Charging System Problem

What is your question?

Hi, I've recently gotten a 2007 Chevy Colorado with 37,000 miles on it. The truck runs great, however I noticed on two occasions it was rough to start. I correlated those two instances, which were the next time I drove if I had sat in the truck or had the interior lights on after turning the ignition off. I went to Advanced Auto Parts to have the battery and everything checked, and the battery came up good, however it said there was a charging system - high voltage. With no load, it's running 15.46, with a load it's running 15.31. I changed the alternator, hoping that would fix it, and I needed to change the belt as well so I figured knock it out. I got an OEM alternator. I replaced it, and got it checked again - the exact same issue came up. All of the wiring, from what I can see, look clean. Any input would be greatly appreciated.

What makes this problem better or worse?

If I have interior lights on at all after turning the truck off.

How long has your 2007 Chevrolet Colorado had this problem?

About a week, just got the truck.