updated 12:00 pm EDT, Wed May 11, 2011
Battery attaches only as necessary
Third Rail has launched the Third Rail System, a combination of accessories for the iPhone 4. At its core is the Slim Case, which fits on any model of the iPhone 4. A slot on the back allows the connection of a 1,250mAh lithium-ion "Smart Battery," which can be removed as needed for charging, or simply to make the case lighter and thinner.
On each Smart Battery is a micro USB port, which allows charging the unit with an AC or PC adapter, or redirecting power to a non-iPhone mobile device. Multiple Batteries can be stacked together for charging with a single adapter, and will even redistribute power amongst each other to produce as many fully-charged units as possible. Power levels are displayed via an LED indicator.
A Slim Case/Smart Battery bundle for the iPhone 4 costs $90. Separately, the case is $40, while the battery is $60. Third Rail is promising to support more electronics in the future.