Stay unplugged longer with the ThinkPad T400s Series 6 Cell Battery. This rechargeable battery uses Lithium-Ion (Li-Ion) technology and can be used as a replacement for aging system batteries or as a convenient spare battery. (For battery life, go to lenovo.com/alldaycomputing)Lithium-Ion (Li-Ion) technology Six-cell, rechargeable system battery, 11.1 V dc, 3.9 Ah Standard system battery specifications Over-discharge protection