I would consider one of these...

http://www.************************/ProductPage.aspx?ProductName=Priority+Start&productid=105113&producttype=10

Will not let the battery drain, it wil leave you with enough power to start the car, so if you leave things on, and the battery gets low, this will cut off the drain and still allow the car to start. I suspect it holds voltage.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U