How to Charge Any USB Device by Riding Your Bike

Step 4 Getting Power

Selecting a motor proved a more challenging task.
Expensive motors provided the proper torque needed to create the braking source, however the cost was prohibitive. To make an affordable and effective device another solution was necessary. The project was redesigned as a continuous charging system, out of all possibilities the Maxon motor would be a better choice due to its smaller diameter.
The Maxon motor also provided 6 volts where as previous motors gave us upwards of 20 volts. For the latter motor over-heating would be a huge issue.

We decided to stick with our Maxon 90, which was a beautiful motor, even though its cost was $275.
(For those wishing to build this project, a cheaper motor will suffice.)

We attached this motor close to the rear brake mounts directly on the bike frame using a piece of a meter stick between the motor and frame to act as a spacer, then tightened 2 hose-clamps around it.

Comments

[lr10cent]

If I was going to do a motor, I'd consider one of the brushless motors used for electric model airplanes. They're really cheap now. Maybe $10 from some sources. What about putting a coil on the bike and a bunch of strong magnets on the wheel? If you set them up so the gap was very small, that ought to generate some power. I'm not sure how to evaluate the peak voltage, though. Maybe with a capacitor that's ridiculously high voltage, and a diode. Also, one could put a micro switch in the brake lever which would turn on the device only when braking. Even better, have a reed switch on the cranks with a delay, so the device would turn on whenever you weren't pedaling. I think this has real promise for a headlight generator, along with a big capacitor or a small battery.

[Fenwick]

I was thinking the magnet and coil thing myself. Although that may only work well at high speeds.

[lr10cent]

It wouldn't have to be all that high speed, I think. You can put the magnet pretty far out on the wheel, if you need to. Not sure about sizes, but you can count on something like 8 or 10 mph if you put it far out enough. I think that's comparable or larger than what goes on in some generators. If you look at some of the wind generator designs, I think it probably wouldn't be that hard to do. I think some of them have direct drive, and in a 15 or 20 mph wind the tip speed of the prop is probably under 100 mph, or certainly under 200 mph. If the generator has a tenth the radius of the prop, then you have similar speeds.

[inquist]

The rim of the wheel has a tangential velocity equal to the forward speed of the bike itself... think of the axle moving relative to the ground, then think of the ground moving relative to the axle, you see the tire (touching ground) must be moving at the same speed relative to the axle, as the axle relative to the ground. (You only even need Gallilean relativity for this, hehe.) So 20 to 30 mph?

[lr10cent]

Um, I think you're overlooking something. The coil has to be mounted on the bike, not the ground. And the magnet probably can't be right on the rim unless we also have disk or drum brakes. So you'll only see a large fraction of the bike's speed, not double that. And we'd like this to work even for people who ride slowly, or in the Midwest (i.e. without a bunch of high speed downhill runs). I average 12 to 15mph depending on the time of year and how much I've been riding, and I know there are slower riders. So let's say 80 percent of 10 or 12mph. That's how I got 8 or 10mph min. On the other hand, if you ride a minimum of 25mph, then you're right. For you. Perhaps if you revealed your real name we'd recognize you from the Tour de France?

[inquist]

Perhaps we could have a small wheel near the rim, and attach a magnet to it, so that the magnet spins inside a coil of wire...

[lr10cent]

Sounds like a regular bike generator, to me. Of course that means we know it would work. Or an inrunner brushless motor meets this description pretty well. But I've used bike generators that use a friction wheel to pick up power from the bike's regular wheel. They can slip when wet, and even when dry if not lined up right can cause problems by sliding, like, if they're against a tire, wearing away the sidewall.

[owf]

This is EXACTLY what I need for my bike, but connected to a dynamo. I have the legendary Schmidt hub dynamo (a true marvel of German engineering) with unnoticeable drag, and would love a way to use that to charge any USB-powered device.

[cdubnbird]

what's a good motor to use if i want to spend $50 or less? - and still get the most out of it?

[Colonel88]

Use one of those rechargable bike generators. You know, those thingies you attach to a wheel and that power a light bulb......

[beartoe]

Most of the standard bicycle dynamos are alternators, running 6v, 3W, so 500 ma is right at their operating efficiency. I don't recall if this setup required a rectifier or not on the "front-end" circuit. But If you're willing to build up a wheel, you can find a brand-new Shimano dynamo hub for about US$85-90. And much cleaner and simpler. Of course, you couldn't charge your USB device AND run the lighting system. Maybe this is a good idea -- at night the bicyclist ought to be concentrating more :)

[beartoe]

I wonder if you could use a "proper" hub-dynamo for this purpose such as a Schmidt Nabendynamo or a Shimano. What kind of amperage is required to run the USB circuitry?

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[Kagetsuki]

The dynamos you linked would probably work much better than this solution. Motors aren't always the best generators. Ideally, you want a high torque low speed motor if you want to generate electricity, but even then they may not be ideal. You also need to consider the orientation and fixture of this motor vs the dynamos you linked. This motor requires friction to move it, and in a natural world you're going to loose or miss out on a lot of possible energy because of that. Not to mention wear may be fairly quick. As for "amperage", that's all handled in the power regulation portions of the circuit. The lines from the motor/generator will fluctuate, but the energy first hits the capacitors which then feed into the voltage regulator which will provide a steady voltage source and a predetermined maximum current as long as it has enough power to drive it. From there they have the whole thing connected to that "minty boost" circuit which will handle the supply to the USB terminal. If you were asking how many amps USB devices usually require that depends on the device. If I'm not mistaken you can pull something like 500mA off the bus. I don't know how much current an iPod sources to charge off a standard bus, but I'm assuming the minty boost circuit can provide enough to satisfy it.