I do not live in an area that is as cold as some of you. But I too had a battery issue with my S500. My mechanic asked me, after all the goodies had been reprogramed back into the computor, if I lock the car while it is parked in my garage. It is not a daily driver as my 500SEC is.
The question seemed kind of odd, but he said if the car is at rest and not locked, the systems within remain "awake". In other words, there is a greater draw on the battery when the car is off and unlocked. Locking the car when not in use will keep the battery performing at it's best. Taking into account you have a good alternator too.

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Selling used Mercedes parts on Ebay...always changing...
cdplayer51 take a look!
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