Hi Cubex...welcome to the Forum
I would check the battery condition with a heavy discharge tester to determine the strength / capacity and if necessary replace with a quality battery.
I believe there are two batteries on the A6 one in the engine for the powertrain and one in the boot for the body control....if so, check both because this can affect the balance.
If all is okay there, then suspect the alternator
Let us know how you get on with this one
Cheers Trevor