The Physio-Control CR Plus LIFEPAK AED and LIFEPAK EXPRESS AED function slightly differently from other AEDs. Instead of a battery, these AEDs use the Physio-Control CHARGE-PAK battery system, which charges an internal, permanent battery inside the AED device. After an AED use or two years (whichever comes first), the battery charger and electrode pads both need to be replaced. The Physio-Control LIFEPAK CR Plus/EXPRESS CHARGE-PAK is designed for use with the LIFEPAK CR Plus AED and LIFEPAK EXPRESS AED. *Currently On Backorder.