i had bought dell laptop 2 n half years ago, it still give me 2.5 hrs on battery. Battery life is not increased by discharging n charging. When u r using AC power, if battery is 100% charge, the battery will not get charged further n so is not overcharged, the AC will then be used as direct power supply to laptop. So better keep AC power plugged in while using laptop.

 However, when u put on hibernate or stand-by, then it's better to disconnect AC supply.