If you happen to be reading this on your iPhone or some other power-hungry device, chances are you are about to run out of juice. Soon, though, you may never have to plug in again. After an early wave of enthusiasm for bulky backpacks with solar-powered chargers, the new bet is on kinetic energy, or energy that can be generated from movement, including the stride of a walker or the turn of a bicycle wheel. The beauty of kinetic energy is that the more something weighs and the faster it moves, the more energy it can generate. Companies want to unlock that energy. In February, Nokia patented technology for a phone able to charge itself from the energy produced by a moderately paced walk. Several start-ups are introducing their own devices. Here's a look at four young companies and their products.

High Tide

Founder Albert Hartman was working on a kinetic energy charger for the U.S. military when he realized that his concept could easily work for consumers. He designed the RollerGen, a generator that sits on the back wheel of a bicycle. A lever moves the generator on or off the wheel, increasing or decreasing the resistance and producing as much as 30 watts of power. A 30-minute ride charges the RollerGen battery, enough for three iPhone charges. The system retails for about $500.

Biking time to charge an iPhone: 10 minutes*

Bionic Power

Bionic Power's device straps onto the knee and converts the movement and mass of the human body into energy. A one-minute walk produces enough energy to provide a cell phone about 30 minutes of talk time. Bionic Power has a contract to develop the technology for the Canadian military, and the company sees potential in supplying disaster response workers as well as hikers. The device also holds potential for prosthetic limbs, which increasingly require battery-supplied electricity.

Walking time to charge an iPhone: 25 minutes*

Lightning Packs

Muscle physiologist Lawrence Rome usually spends his days studying the way fish move and frogs jump. When the U.S. military asked him to find a way to convert movement into electricity, he came up with a backpack that slides up and down a frame as the wearer walks or hikes along a trail, generating electricity. Rome has two contracts to develop a military version of the pack and hopes to have a consumer product ready to sell within two years; it will retail for more than $300.

Walking time to charge an iPhone: 30 minutes*

Tremont Electric

While backpacking the Appalachian Trail, CEO Aaron Lemieux had a lot of time to think about the energy potential of the pack bouncing on his back. He also had the mechanical engineering background to harness it. His nPower PEG is a charger that can be thrown into a purse or backpack to capture the energy of walking. The PEG, which will cost about $150, can also be charged via an outlet. Consumers have ordered more than 2,500 of the devices, which will ship this spring.

Walking time to charge an iPhone: Several hours


The Line: None of these players have made it out of the gate yet. The nPower PEG, which is expected to come to market in June, has an eco-conscious design and the lowest retail price. But because it is small, the PEG does not produce enough energy to quickly charge gadgets that use a lot of power, like iPhones. High Tide's RollerGen produces the most energy but works only on bicycles. Lighting Packs and Bionic Power may have the right strategy by developing products for the military before introducing them to consumers.