Change a normal 9v battery into a life saver. If you have ever run out of juice on your phone and could really use a quick charge then you need a USB battery.
1 5.1v zener diode
1 330-ohm resistor
1 9v battery connector
1 9v battery solder and soldering iron (you can get all of this at radio shack)
1. Solder the Zener diode to the left and right USB connector prongs.
2. Solder the black wire of the 9v connector to the left prong and the right wire to the resistor.
3. solder the other end of the resistor to the right prong.
4. Hook up battery and go to town.
For more, or to get started building your own 9v USB battery, watch this how-to!