awdinut

I have replaced the battery in my car 4 times over the last five years; it seems my car eats them somehow. I have never done the VAG COM set-up after the R&R because the car would start etc. But I have read it said here on AW that the battery does not charge as fully without the VAG telling the car to acknowledge a new battery.

After recent low battery episodes I have decided to have the serial number digit change VAG COM procedure done in hopes of correcting gliches with the battery management system. Couldnt hurt.

My question is...wouldnt such a procedure need to be performed anytime the battery has been disconnected for any reason ?

1Wasserwerks

I changed my battery oh, 1-2 months ago. I had the battery level meter active in my mmi (because of the sluggish starts) and caught my battery on the way out, I posted details in a thread. As soon as I installed the new battery (with charger attatched during r&r) and went and turned the car on, I checked the mmi battery level (as this is where I left it when I shut it down). It still showed a drained condition.
After vag-com reset of the battery (I changed the serial number by 2 digits instead of entering the new serial number, I forgot to check it and it was buttoned up) I restarted and the battery level meter was 100% and everything working nicely.
I think just a disconnection of the battery would not reset the battery level because the same serial number would exist in the system.
Have you noticed any odd-ball system shutdowns? The car would shut down the automatic-grab door handles and some of the draining security systems (which I find odd).

awdinut

What voltage does your system keep the battery at when its topped off ?

Back to the VAG COM question...you say the serial number would not have changed so disconnecting the battery would not necessitate a new VAG COM set-up. But the car would NEVER know one battery from another except--and Im just guessing here--that the car knows when all power has been removed from the system. And that must be interpreted as a battery change...otherwise why would we ever have to VAG COM after a replaced battery ?

And then isnt any total power interruption a 'new battery' ? There must be something Im not grasping to this whole BM system.

RocketSurgeon

The car assumes that it has the same battery until you use vag com to change the battery serial number. That is how the car learns of the new battery and adapts accordingly. That's why it's important to program to tell the car that the battery is new so that it won't behave strangely when it detects a change in battery characteristics.

MP4.2+6.0

The system monitors current outflows, plus it has the ability to shut many of them down as it goes through the various phases of discharge. It also instructs the alternator when/how much juice it wants. Those elements are documented.

Well if you put the pieces together with the coding, and the list of batteries which have rated capacities associated with them, you can infer the system over time is determining/estimating the battery's continuing capacity to hold a charge. As in, if it thinks the battery is getting crappy it makes its power cut decisions earlier based on given flow rates it either sees or knows is associated with a given car system. Thus when you install the new battery, it is the coding that resets the math back to "new" (for the given spec battery), not the act of disconnecting it. If an old one is reconnected, it presumably just assumes it's the same old battery until told otherwise. And if a new one is reconnected instead without recoding, my educated guess here is basically it doesn't realize the new battery now in place can hold a stronger charge so only keeps it at a kind of anemic level through its instructions for charging rate (which it controls) to the alternator, plus it continues to cut off marginal uses more aggressively based on the same outflows as usual but with a mistaken assumption about the battery's actual (new) capacity. Over time, I suppose it might "learn" better in the opposite (more capacity) direction, but that assumes the software/system designer actually considered that scenario beyond essentially the "reset" basic switch they built in with the coding and battery identification approach.

awdinut

Ohhh, so the BMS extrapolates...not having a left brain myself something like that never occured to me. Makes sense though. I just assumed the system is managing the real time energy it is actually finding; hence my conclusion about battery recognition.

Well, I'll have it VAG COMd and see if I stop getting trunk opener, leaving home lights,etc shut down. And if my battery gets charged more fully than 12.2 V. I'll post results.

mishar

Measure the voltage while engine is on. If it is ~14 V and drops to 12.2 V with engine off your battery is dead. If it is always ~12 V your alternator (regulator) is not working properly.

awdinut

The multi-meter shows 14V while the engine is running, so I know the system is charging. Even with a new battery this car has never maintained more than 12.2 on the battery...thats one of the things I hope doing the VAG COM set-up will improve.

This battery is only a year old, but thats about all I canget out of a battery (keep in mind I have never VAG COMd the new batteries) so I suspected the battery when low voltage events started happening again recently. Took the battery out and back to Sam's and had them load test it and they said the battery is good. I had a friend start the car while I held the multi-meter on the battery and it only dropped into the 11+ V range, so I thought that seems like there is not a dead cell.

Is my reasoning sound ?

mishar

14 V while engine is running on the battery? If it is only 12.2 V than something is wrong with your power management.

I am not sure if coding makes any serious difference.

awdinut

Just had the battery tested again at Sam,s (they hooked their NEG test cable cable to the battery post , not the jump start post...was that OK?)and they say the battery tests "good" ie no weak cell I guess. So I'll be re-coding to a one year old battery. But I guess its better that the system thinks my one year old battery is new rather than thinking its a 6-10 year old battery (Ive had the car 5 years).

Two other items : 1] when I went out to start the car this morning after it had been out side in 15 degree weather for 12 hours I checked the MMI battery level and it read 10%. Then I checked the battery voltage with a meter and it had 11.9 V...is that the voltage that one would expect to find with a MMI reading of 10% or is that a case in point for doing the coding so that the management system will have accurate parameters with which to make its judgements ?

2] At Sams they suggested the new Energizer GMS battery line which is a glass mat battery and supposed to be better although he could not explain why. The amps are the same as what I have...what properties do glass mat batteries have that I should pay 60% more money for ?