How To: Charge your Phone from a Dynamo

We show you how to build a hand crank dynamo with a few commonly available parts. Our battery packs are designed to charge from power sources (like solar) that have fluctuating voltages and currents and output nice clean USB power. In addition, they can charge and discharge at the same so you can turn the crank and charge your phone at the same time.

Here’s What You’ll Need:

• 1 V15 USB Battery
• 1 DC motor (select for volts and RPM explained below) – available here
• 1 crank (can be purchased from a hardware store)
• Wire
• Solder

If you do not use a Voltaic battery, you will also need a diode (rated for 2 Amps, Schottky recommended).

Shop Battery Packs

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.

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

The V15 battery already have a blocking diode built in, but if you connect some batteries directly to a motor, it will spin. To prevent this from happening to our dynamo, you’ll need a blocking diode. 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 the Dynamo to the Battery

The V15 uses a micro USB connector for its power input. We modified one of the spare micro 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!

Improvements

This 12v 50:1 DC gear motor will pump 350mA into the V15 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 V15 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?

The post Make a Dynamo to Charge Your Phone appeared first on Voltaic Solar Blog.