Jung-woo Choi is the designer behind the eco-friendly, dual battery, Echo Spin concept phone. Echo Spin's two batteries can be recharged by pressing the circular button and spinning it in a circle using the scroll wheel found on the iPod.
The two batteries of the Echo Spin are designed to be used for different occasions. The yellow battery is designed to power the phone's lower-end functions, while the black one is used for the higher-end ones. The batteries themselves have an innovative design, slipping over the phone like a protective case. While I love the ability to recharge the phone's batteries on-the-go, I do think the dual battery system would be a little cumbersome. Creating a command or a button that puts the phone into a low-power mode would be a lot easier than constantly switching between batteries.