Alternator charging kinda?

Are you absolutely sure the belt is not slipping?

Have you one pulley that you changed between them? If so, any chance the pulley is too thin and not actually tight on the shaft?

Describe your wiring? Trunk mount? What sort of circuit, IE does the alternator go through a solenoid / relay to the battery?

Can you check at moderate RPM check first battery voltage and then right at the alternator stud, and compare the two voltages?

Repeat with a load on the system, IE fans, pumps, lights, etc.

A difference of more than 1-2 volts with the battery a little low means you have a bad connection/ voltage drop between the alternator and battery