I don't know about PCs in general, but when I had my older mac, the one featured below (without the new i7 that my new one does) I left it plugged in all night, and the battery health was still above 95% after a year. However I did use it everyday until it was dead at school or while working, so that might have something to do with it. I honestly think that if a cell phone or my BB gun has a smart charger, (meaning that it stops charging after the battery is full and only supplies enough power to keep the computer going), that a laptop would have one, and therefor not be affected by having it plugged in all the time.