Very dumb question as a disclaimer but please, just try to answer the question lol. How long can one listen to the radio or use the COMAND system while the vehicle is off before worrying about draining the battery to a degree where the car will not start? I had a load test done on my relatively new battery in my 2007 E550 and it checked out - so, I've heard anywhere from 15 minutes to an hour? Any explicit answer? I do know that some batteries have a rating that alludes to the number of hours it can be utilized while not being recharged by the alternator.

 

Most newer benz's will display on the command that the radio will turn off to ensure the vehicle will start. Basically the system won't let the radio drain the battery to a point where the car won't start. I am not sure if your 211 has that or not though.