The standard battery is a size D lithium battery, and should be replaced using the information detailed here. It is a 3.6v lithium battery. The preferred manufacturer is Xeno Energy. They sell the battery under part number #XL-205F (a secondary manufacturer also makes them: Tadiran, under part number #TL-5930 (TL-5930/S)). They can be purchased from several online retailers, including Amazon and Sears, as well as at battery specialist stores.