There are a bazillion solar-powered portable batteries on the market. But they have this little problem: they need the sun in order to work. Inventors and engineers, seeing the need for portable power generation that doesn't require daylight, have been hard at work coming up with some creative ideas for alternative energy sources. Let's call them the Earth element batteries (or just call them awesome). Now you can get a portable battery powered by wind, water, fire, and even mud. Here's the science behind how these mini-generators work.
The FlameStower is a portable device that uses temperature variations to generate electricity. It's based on a simple principle called the thermoelectric effect. To put it in the most simplified way possible: all you need is to put two materials that are effective at moving electricity next to each other and add an electricity-capturing device on one end. Then you heat one side and cool the other. Electrons move from the hot side to the cool side (because they like to be where energy is lower and heat has a higher level of energy, a concept you probably know as diffusion). As they travel into the cool side they release heat energy and voila! You have a battery. Yay physics! This method of power generation is regularly used to power devices in space, where it's easy to generate heat naturally with a decaying radioactive material while subjecting it to the extreme cold temperatures of the vacuum outside.
The FlameStower generator works over any flame or heat source (a cook stove, a campfire, or even the stove in your kitchen). You simply put one end of it over the heat, pour some water into the cold side to keep the temperature there low, and plug in any USB device. They even have a version that can charge your gadgets using a candle. Depending on how powerful your flame is, the FlameStower can produce about 3w of power, which its makers calculate out to about two to four minutes of talk time on your phone for every one minute of charging. You can get one for $70 on their website and their candle charger, which will cost $99, is expected to be available soon.