I have a 2002 X5 4.4L with little only 50k miles, my battery went dead 2 days ago, no warning, just wouldn't start in the evening. The car started just fine in the morning and afternoon. So if your X is around 50K use a volt meter and check the battery often. Anything lower than 12 volt with engine off, you should keep an eye on things. Anyway, I was in the parking lot and looking for a jumb, one thing led to another, the expansion tank pipe broke. Yes the little one on top of the expansion tank, that suppose to be made out of metal but BMW decided to save a buck and made it out of plastic instead. Well guys if your X is around 50k you may want to check it also. It get harden with heat and age, all it took was a little push and it broke in half, and when I picked it up to check it, it broke into 1/3. Nice huh? It is a regular stock item for my dealer, and he stocks a few of them too, that should tell you something about those pipe. Back to the battery, I got a replacement from Sams Club today, it is the Type 94 with 850 amp. It is $65, lowest I could find anyway. Here is the kicker, the Sams' battery book didn't list a battery replacement for the X, but it fit my X just fine. Don't forget there is a vent hose that comes with the battery, you will need it. The first battery I picked up didn't have one, the second one did. Also, my car die at idle and low engine speed during test drive. I thought something else was borken. Well after the test drive I stablized the idle speed with my foot and let it idle for about 15 minutes the problem went away. So if it happerns to you after you replace the battery, try idling it for 15 minutes. I guess the computer have to reset the value of the senors or something.
For the longest time I thought German auto engineering is the best in the world, i didn't realize how wrong i was until I do my own repair on them. I guess engineering greatest vs. bottom line, bottom line always win.