I am reviving this thread to help fellow onlookers and also give kudos to Johnny for this brilliant and accurate post!

I have an '05 CGT and my alternator went a few months ago. Replaced it. However, left side battery still kept dying. I would trickle charge and operate and drive it seldom as a result.
Mechanic was of the opinion that the replacement alternator was faulty also.
I researched some and landed on this Battery Control Module, and got an ebay unit for an '04 Phaeton. It was a direct swap and it was cheap, so I took a punt and it didn't need any reprogramming.
It was indeed the problem! The left battery is now taking charge! Success!

However, main battery's maximum charge now is 12.5ish volt with the engine running. Battery is about 15 months old Varta. The right side battery (starter battery as they call it) is at 13.7v.

So here's another diagnostic question, is it possible that the alternator is weak and is only able to charge one side?

Or is there another faulty equipment at play? Or is it in fact time for a new alternator yet again?

 

Hello @virtual_law ,
I see you joined a year ago, but it is your first post, so, welcome to 6speedonline !

As to your issue,
My understanding is that the battery control module charges one battery at a time, therefore switching between batteries, and being that you are seeing 12.5ish on the house battery, is it possible that it is because the BCM in charging the starter battery at the time of your reading, what is the reading of each battery voltage with the engine off, because, if I am wrong as to the charging separately, and you have 13.7 on the SB, and only 12.5 on the HB, the house battery may be failing, the voltage reading of both batteries with the engine off will help in this determination.

Johnny

 

That is fascinating. I've been reading these threads for the 4 years that I have owned my 05 GT and found them very useful. I'm posting today as I failed to proceed yesterday. I replaced both batteries a couple of years ago and use a conditioner whenever I am near an outlet (also a large PV cell on the back window ledge when I leave it). The car was exhibiting signs of low House Battery for a few days and I have been trying to use it as much as possible. Yesterday I started the car and within 10 seconds the engine cut out and the displays went blank. Re-starts brought about the same thing. I called for recovery, thinking bad battery or alternator. He read 10V on the House Battery with engine running (before it cut out). Then removing the House battery to read it off the car - gives 9V. He then put it back on the car and connected everything up again for a final try. Car started and and kept running. The House Battery was now reading 12.5V so it was receiving charge once more. It's at my local garage at the moment where they are reading the voltages for both sides and checking the alternator. I've asked him to check for fluid leakage on the alternator too while he's at it. It is interesting to read about the Battery Contoller Module though - I saw it while the HB was out and wonder if it was not working quite right and bumping it with the battery to get it back in brought it to its senses. Our readings yesterday were the same as @virtual_law 12.5 HB and 13.7 SB Maybe this is indeed normal. I read somewhere that there is some kind of inverter which will boost whichever battery needs it at whichever time (up to 16V). Thanks to Johnny for all his advices on these forums. I will keep coming back
Jonathan

 

Hello all,

To be clear the continuous under voltage of the battery(s) will eventually ruin the alternator. This situation causes the alternator to deliver max output continuously. A qualified load test of the batterys is a prerequisite. An output test of the alternator is also advised.

 

I did some further tests by removing the battery and Cteking it overnight. My ctek has the Recon option for the battery. I even tried that for 8-10 hours outside of the car. My findings were:
House battery will not go above 12v outside of car. A good battery apparently has to show 12.6v or above on standby. Most importantly, battery would get hot to the touch on ctek. On idle with engine on, it would hit 12.5v as earlier.
My ctek also tests the alternator I discovered. It has numbers 4 5 6 to determine health of alternator. I did that and received 4 (which is not a good number) with engine running. Ctek also recommends raising the rpms to 2500 rpm if the read out says 4, at which point a good alternator would rise to 6. I did that but mine stayed at 4. If I believe the ctek, my alternator is weak also.
I then dove further and swapped a working used battery from my w124 to the Bentley. That battery registered 12.9v at idle. This confirmed my suspicions that my 15 month old Varta had kicked the bucket.

I have since swapped for another Varta, which is now holding 13.05v at idle. AC compressor is working as it should with this new battery (it would cycle on and off earlier with the faulty battery), all other modules are working fine too, most noticeable of which was auto up down of the driver window.
So the correct diagnosis on mine was:
Battery not charging = battery ecu shot.
Battery volts below 12.6v at idle (+ hot to touch) = battery faulty
Alternator weak (according to ctek)
​​​​​​
​The last bit will be confirmed if the car misbehaves again. I parked it for 2 days and will drive it tonight to see if new battery is still holding.

@Erebus hope this helps you somehow. lol

 

How does one do the output test reliably?

 

Yes you could try VCDS. However I was suggesting a proper voltage output reading with a voltage meter at various RPM's. You should see 14.1 volts at 1000 RPM's. However the battery(s) should be tested first. Test can be done at the battery terminals. A 12.5 volt static charge level is discharged battery. Look online regarding battery charge levels.

 

Apologies for reviving an old thread but this is extremely relevant. Johnny, would you know whether the 3D0915181D supersedes the 3D0915181C?

My 04' has the 3D0915181C and recently displayed fault code "Control Module Defective" on 071 Battery Charger channel. No symptoms but can't seem to clear the fault so thought best to get the control module changed asap to avoid any potential damage to alternator or batterys.

I have seen a few B and D variants up for sale and wondered if these were compatible or whether I should stick around and wait for a C to come up?

If anyone else has any experience with this, please let me know. Thank you!

 

Generally speaking different suffix letter parts are not interchangeable. But I don't have any data for this particular case.