Third Rail Trio Slim Protective Case and Fuel Universal Rechargeable External Smart Battery Combo Kit for iPhone 4 and 4S Universal power solution to charge 100s of devices (mobile phones, Bluetooth headsets, MP3 players, MiFi Cards, gaming devices, etc) using the included adapter cable; Up to 6 Smart Batteries can be stacked together, creating a power hub that allows you to charge multiple batteries and devices through one power outlet (AC or PC) Contrary to bulky traditional power cases, in conjunction with any Third Rail Case, the Smart Battery can slide onto case when you need power and slides off when you don't The low profile and feather weight (1.4 ounce) Smart Battery is one of the most compact charging solutions on the market, making it perfect for on-the-go power Innovative Third Rail technology allows Smart Batteries to be stacked, automatically consolidating available power among partially charged Smart Batteries Easy-to-read indicator lights let you check the battery level of the Smart Battery with the touch of a button; Contains a 1250 mAh Li-Poly Battery Thinnest battery-enabled case on the market Universal Smart Battery slides onto case when you need extra power, slides off when you don't The Smart Battery can function as a standalone backup battery to power the phone or other devices via USB adapter (included) Smart Battery is compatible with all existing and future Third Rail cases - use the same battery even when changing devices Easy-to-read indicator lights let you check the battery level of the Smart Battery with the touch of a button Battery life will vary based on usage