I was driving the truck the other night; I accelerated and immediately after the battery light came on when the truck returned to a low rpm. However whenever I reach 3500-4000 rpm the battery light goes off and I get a good charge. So essentially the alternator will only charge at high rpm it seems and it is a new alternator ( about 3-4 months old). Truck has 100,000 miles

 

Alternator is bad, if it's only 4 months old it should have a warranty.