Alternator output

[Somerandomguy]

I'm struggling with poor battery life still, and had assumed that I had a parasitic drain which Ive not got to the bottom of.

After checking all of the fuses for current, i found nothing so am wondering if I have a problem with the BEM or alternator. Ive been monitoring voltage whilst I drive, and it seems a bit low, ranging from 12.7v up to 13.6v depneding what's switched on.

So im thinking that rather than a drain, the battery isnt charging - do any of you know what your normal voltage is?

Cheers

[snapdragon]

The same as you - lower than expected.

[JulianHicks]

Originally Posted by Somerandomguy

After checking all of the fuses for current, i found nothing so am wondering if I have a problem with the BEM or alternator. Ive been monitoring voltage whilst I drive, and it seems a bit low, ranging from 12.7v up to 13.6v depneding what's switched on.

So im thinking that rather than a drain, the battery isnt charging - do any of you know what your normal voltage is?

Cheers

Some thoughts:

1) Have you checked ALL the fuses? They're all over the car; 8 places by my count

2) What is the version of the BEM in your car. Some of them have a flash update to correct issues. Sadly, Audi never said what the faults were, only that BEM firmware above 240 and below 510 should be flashed to 0510.

3) The alternator is 'clever' in the A8 in that the voltage output is selectable - by the BEM. THE BEM selects the appropriate voltage and sends that value to the alternator. Thus there is no 'normal voltage'; it changes in each car as the conditions dictate. In drastic conditions, the BEM will also speed up the engine idle if it thinks the alternator is not up to keeping the charge up at normal idle.
If you have VCDS, you can monitor alternator output in real time using BEM Measuring blocks.

4) A fully charged battery is 12.6V, so any output voltage from the alternater over 12.6 will be charging the battery.

Julian

[Somerandomguy]

I thought I had checked them all, but probably not, as i had only checked 4 locations- left and right front dash, pkus left and right boot alcoves. Where are the rest?

BEm is H12 0550, which looks like the latest version. I realise that it is a smart system, but sometimes after a long drive im sure I have even seen the voltage drops below 12.6v, so Im trying to establish whats "normal" behaviour.

I had a look at the measuring block today after charging overnight followed by 25 miles driving and at idle, the BEM is reporting positive current flow between 2A to 10A, dropping to -14A with the engine off. Interestingly the requested voltage was over 14v, but the battery voltage was 13.7v.

Ive spoken to an audi mastertech friend and hes offered to monitor it over a few days with a scope to see if he can see anything

[JulianHicks]

Originally Posted by Somerandomguy

I thought I had checked them all, but probably not, as i had only checked 4 locations- left and right front dash, pkus left and right boot alcoves. Where are the rest?

BEm is H12 0550, which looks like the latest version. I realise that it is a smart system, but sometimes after a long drive im sure I have even seen the voltage drops below 12.6v, so Im trying to establish whats "normal" behaviour.

I had a look at the measuring block today after charging overnight followed by 25 miles driving and at idle, the BEM is reporting positive current flow between 2A to 10A, dropping to -14A with the engine off. Interestingly the requested voltage was over 14v, but the battery voltage was 13.7v.

Ive spoken to an audi mastertech friend and hes offered to monitor it over a few days with a scope to see if he can see anything

1) As well as the obvious locations at left/right dash ends and left/right boot there are apparently 4 other locations: Plenum chamber, behind left dash panel, right A pillar, front right footwell. [See PDF - A8 Current Flow diagrams - Section 802]

2) That's a good level of the BEM - latest [and last] version, as you suggest

3) Battery voltage with engine running is actually alternator output voltage. You can only measure true battery voltage via VCDS with ignition on / engine off and no charger. Were those voltages with engine off?

4) I've come to conclusion you can only really measure battery drains with fancy £1000+ kit that dealers have. You need real time measurements with an inductive connector on the battery lead with all the car's systems shut down. Sadly I don't have access to one of those.

Julian

[Mechcanico Lee]

Regards the parasitic drain , what is the amps draw reading you get when vehicle had been asleep for over half hour , how have you measured this reading

[Somerandomguy]

Ive not been able to measure the drain as I dont have a clamp meter, but didnt find any voltage drop across any of the blade fuses, though looks life there's a few I havent tested.

Engine off, the voltage is around 12.6v after a good drive, though I have seen that low a voltage whilst I was driving- 4 hours in to a long journey which I suppose is what has made me wonder about the charging side of the system.

Hopefully, a few days watching it with the proper VAS tool should help get to the bottom of it, as it seems like its normal for the charging voltage to be lower than the 14v I would expect to see.

Thanks for all your advice

[Mechcanico Lee]

I'm not worried about your charging volts , I've seen low voltages before with smart charge systems .

I'm going to do an about turn on some of the info I have posted before on finding drains , we have Vas we have Pico scope and various load rating inductive amps clamps .....they all have there uses ,Pico monitoring over long term for strange drains that can wake up after lots of hours ....it all depends on the fault that what tool you need to use .
What I have found when you are looking at drains at let's say under an amp or less, inductive Amps can ' drift ' they can be over or under the real ' in line 'drain

For your fault ,I think a basic meter inline with the battery will surfice , I use a shunt to ' tap up 'off as if you inadvertently wake the car up a big surge of amps will pop the fuse in most meters .

Roll the boot latch over , have your meter in the boot and lock the car , give it twenty mins and have a look at what amps you are pulling out , knowing what is being pulled out can give a clue as to work to start looking .

I have had instances where there has been an obvious drain I have volt dropped every fuse and not found any drop , alternators are not always fused a drain can pull through a fuse box shunt or a fet ( field energised transistor ) but they always end up back at the source .....the battery .

In some cases where positives start to branch out from battery to say a fuse plate you can lift an eyelet or pull one of the heavy fuses and issolate a whole section of take offs , if your drain stops case of looking what on that branch .

I would say it's more rare for something to power back up after shut down , it's more the case that when it shuts down there is still amps being pulled ....every instance can be different .

They can be an arse but start at the battery and work through methodically , I have been tied in knots with them over the years !

[tc4332]

As always Lee, good sound advice.
These battery drains can be an absolute dog to locate especially on "intelligent" cars like 8s. Starting at the beginning (battery) is really the best way.
I know, electricians are trained to start in the middle and decide which direction to continue in. Splitting the resultant in half each time a sample is taken. But parasitic drains are a different kettle, you can spend hours twiddling your thumbs waiting for something to happen. Isolate chunks is the only way.

[Somerandomguy]

Cheers Lee - that's good advice - I'll have a go at hooking something up today on the battery and see where it takes me.