Similar problem with my 06 X5, no warnings or nothing. I connected a Voltage indicator & the alternator only charges when volts drop to about 10, it then charges ok for about an hour & then stops charging again until it drops to 10.
Have ordered a new voltage regulator & a new alternator - hopefully its just the voltage regulator and then I will have a spare alternator.
Cheers

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X5 4.4i 2006 sports package, Titanium silver, Sports exhaust with Xpipe, Limo tint, 20" wheels 168's (Black), 25mm H&R wheel spacers& arch flares all round - Loud........

Work in progress -:
Replace BMW Ipod system with DICE - installed but no sound yet??