Pavilion zv6000 Monitor dims when charger attached

[Sizzor Bill]

Recently my HP pavilion zv6000 started diming the monitor when it's plugged into the charger. When I unplug the charger cord, the monitor brightens back to normal? It use to be the other way around (for energy saving I assumed). I can't figure how to brighten it back to normal when it's plugged in? Any advice would be appreciated.