I have been working on my 1982 dodge d 150 truck with a 318 and my alternator is not completely charging my battery. It will run for a little bit but when I turn on the lights it dies with in 5 minutes. I replaced the alternator then took it back and had it checked, it was good. I noticed tha the belts didn't seem to get as tight as I thought they should but I put a screw driver on the back and it sucked it in so it is charging on the truck. So what else could it be?

Have them test the alternator while it's still in the truck. Also have them load-test the battery.