I have a 2008 X5 3.0D and it would seem that the s/ware on this vehicle is a little flakey to say the least.
I got the Battery Discharge message this morning for the 1st time..... so lets wait and see ..
Initially I had the issue where the trip meter (not the odometer) would sporadically reset or increase more than the odometer. it took BMW 7 months and 3 re-programming sessions later before they admitted that it was a "known " issue and it was fixed in the Sept 08 release of their software. So it would not surprise me that the Battery issue is also a software "un-documented feature"
Please let me know if you've experienced similar issues.
Hans (otherwisehappyBMW owner)