Car is now 5-6 years old with the original battery. What I've noticed is lately when in park after going for a drive, probably after 5 minutes of listening to music, I get "COMAND will shut off in 3 minutes". This never used to happen before, so I'm wondering if that's the main battery that's powering the audio, and not the auxiliary one? Also, what is the auxiliary battery responsible for (that has also never been replaced)? It's also not in my trunk where the spare wheel is, so I guess it's somewhere under the passenger footwell. I've never had any problem starting the vehicle though.

 

from what I understand it controls the start stop and I’ve also heard it allows to the car to go into nuetral if the main battery dies. I’ve heard it can affect you command unit as well. I just changed mine only because the car told me too lol but from what I’ve read and heard that’s the main function of the aux battery. Mine is a 14 550 rwd and it’s by the trunk and if it’s not by the trunk I think you are right it’s by the front driver footwell, easy to change I got the battery from Mercedes and easy install myself, no more light

 

I think the aux battery is for the S/S and also all the electronics these cars have.

 

If the aux battery isn't in the trunk, it's under the dash on the DRIVERS side. Search the site for threads on it.

 

S/S? So it's the aux battery that would be powering the COMAND system when the engine is not started?

 

Start/stop.

 

This prompt is a normal part of the operation of the vehicle; in addition to it being in my 2016 CLS, I had it in my 2013 E-Class.