uNu Exera Modular Detachable Battery Case for iPhone 4S 4 - White/Purple Innovative Detachable design: Double the battery life for iPhone 4 4s without adding bulk. Choices of 8 Color bumpers, choices of black and white back house. A distinct and stylish case for everyone. 1700mAh Swappable Battery Module, which enables unlimited power with addtional battery cells (sold separately) Exera adds additional 7 hours of talk time, 10 hours of music time, 6 hours of web browsing, and 270 hours standby. Smart charging switch. Accurate remaining power indicator. 1 Full year warranty from manufacturer.