2003 Oldsmobile Alero 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