MMI Battery Gauge Stuck on 10%
 

I had a flat battery the other morning after a night shift, not sure if it was the driver's door handle or running VCDS the day before. 20 mins on fast charge and it was enough to start and get home by which time MMI was at 80%.
Went to bed and battery was flat again in the afternoon, so charged fully off the car. All was OK, but same again the next morning but battery gauge in MMI never moved off 10% even after 90 mins driving and finishing the charge on the charger with the charging post. I disconnected again to reboot the BEM. Still 10% after driving to work this evening, I brought my charger and 50m extension real this time, but just been out and tested an it was perfect. Still showing 10%. VCDS shows battery internal resistance 9OHM which doesn't sound right.

DO you think I fried the BEM - maybe try recoding as new battery or is it telling me the battery is junk status? Even though the battery is now fine (cranks really fast) , it is shutting stuff like the power boot and alarm off.

I would try recoding as a new battery first by changing the last digit by 1, that will tell the BEM you have changed the battery and it will try to reset, I had issues with a D3 doing the same thing and all I've ever needed to do is that to correct it. Failing that I would change the BEM module unless youre aware of any drain possibility +++

9Ω internal resistance is way too high so the battery monitor may be correct if it is reporting battery capacity rather than state of charge. It should be 1-2Ω really. 4-5 is a bit iffy.

Internal resistance is caused by lead sulphate crystals building up between the lead plates and causing tiny short circuits between the plates. As this happens there is less usable surface area to hold charge so while the battery may be at 12V and appear full to a charger, its actual charge capacity could be very low. It'll appear to be ok after a charge, but probably won't start the car in the morning. I have a couple of old batteries in the workshop like this so I keep one of them on a trickle charger so I can use it for jump-starting. If I take it off the charger it goes flat overnight on its own.

Smart chargers like the C-Teks have a desulphation stage which pulses AC in to the battery to dislodge the crystals, but they don't break down - they just pile up in the bottom of the battery casing. Eventually enough sludge will build up to touch the bottom of the lead plates and then they cause the same short circuits but on a larger scale, so desulphation can prolong the life of a battery, but not recover a terminal one.

Thank you both so much. Would you know it, it did correct itself on it's own today.
Previously, even though the battery was full according to my charger and should be full as it had been charged so long, it stayed on 10% and the alternator output was high compared to uaual.

The alternator never charged it more than ~20A, neither did the external charger by much. I have used a clamp meter to test the current a lot.

I will check the internal resistance reading again when it has settles. It is an AGM battery, apparently they should be lower internal resistance than flooded cell, but mine has always been 4OHM.

Mmm yes AGM are supposed to prevent sulphation by virtue of having glass fibre between the lead plates which prevents the build up of the crystals.

It might be worth a trip to an indy garage to see if they can stick a battery tester on it to validate what VCDS is telling you. 4Ω is high according to my tester, but thats for conventional lead-acid. Not sure what the tolerance for AGM would be.

4Ω or 9Ω sounds unrealistically high to me, it should be well below 1Ω, more like 0.01Ω for discharging and 0.05Ω for charging.

Hi guys! I have a bit of a problem with coding a new battery, cause some "brainiac" in the past entered some fishy BEM codes and I have the PART Number quite normal, but the SERIAL number is a mess "N311111111":Confused:

Dont know how they even got the car to accept this serial number but OK,

I have installed the Varta battery 110Ah, I didnt buy it at the dealership, the car is 2004 A8, 3.0 TDI,

Does anybody have some helpfull tips to how to make this right?

Thanks in advance,

Regards from Slovenia,+++

Yes, the part number is the important bit that tells the car the Ah of the battery and maybe other characteristics such as AGM.
The serial is irrelevant it just tells the BEM it has had a new battery when the serial number has changed from the previous value and doing so resets all the parameters. So they probably are a genius afterall.:D
What is your part number? Something like 4E0915105A is correct but first 3 digits may be 000.
For 95Ah AGM I would go with 3D0915105G which is for the VW Phaeton.

Thanks for the info +++

Yes, you are correct about the part number, it is exactly like you said :)

So, I leave the serial as is, but put in the part number 3D0915105G,

Eventhough it is a 110 Ah battery?

Regards,

No, leave the part number and change a digit in the serial number to tell the BEM a new battery has been fitted.
It is customary to add 1 to the serial, so N311111111 becomes N311111112

I was just giving the 3D0 part number as an example for an alternative battery.
You have a 110Ah lead acid so the current part number is correct.