FWIW, I left my 2010 X5d (with no prior battery issues) in an uncovered parking spot at the airport for three weeks in June. Came back and it started from the first try with no problems whatsoever. I did remember to press the start button twice when I turned it off though, just to make sure EVERYTHING is really turned off, not just the engine.

Now that I've read the above posts, however, I'm thinking maybe I got lucky. So, guys, any recommendations for a specific battery tender that you've used and liked would be appreciated. Thanks.