There's a mobile power bank device out there called Quarter. It's pretty neat. You may have been confused by what it does if you've already heard about it. Let me tell you what it's all about. Quarter is a battery, one that you charge up - fill up with power, that is to say - then stick in your backpack for later. Later, you take it out, plug in the cord that charges your iPhone, and let this battery power up your iPhone instead of looking for an outlet.