I've always kept the RS6 outside and, with my work travel schedule, have left it sit for weeks at a time without starting it. Never had an issue with starting the car, dead battery, etc.
Just recently I put the car away in the garage since June 11 (20 days) and tried to start it. Battery was dead.
Is this a coincidence or do I have a weak battery that the solar roof has been keeping alive for me?