I would try and get a quality battery maintainer.

Ctek is one of the best. The ones Mercedes sells as accessories appear to be renamed Ctek chargers. Amazon has better pricing. Don't worry about the amperage rating - these smart charges adjust to what is needed.

But do you need one for winter storage? Why not charge battery and then disconnect it? I do that on my two older cars and sometimes even on our 98 E320. At most you have to reset your radio in Spring.

I prefer disconnect option, because I hate leaving anything electrical plugged in that could malfunction and perhaps cause a fire.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5