I think your voltage is too high for an X5. The usual voltage is in the 13.85V range for the highest point with these alternators. I would suspect the regulator pack but would change the whole alternator if I were doing it. In your case with the 4.4 engine that is not a cheap repair.
GL

__________________
Sent via Telegraph Wires using my Morse Key.
2015 X5 xDrive35i MSport
2013 X5 xDrive35i