Third Rail Slim Protective Case for iPhone 4 and 4S Compatible withThird Rail Fuel Universal Rechargeable External Smart Battery Lightweight, sleek everyday protective case with a soft-touch feel Thinnest battery-enabled case on the market - Third Rail Smart Battery sold separately Sync/charge every iPhone 4 model without removing the case Third Rail Slim Case is compatible with all Third Rail Smart Batteries Third Rail System - Slim Case and Smart Battery bundle also available for purchase on Amazon.com