alternator is good but battery isnt charging

First make sure you have the alternator checked with a meter to make sure it is putting out enough voltage. It should be putting out 13 to just under 15 volts. If the battery is not charging I would say you have a bad battery.

Could be a wiring problem between the alternator and battery or a bad battery. How much is the charging voltage from the alternator with and without load? Zee