Why do games run better off my laptop battery and choppy when its plugged in?

when my laptop is plugged in it runs games all choppy. like playing hellgate it seems every step forward i take my character stops for a split second. but when i have it running off battery its smooth as ever. same with other games like world of tanks. it runs at 12 fps while its plugged in and 30 fps off the battery. is there any way to change it cause its annoying only playing for 2 hours and then needing to recharge it for 2 more.