Alero 3,4L charging problem

Hi, first of all thank you for helping me. Here's the problem, the battery's voltage drops while the cars is running. I'm using a brand new battery (in fact I tried 3 different batteries) and I just bought an alternator from a scrap yard. The guy there tested the alternator and told me it was charging at 14,87 volts... I verified all the connections and the pulley and everything seems to be ok... All the electrical components in the car (radio, windows, door locks, lights, ect.) are working. I checked every fuses in the three fuse boxes and none is broken

Here are some things to consider:
- The car remained park for the winter outside but got started every here and then
- Because the car wasn't starting (battery was dead) I boosted my car, but I accidentally plug the wires backward on my old battery (red on black and black on red) and white smoke came out of my hood). I then changed the alternator and battery after...

My car:
2003 alero gl2
v6 3,4L
stock tranny
stock everything

Welcome to AF.

What voltage are you reading while the engine is running. What does the voltage drop to or the low? Use a multimeter and go across the battery positive and negative terminals. Is the instrument panel alternator/charging system light on?

The battery isn't charging at all, so it depends on how much I charged the battery with my charger before. Usually I charge it to 12v and when I start the car, it start to go down and at around 9v
I stop stop car. After the car has stopped, the battery voltage goes back up to almost what it was.

oh and no lights are on