[safiu]

Bought my mercedes privately 6 months ago with the same message on the dash and all the convenience were truly not working. It did not bother me as it was summer. now all of a sudden the battery drained over night, i replaced the battery with starting voltage of 12.8 but it quicky drained it. My worries is that the alternator and the battery appeared newly fitted.
How do i check if the draining was not caused by somethine else.

 

[Tschuss_Bill]

Assuming the alternator/voltage regulator are fine, could be the battery control module or a function did not 'go to sleep'. There are some DIY diagnostics that you can do with regard to the latter, but not helpful for intermittent problems.

 

[philesk]

A new fully charged battery should measure 12.8-12.9v. with car not running and all options turned off. With the engine running, a good alternator should result is a battery voltage reading in excess of 13 v. Mine with new battery was 13.18v.

Some of the newer cars will just shut down if battery voltage falls below 12v. Older cars will often crank with voltage as low as 10-11v.