So what's going on inside a wind-up cell phone charger? How does the simple turning of a crank lead to extra juice for your phone's battery?
To understand the basics of wind-up cell phone chargers, it helps to look at a much bigger, yet surprisingly similar, technology -- wind turbines and wind power. When we use massive fans to generate electric power, we're relying on the motion of wind. In the simplest terms, the blades of the fans capture kinetic energy, or the energy of motion, from the movement of the wind. As the blades spin, so does the shaft that the blades are attached to. As the shaft spins, it creates rotational energy, and it transfers this energy over to a generator. A generator, in the case of a wind turbine, is simply a set of magnets that spin around a coiled wire. The magnets spinning around the wire create an electrical current, providing us with power.
A wind-up cell phone charger is like a miniature wind turbine, except instead of using wind to power a generator, your arm and hand provide the kinetic energy necessary to move the charger's crank and add battery life to your phone.
Although there are various types of designs, most wind-up cell phone chargers use a similar design and basic principles to generate sufficient power. A common element among these devices is a simple crank, a graspable lever that we can easily hold onto and turn in circles while holding the base of the charger. The crank usually has an easy-to-grip handle to make it simple to hold and spin faster. The center part of this crank is a short shaft -- remember, just like a wind turbine. The kinetic energy from your arm and hand is transferred to the shaft, which turns into rotational energy as the shaft spins. Connected to the shaft is a set of gears, which transfers that rotational energy further to the charger's generator, creating anywhere between 4 and 6 volts for your phone's battery.