Make a Dynamo to Charge Your PhoneBy Phillip Stearns
|Voltaic System’s battery packs will take charge from solar panels, AC adapters, USB ports, even other battery packs. You can also charge them from kinetic power sources like hand-cranked or bike driven dynamos. Here’s how we did it.|
1 V11 USB Battery
1 DC motor (select for volts and RPM explained below) – available here
1 Diode (Rated for 2Amps, Schottky recommended)
1 mini USB power connector
1 crank (can be purchased from a hardware store)
|What’s a Dynamo?
A dynamo is an electric generator made from coils of wires that are made to turn in a permanent magnetic field by way of a commutator. Essentially any electric DC motor with a permanent magnet can become a dynamo. Rather than applying current to the motor to cause it to spin, you can turn the shaft of the motor and generate a current.
Selecting the DC Motor
We used a 12v 50:1 DC gear motor. The reason for the gearing ratio is so that we don’t have to crank the motor at 3000 revolutions per minute (RPM) to generate the power we need. Motors are rated for certain voltages and will give you the RPM for those voltages. Sometimes you’ll get a current rating as well. If a 12V DC motor turns at 100 RPM and draws 100mA of current, it’s consuming about 1.2W of power. When used as a dynamo, we can expect to generate something close to 1W when turning the motor at 100RPM. Basically, a motor rated for 60-120 RPM at your desired output voltage is good for hand cranked applications, higher RPM ratings are good for use in bike driven applications.
Determine the Polarity
This is a bit tricky; you’ll get different results depending on which way you turn the motor shaft. Measure the voltage across the terminals while turning the motor in the desired direction (I chose clockwise because it’s like winding up a fishing reel). Label the terminals of the battery positive and negative and mark which direction you turned the shaft (CW or CCW).
|Preventing the Battery from Driving the Motor
If you connect any battery directly to a motor, it will spin. To prevent this from happening to our dynamo, you’ll need a blocking diode. The V11 battery already has one built in. If you’re trying to charge your own battery, connect the diode in series from the positive terminal of the motor to the lead that will connect to the positive terminal of your battery. Any diode rated for 2A should be fine. We recommend the use of schottky diodes because the lower feed-forward voltage adds just a touch to the overall efficiency. Connect the negative terminal of the motor to the lead that will connect to the negative or ground terminal of the battery.
|Connecting to the Battery
The V11 uses a mini USB connector for its power input. We modified one of the spare mini USB cables to connect from the dynamo and used a breadboard to make our connections easy (but not permanent). The positive output of the motor (red wire) connects to the positive lead on the modified cable (again the red wire). The negative leads (both black) are connected together as well. Once you’re all connected attach the crank and give the dynamo a whirl. It’s a lot harder to turn that you’d think; you might have to rig up a geared crank system!
This 12v 50:1 DC gear motor will pump 350mA into the V11 at a modest 45 RPM. If we do the math, 350mA into a 3000mAh battery will take about 10 hours of hand cranking to fully charge. We could make things easier by putting a bigger crank arm onto the dynamo. To get even more power out, we could build a pedal driven crank system. It’s definitely a lot of work, but hey, when the sun is tucked away behind the clouds, the V11 battery can be charged from a dynamo, perhaps one that you’ve attached to your bike, or an elaborate system of gears and pedals hidden under your desk at work, maybe even attached to some fan blades and driven by the wind?