Replaced the alternator on a 1989 Ford F-150 but the battery will still not charge. How do I fix this?

Silly question but did you check your battery?? I have put a high out put alt and still no charge battery could be toast or even a bad ground

It was charged to 90 percent before we replaced the Alt. Haven't checked the ground yet

Also, check the "mega" fuse between the alternator and the battery. It may be located at the end of the under hood fuse box. Be sure to clean both ends of the "B" wire coming from the alternator. Corrosion, here, can be a big problem.