Dimmed Screen

I always plug it in when I play my sims (I play for hours on end) 
I did bring the game up without the plug, it stayed bright, minimized and went back (IK some people have a problem of it going dim) and it did still stay dark.
Windowed screen usually stays bright but my laptop keeps changing back to fullscreen when i do it plus its a lil annoying anyway.
I don't know what settings make the screen go darker with the plug in (I thought I did all of those already)