC generation => Main batts charging via AC bus, TR and DC bus

APU batt for APU only (in normal operation). Check your FCOM1 in Electrical and APU, you should find all the answers.
Regards.

[9 points]

OK here are some more thoughts:

The APU Batt is only for the APU, at all times.
In normal ops the APU start is also assisted via the APU TR drawing AC from AC bus 2/external or other engine etc.
In emergency config the APU start is from APU Batt only, the start is limited to F250 max.

So all this is great in terms of how it works. Thanks for the help.

But still, did Airbus design the 330 with an APU Batt for
1. Starts in emergency elec config.
2. Preserving the main batts for take off.
3. Allowing the removal of the batt for cold soak.
4. Combination of the above.
5. Other?

[grounded27]

Weight savings, placing the a/c battery close to the cockpit and the APU battery close to the APU eliminates lots of heavy gauge wire. The longer the distance the larger gauge wire required for the desired current draw.

If Airbus could use 26 gauge wire for everything they would.

[Swedish Steve]

The BA B737-400's were all delivered with two batteries. One was the APU battery, but when standby power was on, this battery was coupled to the main battery to give better time. The rest of the time (all the time) it was isolated.

Then we acquired the B737-400's from Dan Air. They only had one battery.
So we had 10 734s with one battery, and 25 734s with two. But all batteries had to be serviceable, we cannot MEL the APU battery, even though the other B734's did not have one!

[fantom]

Shoot me down but I don't remember APU being required for 330 ETOPS. Might have changed but...

[9 points]

APU not required for ETOPS on A330 (up to 10 days etc etc).

[TURIN]

Shoot me down but I don't remember APU being required for 330 ETOPS. Might have changed but...

From memory, APU is not required but the a/c can be dispatched with an engine generator inop providing the APU is servicable.

I think.

Maybe this is also one of the reasons for a seperate APU battery.

I had a look at the 330 MEL and you can't dispatch with a main Batt inop.
I guess you might be able to swap with the APU Batt if they are the same but I don't know if they are and I doubt Airbus would have installed an APU batt just for spare parts.


Batteries are the same. As are the battery chargers which tend to go tits up more often in my experience. :hmm:

You are quite right, airliners do not lug batteries around as spare parts, but when you consider all the other advantages, increased redundancy and ability to dispatch away from main base must add to the pros and overcome some of the cons. After all, many airlines fly spare main and nose wheels along with a parts/tools kit in the bulk hold just in case. :ok:

[Chopper OZ]

It is there for most of the reasons already stated.

1. The closer the battery to the APU the less cable required - to save weight.

2. The initial current draw on the battery for APU start is 1000A+. To drive that from the front of an aircraft the length of an A330 would kill aircraft main Batts pretty quickly.. It would also need bigger batteries to feed the extra power req for the start.

B737's had them to start with but were removed at operator discretion to save weight and maintenance costs. Even if the main batt takes the extra work and needs changing sooner, the overall cost benefit is worth it for an A/C of its size.

ETOPS is irrelevant as main BATTs are an emergency power source in case of all other failure. They are required for every flight for obvious reasons and can't be dispatched u/s.

BATT's don't generally do a lot of work in flight. They sit idle being babysat by the BATT chargers for emergency use when required.

[Junkflyer]

The 747 classic has a seperate apu battery in the tail area. It would be a very long wire run (and voltage drop) from the cockpit with only a single battery.

[9 points]

A330 APU BATTERY
My understanding:

The APU Batt only supplies the APU, no exceptions.
In emergency config the APU start is from APU Batt only, the start is limited to F250 max.
In normal ops the APU start is also assisted via the APU TR drawing AC from AC bus 2/external or other engine etc.

Airbus designed the 330 with an APU Batt for:
1. Starts in emergency elec config, preserving the mains for PRIM’s etc.
2. Preserving the main batts for take off.
3. Allowing the quick and easy removal of the batt for cold soak.
4. Can be used as a spare for a failed main Batt.
5. Short wire to APU starter motor = easier to start and avoid hugh batteries for that job.
6. Short wire saves weight.

Waiting for further...