Assistance requested - BEM / Battery voltages ...
 

Guys; having had long time battery charging issues, with supposedly 'good' battery and alternator, I'm now determined to try and get to the bottom of it.

I've had my 2005 A8 for 10 years now but VCDS only for about 2. Sadly, I've never tried to use VCDS to monitor what's going on until now. I'm focusing attention on the Battery Manager ECU's measurement blocks, as Group 1 - fields 1 and 2 - show the 'alternator output [requested]' and the 'battery voltage [measured]' in real time.

I'm interested in the BEM values when the engine is idling after a start.

I've looked at my numbers several times in the last few weeks and they consistently show Alternator 14.3V and Battery 13.7V

I've directly asked one forum member for his numbers - Thanks Ray - but I'd be grateful if a few other forum members [4 or 5?] with VCDS could do a quick check of those ECU 61 Group1 fields when convenient and enter their values here for comparison.

However, if you'd like to contribute to the thread and you haven't got VCDS, I'd still be interested in seing what your battery voltage is, engine on and idling, measured with a DVM.

Thanks - Julian

Julian,
My battery is two years old now

Mine are 14.30v and 13.20v and is stating 50% state of charge
Battery was installed december 2016, but this will be my third battery in 6 years.
First one was killed by a faulty alternator, the second two I think is down to excessive starting drain due to leaking injectors (fuel collects in the piston bores)

I dealt with a parasitic drain on my D2 for a long time before I eventually solved it.
I had a bung voltage regulator on the alternator.
It 'tested fine' with an auto elec.
It measured 13.7V running with lights on full beam and aircon on.
Parasitic drain remained until I went with my gut and replaced the Voltage Regulator (and bearings and slip ring).

So you just have to figure out now whether the D3 battery monitor is capable of picking up a failing voltage regulator?

Thanks for checking your numbers 'SRG'.

Those numbers are even stranger than I think mine are :Confused:

I know that the Battery Manager [BEM] is only 'requesting' 14.3V but I can't understand the voltage drop to 13.7V in my case [and 13.2V in yours]. Those numbers aren't really good enough to be charging the battery.

Interestingly, Ray's was over 14V.

The trouble is that although the BEM can directly measure the battery as it's 'right next door', it's not 'seeing' the Voltage level of the output from the alternator. So I guess some sort of test at the alternator is required.:(

Hopefully we'll see some other results from other cars ...

Julian

Marty; thanks for your comments but I believe the D3 alternator is more advanced than the D2 one; the D3 one is water cooled and has variable Voltage output based on what the Battery Manager [BEM] asks it to produce.

The attached diagram shows only one connection from the BEM to the alternator [in blue] and that's its 'voltage request line', I believe.

As per my comment to SRG above; I guess it needs a voltage check for the alternator output to see if it's giving the 14.3 it's asked for. If it is, I think I might have 'voltage drop' but I've no idea how that battery current gets to and from the front of the car.:(

Julian

Somebody on here, sorry can't remember who posted a photo of the big positive cable from the battery to the engine area which had a connection around the driver's footwell which looked a bit poor and had overheated due to contact resistance and high current and it needed fixing.. Maybe that is causing the voltage drop?
Sorry, I can't find the post now.

That’ll be this one I assume
http://forum.a8parts.co.uk/showthread.php?t=13499

Well done! That's the one, now you find it I can hopefully check mine soon.
I have always suspected lhigher-than acceptable osses between the BEM/battery and alternator and vice-versa to starter. (Low charging voltage at battery when you would expect it to be high, and somewhat lazy starter)

Attachment 18841

Attachment 18842

Its worth doing, I checked mine at the weeknd, and although no sign of any heat it seems a little less lazy having sanded the contacts clean.