Charging system stuck st 13.4v

[ReiSabre]

This may be dumb questioning, but I have an 08 grand prix gxp, ran fine and let it sit for a month with some on and off driving in between. At some point, charging system stopped working, and charging doesnt go above 13.4-13.5v, and won't give more when going lower from accessories or cold starts. I do have a b1516 code and current sensor has correct sensor replaced, only other things found are the ground wire to bcm from battery shows about 3-4v when reading, and not sure what the current sensor pins should read. It does charge higher and adjust if i clear the b1516 code until it comes back as well, and drops back to 13.4v. I'm still very much novice, so some insight and help would be great.

Edited 2 hours ago by ReiSabre