This post as it happens is similar to one just below but a few pointers may help. 2.0tdi A6 was draining battery totally flat in 2017..so flat it had not even a warning light. I changed the battery and it seemed to remedy the issue although it has to be said the battery I still use a a jumper in the garage, it may not be as new but it holds charge!

Now the car periodically flashes up the warning light for low battery, I can look into the MMI and whilst driving see it go up from say 70% to 100%. Next morning it may be 80% again.

The new battery may be faulty, it had onl;y 11.5V this morning.

However, what should the regulator on the alternator regulate the voltage to on a modern car like this one...I have tested its charging running with all electrics on and off and I feel it is probably not a high enough voltage?.