Happy new year to you guys.
I fully agree with Joe (thanks for your help) not to replace anything if not defective.
I did some observations during the last days:
1) Production date of the battery is August 2016 => it has already more than 5years.
My car was delivered in Mai 2018.
2) Voltage drop
- I measured 0,5V drop in 30seconds after pressing door unlock key, charger not connected. I assume during these 30sec the draw will not be that high as car unlocks the doors, switched headlights on and energises the display inside the car
- when doors are locked (no charger connected), the voltage drops ~0,5V in around 1 day (30mA draw as mentioned by Joe I assume)
- one start of the car is still possible, the voltage drops from fully charged 13,6V to 10,6V in this case
I didn't find a possibilty to open a wire from the battery to measure the current as proposed. I do not have a current clamp which would allow this.
This leads me to the conclusion that my battery needs replacement as it doesn't keep the charge. As a final check I will connect a healthy battery in parallel and see what happens with the voltage drop.
I noted that accessibility to the front fixation screws of the battery holder is "more than terrible" as far as I can see, so my dealer will get the job.
Hope this helps other members to evaluate their coming battery issues a little bit, sooner or later it will happen.