A4 B8, battery won't charge?

Hi everyone, I installed a firmware and map update for the MMI in my B8.5 from mid-2014. The procedure takes about three hours, so I had already connected a charger, something like a Eufab CTEK clone, to the battery (95Ah AGM). It showed that no charging current was flowing, and the battery and charging voltage were displayed as only 12.2V.
After that, I connected my dumm&stark charger, a Helvi Rapid 380, to the battery. After a while, the ammeter gradually decreased from an initial charging current of 25A, and I started the updates.
Right in the middle of it, the car's AI system displayed a message indicating that the battery voltage was low and that the infotainment system would be turned off to prevent the car from being unable to start. This was happening despite the fact that a charger was actively connected directly to the battery terminals.
I then started the engine in order to complete the update.

In short and to the point, what's going wrong? Is the battery status only being calculated, or estimated by the Battery Management System (BMS), or does the charging current have to flow through the BMS?
Unfortunately, I didn't have access to either VCDS or a multimeter to be able to collect any concrete data.

Hello,

So, did you measure the charging current using the vehicle's onboard network control unit?

If the battery voltage drops when starting the engine, it means either the charger is too weak or is defective.

Depending on the vehicle (type and number of control units), you need a significant amount of power to maintain the on-board voltage (and thus the battery charge level), and I would consider 20A to be more of a lower limit for this purpose.

Ventilation and radio, interior lights, TFL (traffic flow light) etc. – of course, turn everything off.

Best regards, Rainer.

Hello, thank you for the reply. I didn't expect that the vehicle would have such a high power consumption with the ignition on. Of course, everything that could be turned off was turned off, but the infotainment system had to be running for the update.
The chargers are otherwise functional. The Helvi is a starter charger and can operate at a low setting of 25A, a high setting of 38A, and provides a starting current of 200A.
I only dared to charge it at a low level, which was probably not enough.

"In situations like these, many control units have been damaged or destroyed."

Yes, that's why I initially used the smart charger, but it wasn't enough.
Workshops should ideally have some way to buffer power for larger operations like this. I probably need to buy an MXS 25 from CTEK.

You wrote that you charged directly at the battery terminals.

That shouldn't be done with modern vehicles anymore, as the onboard computer then can't measure the charging current (but it can measure the discharge).

There is usually an additional grounding point near the battery. The charger should be connected to this point (or another point on the chassis) in order to be registered by the BnSTG.

Purely speculatively, it's possible that the battery charge level was actually fine. However, the vehicle measured that the battery was being discharged continuously at 10-20 amps. The vehicle couldn't detect that this current was being supplied directly to the battery from the charger. Consequently, the vehicle assumed a long-term discharge of the battery, displayed this information, and shut off the infotainment system.

Das das Fahrzeug dabei "BatterieSPANNUNG niedrig" angezeigt hat, würde ich nicht auf die Goldwaage legen. Solange die Spannung dabei nicht extern gemessen wurde, würde ich dem Auto den Wortlaut nicht glauben. (Ladestrom und Entladestrom geben wesentlich genauere Auskunft über den Ladezustand als die Spannung. Wenn das Steuergerät wirklich erst abschalten würden, wenn die Spannung deutlich fällt, dann wäre danach kein Motorstart mehr möglich. Das STG geht daher primär über den Strom. Die FeThe warning message, however, must be short and easy to understand for the average driver.

Regards,
Guste.

"Thanks, that's probably the point. I had already suspected something like that, but I wasn't sure whether the actual voltage is used as a reference for the system shutdown, or if it's a calculated value. I'll check the user manual to see if it provides any information on that."
Thank you for your input.