The Lenovo ThinkPad 81 Battery is a 6 cell Lithium-Ion battery which is often used as a replacement or as a spare battery. The battery lasts for 44 watt hours* on average.  It charges via the machine, as well as the external battery charger (40Y7625, not included) *Battery life varies due to many factors. These include: applications, battery conditioning, features, screen brightness, power management and other customer preferences.