Have had a 2002 SLK for 3 months. A couple of times battery has flattened when I've left it for a couple of days. It recharges easily and restarts fine. Have had conflicting professional views as to cause, including seat memory, which it hasn't got!! No one says it's a dud battery though. Mercedes garage says something, and they don't know what, is draining the battery and it needs further testing as there is a fluctuating load across it. Anyone else had this problem to short circuit diagnosis and save garage bills?.

 

could be the PSE unit in the trunk by the spare tire...it should be covered with soft styrofoam. they get water in them cand can kill the battey and many other things. are the interior lights working correctly? start pulling fuses for certain devices and see if problem stops.