Early this Spring I went to start my wife’s MB and the battery was dead. I jumped it and it started but since then, when I leave it setting in the garage for a couple of days the battery goes dead again. I put a new battery in but the problem remained so I took it to the local MB Dealer and he ran a “drain test” for two days. They couldn’t find anything wrong but did charge me a few hundred for the experience. Anyway, it still does it. If I don’t start it every couple of days the battery will go dead. Any suggestions?
Phantom battery drains can be time-consuming to track down and many mechanics don’t have the experience or the patience. I recommend doing it yourself.
http://www.diagnosticnews.com/featured/parasitic-battery-drains/
Here’s another helpful article:
You may have to find a better dealer, or a better mechanic
While it is possible that a control module is intermittently “waking up” overnight, I’d wager that your problem is more simple.
I’ve had many cases where the draw is actually the radio or the cluster, even though they were off and the display wasn’t on.
A few months ago, I fixed a Ford focus with a draw. The a/c on/off switch was the problem!
My point is you have to be willing to consider all sorts of things, when it comes to parasitic draw
How old is the battery? Over 3-4 years, and the shop tests show no unusual phantom current drain, then a failing battery would be my first suspect.