'88 Fiero Formula keeps losing battery power

The car is a 1988 Pontiac Fiero Formula…new alternator, new battery recently, and now the battery is showing signs of weakening again (squeaking belt, turn signal weak and gauge in dash indicator is lower.)…Any ideas about what the problem could be?

Get a multimeter (harbor freight, under $5), learn how to read DC voltage and measure across the battery with the car running. It should be 14.4 volts or there about.
It may not be the battery or alternator - You could have a bad connection on the battery or from the battery to the frame.

From The Easiest, Cheapest, Most Obvious Department:
That Squeaking Belt . . . It’s A New One That Was Replaced With The Alternator And Correctly Tensioned, Right ?

I’m just trying to eliminate some possibilities, the easiest, cheapest ones. I believe this vehicle uses a V-belt. Old belts (especially slipping belts) lose their grip and so do old pulleys.

I didn’t see belt listed with alternator and battery and saw “squeaking” in the post so I had to ask.
Also, some of the Fiero alternators can suffer damage from prying in certain places to tension the belt and have a square lug that takes a 3/4" (I believe) open-end wrench for tensioning.