I concur with Woody on this if you've had the battery tested by Batteries Plus. There must be a small current draw from the car while sitting idle. As Woody stated, this can be verified fairly easily. If you don't have the knowledge to do this, take the car to a local independent service shop that handles Mercedes-Benz cars & have them diagnose your issue. I'm not saying to avoid a dealer for this (most of us don't take our cars to a dealer), but if you can find a M-B dealer that would look at your car (quite a few won't look at Crossfires), I would recommend that over going to a Chrysler dealer. An independent M-B service facility is probably your best bet.