I have a 2003 1500 Chevy Silverado I replaced the alternator with a refurb. and the battery is 6 months old. my dash the battery light is still on and dash still reads "battery not charging" what is my problem?

 

confirm the voltage at the battery with the engine running. it should be in the range of 13 to 15.5V. if not check at the large post of the alternator.

it you below that, you should do some voltage drop tests of the insulated and grd circuits. eric the car guy has a good youtube video on how to test this.