17 R3, charges battery while gaming

Hi,

Alienware 17 R3
i7-6820hk
nvidia GTX 980M

Battery is fully charged and the charger is plugged in. Everything runs fine. When I start playing a game (but obviously only games that need a lot of power) the battery starts charging and FPS drop down. I noticed this issue in For Honor and in Quake Champions. But that are the only intense games I'm playing atm. I lowered down the graphic settings in For Honor a little bit and it works much better now. There are also little FPS drops in Rocket L. But there are no problems when playing Mirror's Edge (but that's an old game). I have this issue for approximately 2 weeks now - never seen that before. What could be the problem with my laptop?

Thanks,