Now for the iPhone 4 and 4S (and subsequent phones by simply swapping out the Smart Case), the US$89.99 Third Rail System ( is purportedly the world’s thinnest battery-enabled protective case that allows you to add extra power when you need it, and remove it when you don’t.

Initially for the iPhone 4, and compatible with both GSM and CDMA models, the Third Rail System is comprised of a protective case (Slim Case) and a universal, removable battery (Smart Battery). The super lightweight Smart Battery slides on to the Slim Case when power is needed, and can also serve as a stand-alone universal charger for other devices (via micro USB).

Functioning as a protective case, a power case, a portable external charger and a power hub for charging multiple devices at one time, this universal power system replaces all existing charging accessories.