MIT researchers develop electrochemical-based batteries able to harvest power from a wide variety of human motions and activities-- in other words batteries one can charge by running or even walking.
Motion-charged batteries already exist, but these are based on the tribolectric effect (the principle behind producing sparks by rubbing a balloon against a wool sweater) or piezoelectrics (crystals producing a small voltage when bent or compressed). Such principles work well with high-frequency motion sources, but are limited by regular human motions. The required materials can also be quite expensive, and have high electrical impedance and bending rigidity.
On the other hand the new MIT-developed batteries promise to be inexpensive to produce at large scale while demanding less motion to actually produce power. The technology is actually similar to lithium ion batteries, since it consists a porous polymer soaked in liquid electrolyte sandwiched between 2 thin lithium alloy electrodes.
When bent, the "sandwich" squeezes lithium ions through the polymer. It also produces a counteracting voltage and an electrical current in the circuit between the two electrodes-- a current one can use to directly power other devices. A "small amount" of bending can produce a voltage, and MIT says adding a tiny weight to the end of such a device bends the metal through everyday activities.
Oddly, unlike batteries and solar cells the output from the new system is in AC, with the flow moving first in one direction and then the other as the material bends one way and back. The system is also "very stable," with little performance reduction after 1500 test bend/unbending cycles. Current system efficiency clocks at around 15%, but the researchers say it can reach up to 100% since, being electrochemical, “it is not limited by the second law of thermodynamics.”
The technology is still in early days, but it appears to be scalable and useful for a variety of applications-- including a future motivation for some much needed exercise...