Is there a problem in charging laptop all the time

I am curious to know whether there is enough technology to stop charging of the laptop when connected for prolonged hours. I never remove charger cable from the laptop. It will always be on mains. My friend insists me to work for some time on laptop without mains so that battery gets discharged. But, I never listen to him. Is it really important to charge and discharge the laptop battery frequently? If yes, how frequently? If yes, up to what percent the battery should be discharged?
