hi, my values are from VCDS j644 MVB. I started to read many german forums about this issue and my new infos are:
- many prefacelift A6 with j644 have the same problems with weak charging, also MVB data from other people were similar to mine, (weak DF signal, only 14,3V specified voltage)
- facelifted A6 has battery management embeded in gateway and there are no problems. Also the battery coding can affect the requested voltage for regulator (14,7V if AGM, 14,3 if flooded...) also i saw 15V as a specified voltage :O and 87% DF signal (maybe during winter)

It really looks as a software issue, this told me also my mechanic, very experienced with Audi cars.