2009 535xi. My battery went dead while I was sitting in a car wash listening to the radio. After jumping, the car seems to run/charge fine. A friend suggested you can run the battery down by leaving a key in the car a long time while car is off. Is this true? I never leave the car undriven for more than 2-3 days, and, in Montana, just starting the car means at least 30 minutes driving. Why did the battery run out? How do you find out if the battery needs replacement BEFORE it goes dead. I'm 150 miles from a dealer.