Why is my alternator overcharging?

I have a 2002 Ford Focus with only about 64k miles. About 3 weeks ago while driving my battery light goes on.Immediately I assume my alternator is shot. I borrow a friends jump box in case with plans to have it tested after work. The next day while on my way to work I noticed the battery light would sometimes disappear randomly for less than 30 seconds then come back on.So I have it tested, and to my surprise both my alternator and battery passed. However the employee informed me that even though it passed the reading on the alternator read 15? Which is apparently too high.Being a parts store no one knew what would cause this. We verified the right battery that s less than a year old. I ve been driving it short distances.To work 5 miles away etc. And it was fine for about 2 weeks.Light still goes on and off. But sometimes while waiting at a stop light or the car is idling the car will vibrate slightly. Now just Recently I drove my car from my home to a nearby gas station less than a mile away. When I started the car to leave it immediately died. The car started right back up and all was good.It s happened 2 other times but when I first start it not while I m waiting at a lighy etc. And it starts right back up.So my question is what needs to be checked,replaced, or looked into. The alternator is good but something is telling it to send more power than needed.

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