Alternator problem

[masurati]

Hi have 05 x5 4.4. When i first start driving in the morning in about 10 minutes
I get 4x4 light and flat tire lite. I had codes read and they point to voltage being low.
So i started to monitor voltage by a voltmeter.
When i start the engine in the morning a voltmeter shows 12.55. After about 10 minutes i hear some noise from the alternator and voltage jumps to 14.5. At this point engine rpms drop momentarily. Feels like the alternator was not engaged and suddenly it got engaged. If is start driving now i never get 4x4 light. Any idea why is it engaging so late? Could it be that the belt is loose or is there
A built in mechanizm in the alternator to activate/deactivate it? Or is it just alternator gone bad?

Once it "activates" and voltage reaches 14.5 it is good for rest of the day. I can shut down the engine and then start it and voltage is good. It only happens after the car has sat overnight.

Really looking forward to hearing from you
Thanks