If you have a bicycle dynamo, your device can be as easy as going for a bike ride in the park. The PowerCycle by Emile Nijssen and Joel Edelstein is an external USB battery pack that gains energy when hooked up to your bicycle wheels.
Electricity-free charging is a new concept in the external battery industry. The PowerCycle also has an eco-conscious aspect since it feeds energy off of simple pedaling motion. This product also lets you stay active while charging it; you won't ever have to sit and wait by your wall charger.
Once fully charged, this compact battery acts as a power source for any electronics using a USB cable. If your phone is ever low on battery while on the road, you can hop on a bike and rely on the PowerCycle.