One other limitation is that neither of the current U-Charge models works with the batteries for the latest (unibody) MacBook and MacBook Pro. A FastMac representative told me the company is currently working on a new version for these models. Although the U-Charge's battery-connection cable is interchangeable--a design that, in theory, lets you change just the cable to gain compatibility with newer batteries--it's not yet clear if either of the current U-Charge models will work the latest MacBook and MacBook Pro batteries via a simple cable swap.