Powerful Lithium Polymer Rechargeable Gomadic Brand Battery Pack High Capacity Portable Charger Compatible with Microsoft Zune HD with upgradeable TipExchange Technology Integrated circuitry tailors output voltage and charging current to exact specification of the Microsoft Zune HD (accurate to 0.1v). Very easy to operate. Simply connect Microsoft Zune HD, press activation button on pack, and charging begins. Quick start guide included. Specification: Internal Battery (7800mAh lithium polymer), Input port (microUSB), Output Port (Standard USB), Onboard Circuit Design (Microsoft Zune HD Specific), Dimensions (2.75 X 3.25 X 0.75 inches), TipExchange enabled (Yes) Built with high capacity lithium polymer rechargeable battery technology (28 Wh / 7800 mAh) in a compact design. Elegant, compact design is approximately the size of a standard deck of playing cards. Rugged exterior is built to last and backed by Gomadic Lifetime Warranty