Get a "Battery Buddy" for $30 or so at your friendly auto supply store, charge it up and keep it in the back. Then if your X3 lets you down you have a ready made jump to get you out of the long term lot and back home. Practice hooking it up in the daylight using the terminals in the "box" located at the righthand side of the engine compartment (read the manual) so it won't have to be a "learning experience" at midnight during a snow storm. BTW your X3 shouldn't be discharging the battery this way, you almost certainly have something wrong that your dealer/shop can't find. You need to get another shop's opinion IMHO.
Last edited by UncleJ; 01-22-2010 at 08:24 AM.