Hi Markku
Did you get to the root course of the issue on your Q5? I have the exact same issues. Voltage dropped and car went on safe mode. The battery was 8 years old so I decided to change it. coded it to the code and all.
The only exception in my case is at time the battery is charging. Like on the 3 or 5th start it seem to charge at 13 to 13.7 Volts.
Thanks