I seem to have a somewhat related issue with my '99 E300 @ 153k miles. The car runs fine with no apparent charging issues as long as I run it daily. After several days of being away and not driving the car, I went to start it and there was absolutely no battery power....the interior lights would not even illuminate. The battery was about three years old so I replaced that first. Seems my issue may be more of a current leakage issue (electrical short or bad component somewhere) than a charging problem. As long as I run the car there is no problem. If I let it sit a few days the 6 week old battery is dead. Any suggestions appreciated.
Thanks,
Ken

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2014 E250 Bluetec 4-Matic
2014 GL350 Bluetec 4-Matic
1996 Chevy Tahoe 4 x 4
2009 ML350 (sold)
2005 SLK350 (sold)
1999 E300 Turbodiesel (sold)
1998 ML320 (sold)
1991 300D 2.5 Turbodiesel (sold)

Other members here have reported a few things that have caused them repeated battery drain.

Door Lock pump- Continues to run due to leak in the system
Trunk light- stays on with lid closed
Power Antenna- slipping and continuing to run (more of an overnight failure)

Usually they were found by removing a battery cable and placing a meter, on the amps setting, in series. Then pulling a fuse at a time to isolate the problem. Look for something pulling 1 amp or more.

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Sam

84 300SD 350K+ miles ( Blue Belle )