vettebk

W221 is particularly finicky about low voltage. When it senses the rear battery voltage is too low, it sheds the loads by arbitrarily shutting things down. I suspect that rear battery is not fully charged. The cause could be a bad alternator, a loose serpentine belt, bad tensioner or pulley which is draining the new battery. You may also have a parasitic drain especially if the car refuses to go to sleep after three or so minutes.

Best procedure is to charge the rear battery fully, start the engine,check voltage with a VOM or with a scanner plugged into the port. You need 14+ volts when running. You need 12 volts at rest. Check to see if the car goes to sleep (you should see a voltage change). Then report your results back here.

I am betting your alternator is suspect. Good luck.

Arrie

Where do you get the misinformation that the voltage needs to he 14+ when engine runs?

My both cars, 2010 E550 and 2012 S550 show voltage below 14V most of the time.

Above 14V happens if very heavy electric demand is present, like when the FOG light is ON or rear window defroster runs.

The other occurrence for above 14V is when slowing down in traffic. This is due to the smart charging system.

BOTUS

most cars should reach about 14.2V with the engine running - but it can vary - mines hits a steady 13.9V at idle if you start it

a fully toy laden twin battery 221, uses 37 amps key on (engine off, exterior lights off) so few batteries will ever be happy for very long