Cell phone or mp3 portable spare battery

We always encounter the situation that mobile phone or mp3 player runs out of power. But as long as you have this PowerBlock Reserves portable back-up battery worth 40 dollars (about 273 yuan in RMB), you can continue to communicate with others. Oh, this back-up battery uses USB interface and is available for iPod, iPhone, or other mobile phones and MP3 players that can provide this interface.