New battery not charging. The alternator is OK.

[Mihai857]

I have a 2009 A5 Coupe without Start/Stop and my troubles started in January when my old battery (a Moll M3 plus K2 110Ah 83110 battery from 2015) started to show sign of degradation, but since I don't drive my car that much during winter, I decided to postpone changing the battery until March. At the same time the battery started acting up I noticed that the alternator only charged at about 13.3V - 13.6V (value from OBDeleven and also measured by me between the charging ports under the hood), before it was charging at around 14.5V (also from OBDeleven).
Because of this I went to a shop to check if the alternator is faulty, however when measured right at the output of the alternator the value is 14.6V - 14.7V. They suggested to check the coding for the battery and we found the following code: "8K0915105F VA0 280829T1CB", so they changed the code to a generic Moll code: "8K0915105F MLA 190629T01B" but that didn't change anything.

Well two weeks ago I put in a new Varta Silver Dynamic I1 110AH 610402092 battery but the problem still persists, so the guys from the battery shop tried a bunch of different codes but they couldn't get the charge past 13.5V.

Some of the codes they and I tried:
8K0915105F VA0 280829T1CB - this only charged at a maximum of 12.3V (the initial code the car had);
8K0915105F VA0 201120T1CA - charges at 12.6V;
4F0915105D VA0 280829T1CB - charges at 12.8V;
000915105CE JCB 420412C09T - charges at 13.3V;
000915105AK VA0 402092T1CB - charges at 13.4V (this is the code I'm using right now);

Basically I've tried a number of combinations of codes for the 110 Ah battery:
110 Ah - 4F0915105D / 8K0915105F / 000915105AK / 8KD915105A / 000915105DL

So right now I have two issues to check: either the codes that have been tried are not good or there's a faulty wire between the alternator and the battery that cause a voltage drop. Or there could be a different issue?

Do you guys have encountered a similar problem?

Any help will be much appreciated!

[Ulrict]

From what I understand the coding for the battery doesn't actually do anything other than tell the ECU to reset the power management system. You could just change one of the numbers and it should accept it. I didn't change the coding when I changed the battery and didn't see any negative results. I went ahead and changed one number anyhow just because. I've had no problems and everything reports 14.4-14.6 depending. When checking with OBDeleven the battery will show lower voltage (12ish) until it's been running, needs to recharge off the alternator. My experience is from my 2016 S5 and may be different for you.

[Mihai857]

All the voltages I measured where with the car running and checked both with OBDeleven and by measuring the voltage at either the charging terminals under the hood or directly on the battery.

Update:

I even tried to remove the Battery Management System sensor from the negative terminal of the battery, which should bypass the BMS and charge the car with supplied voltage from the alternator, however even with the sensor removed there is no change in the voltage supplied.

[RockJGC]

It's pretty common to see charging voltages drop when you install a new battery and then do battery registration (coding). The idea is that the older, worn battery needs more charging voltage then a fresh new battery.

A fully charged battery is 12.6 volts. As long as you're seeing more than that at the battery terminals with the engine running, then your battery is being charged. Rather than use the jump start terminals under the hood, definitely check at the battery terminals to see what the battery is really receiving.