I'm not even going to tell you how many KM we have on our E90 335i (it's ridiculously low). However, we still have the original battery on our '07, and I use the battery tender whenever the car will be sitting for more than just a few days, even in the summer driving months.
2015 Porsche Macan S
2011 E90 M3 Alpine White on Fox Red
2007 E90 335I Graphite on Terra