when playing video games your system needs full power to CPU and video card.
when not plugged into wall, your on battery save mode, which means CPU variable power and GPU Variable power to save as best they can energy.
when playing games (not Facebook games here, real games) and while on battery, then expect the battery to last maybe end up 1/3 time expected to last normally as cpu and video card will eat up power like candy
I recommend to plug in wall. when playing video games
It is ok to play games while games as long as the battery is working. If it has a habit of dying while on battery while doing normal things, get it replaced as this can make it worse while playing games.