My battery went on my 2005 Z4 after about 2.5 years. Car is a garage queen and only had about 12-13K. I do have a V1, icelink, alarm system, and a gps hooked in to the system. I've been using the battery tende+ once a month just to make sure that the battery stays charged, BTW, when my battery went dead and i was still under warranty, BMW said tough sh*t, you don't use the car enough to keep the battery charged, and they wouldn't replace it.