I am experiencing a near identical situation to the original post. I was finding my battery dead during the winter if I was gone for a week and figured it was time for a new one anyway, so got the middle priced Autozone one about 2 weeks ago. Then yesterday after a morning of stopping and starting the car from garage-sale-ing the x5 would start without a jump, but then it would run perfectly (making me thing it wasn't the alternator.) Since then the battery won't keep a charge even with a slow charger on it overnight, but the lights/alarm/gate all work - just not enough power to start the thing without a jump.

So tomorrow I have to start troubleshooting this thing. Looks like I need to start with a trip back to autozone and get some readings.

As a side note - is there a way to turn off the climate control? When I am trying to minimize battery drain it is frustrating to hear the fan running and the gps screen start up. Not helping!