Plan V, not just another battery back-up for you to leave at home or forget to charge, it is a failsafe charging device which is always on you. So small you won't know it's there, until you need it.
Plan V is a failsafe device you don't need to charge and is so small you can take it everywhere (on your keyring). It works much like a car jumper cable. You simply connect a 9 volt battery to the Plan V device and then hook it up to your phone. And since these are the square batteries you find in smoke alarms, they’re about as widely available as chocolate bars.
Read more on project kickstarter.