I have posted your question about the battery replacement on the Mercedes Benz Club of America website. Hopefully someone can provide specific details.

THANK YOU!!!

__________________
2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)

Just to add a bit of info for future battery-changers;I found this out on my 2015 ML350:

YOU CAN DISCONNECT THE MAIN BATTERY FOR AT LEAST 8 HOURS AND NOT LOSE ANY OF THE MYRIAD MEMORY SETTINGS.

I unplugged my engine computer to access the spark plugs, and decided it would be wise to disconnect the battery first.
Just reached under the seat and popped off the plastic cover and the negative terminal - just going that far was really easy.
When I hooked it back up 8 hours later I didn't have to reset windows, seats, or anything.
Only thing that reset was the trip computer - no biggie.
I assume the small auxiliary battery kept everything alive?????

Anyway, appears to be no need to provide external standby power when changing the main battery.
Now, what happens when you change the little battery in the rear is anybody's guess.

Cheers,
DG

Adjust the seat forward and up all the way, remove the push pin plastic rivet
and slide the vent to the right and up out of the way, removed that long cover over the battery by lifting and pulling rearward, then remove the battery hold down bracket a 13 mm socket is needed, then disconnect the negative battery cable , then use a 10mm socket on a long extension and remove the positive cable from the battery, then you can lift the battery out
I use a large pair of visegrips and clamp it to the negative post and heave ho, tilt it out , it weighs about 45 lbs you do not need to remove the seat, cutting the carpet is part of replacing battery, its no fun