Other than the reported random electronic issues, I did see the battery icon pop up on my dash (not idrive) once. This was a few months ago, also an '08. It happened when I turned the engine off but left the radio on - which is not something I do often. I was just wait for someone to come back. It turned off a lot sooner than it normally does (15-20mins) - this time it was like 5min. So I push the start button again to turn it back on and shortly it went off again but with a battery icon. Maybe you should try that simple test. The battery icon is unmistakable.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr