From what I remember from my schooling, the pcm dictates the how hard the alternator works based on the needs of supporting the electrical load in the vehicle. For example, if you have your stereo, a/c, lights, blower motor, ect. All working at the same time, the pcm will tell the alternator, "hey work harder, we need it." but when there isnt a need for heavy loads as such, the alternator shouldnt be working nutso trying provide the electrical energy it needs.
This is all processed through the voltage regulator in the alternator too. Once the voltage ragulator gets the message from the pcm that it needs to provide more or less enegry, it does so. But if the regulator has gone haywire, which is common, it will often times, do the opposite of what the vehicles needs, ie. Provide way more enegry, or not enough.
If you are getting over 15v from the alternator, to me, it sounds like the regulator has gone haywire, but there is a way to test this, called full fielding. Its model specific but idk if ours cars has it. May be worth reserching. In the mean time, have you checked this video out? May help you if your diagnosis.