Bicycle iPhone and USB Charging – The Generator

Before I talk about how to generate power, I need to respond to a more basic question first: Why bother generating power at all? With a big enough battery, can’t I power my gadgets for a full 48 hours, and just recharge when I get to civilization?

Time to do some analysis with Ohm’s Law. If you’re going to be designing electric circuits, you need at least a basic understanding Ohm’s Law. On the other hand, if you don’t care about the details, then skip this section. (The answer is, "Yes, I need to make power.")

Let’s work with the most relevant example to me, the iPhone. The iPhone charges at 5 volts, the voltage supplied by the USB ports in a computer. The power supplied by a typical USB port starts at 35 milliamps, and a device that’s plugged into the port can auto-negotiate by sending messages over the "data" lines to receive up to 500 milliamps (half an amp). This is what the iPhone does. Whether you plug it into a USB hub, or a USB port directly on the computer, the phone starts drawing 500 milliamps at 5 volts and slowly begins to charge.

The iPhone also supports a "quick-charge" mode. Plugged into an iPhone wall adapter (which has special circuitry in it to identify itself), the iPhone can draw up to 1000 milliamps (1 amp). Drawing 1 amp, an iPhone will quick-charge from empty to 80% full in about 90 minutes, then switch to "trickle charging" mode to charge the rest of the way.

Do you need an Apple-branded USB hub to charge the iPhone?

No, the iPhone will charge off any port that supplies 5 volts. (Besides, there’s no such thing as an Apple-branded USB hub.) When testing their iPhones on USB hubs, many people make the mistake of just plugging the phone into a hub that’s not connected to a running computer. Almost all USB hubs will supply no power unless they are uplinked to a computer that is turned on and awake.

Do you need to supply 1 amp to get the iPhone to charge?

Nope. As long as an iPhone can see 5 volts on the connection, it will attempt to charge on it. In fact, the power level the iPhone prefers for trickle-charging is the standard USB level of 350ma.

Suppose I purchase two LARGE batteries, wired up to supply 6.4 volts at 1 amp for 10 hours, and then attach a circuit called a "regulator" to turn the 6.4 volts into 5 volts. I could then use the batteries to quick-charge an iPhone, two hours at a time, for a whole ten hours. If I attached these batteries to a bicycle, but didn’t include any mechanism for recharging them, I could recharge the iPhone from empty to full somewhere between three and four times. If I include the first full charge I have inside the phone when starting out, I have between four and five complete cycles for one bicycle trip. (About 11 amp-hours of 5 volts, downstepped from 6.4 volts with a 10% conversion inefficiency, equals about 3-and-a-half full charges.)

That sounds like more than enough to ride out camping for a day, then ride back. But a road-test with the iPhone proves different. If I use it to play music while riding, and check my location on a map every half an hour or so by pressing the "locate me" button, and take the occasional photograph, and make the occasional phone call, I get somewhere around six hours from a full charge. Much less if I’m using the 3G phone with GPS active, constantly downloading map data (which keeps turning the antenna on). A full day of biking followed by enough campsite use to chat with friends and upload pictures could easily surpass two charge cycles. Now add another USB device, for example my GPS tracker. And a USB-charged flashlight to set up my campsite.

Still, this seems like it might be enough for a single day of camping. And it’s more than enough for a day-long ride. But what if I add a battery-powered bicycle headlight into the equation?

A nice, bright 3 watt LED headlight designed to operate at 6 volts (matching the battery pack) would drain the battery just as fast as a quick-charging iPhone, cutting my available time in half.

Why such a fast drain?

It’s because of the watts. The relationship between volts, amps, and watts is tricky, even though the equations look simple. You need to pound your head against Ohm’s Law for a while to know what’s going on. To shine at full brightness, the 3 watt bulb in our example will draw close to an amp of power from the battery. The batteries are only good for 10 amp-hours – slightly less, in fact – and when you combine this with a charging iPhone and a GPS unit, we’re drawing over 1.5 amps. And the faster we draw power, the less efficient the batteries will be.

In fact, after you factor in all the inefficiency, it turns out that one hour of headlight use would cost me more than one complete charge cycle of the iPhone.

Now it’s not looking like such a good idea. I could avoid this by powering the headlights with the bicycle, and charging the iPhone separately with the battery pack… But that means installing a generator of some kind for the headlights. And if I’m going to do that, then why not use the same generator to charge the battery pack during the day, when I don’t need the headlights?

So yes, I do need to generate power. (Either that, or haul around a huge battery.) Now I can finally ask the question: How do I make it?

At first, I looked for easy solutions. I was expecting that any solution involving a generator would be hugely inefficient, or heavy, or require welding experience, so I went looking at solar power and wind power instead. Both had problems. Then I did some more research and realized that using a generator would be easier – much easier – than I thought.

Solar panels:

There are several devices on the market right now that combine rugged solar panels with batteries. You can mount these on the rear of your bike or even your helmet. In good weather, when you’re out riding under the hot sun, you can put some of that light to good use.

Even so, the real advantage of solar panels comes when you are off your bike. Assuming you set up camp somewhere without electricity, and feel alright about leaving your gear in the open to soak up some sunlight, you can store enough power to charge your gadgets. But you’ll need a large panel to store enough for a sufficiently bright headlight, and you’ll need to consider how heavy that panel will be as you haul it from place to place.

I probably would have chosen a solar panel, except for one thing: A solar panel is useless at night. If you’re riding at night and your battery quits, there is nothing you can do.

Wind power:

Unlike solar, at least wind power takes advantage of the motion of the bike, but wind power is unpredictable and the few approaches I’ve seen for putting a turbine on a bicycle are very awkward. And like solar, the power is dependent on the weather conditions.

I have seen a prototype of an invention that acts as a generator while the bike is in motion (in the form of a wheel rolling on the ground, tethered to the back of the bike) and acts as a wind turbine while the bike is at rest (if you unfold it and place it on a flat surface nearby), but it was a prototype only and appeared too flimsy for regular use.

If you put a plane on a long conveyer belt, and ran the conveyor belt backwards, and started the plane so it began moving forwards, could you eventually take off?

Yes, and quite easily. People are too used to thinking about cars, where the wheels transfer the driving force. On a plane the wheels just keep the plane from hitting the ground. It would be trivially easy for the plane to overtake the speed of the conveyor belt, and fly right off it, since the plane is pulling itself along the wind, not along the ground. The wheels would spin faster but the plane would barely notice. An equivalent question would be, is a speedboat able to cross a lake when there’s a headwind? Of course.

Dynamo power:

Then, there’s the obvious choice: A dynamo attached to one of the bike wheels. (These devices in general are called "generators", but when attached to a bike, a generator is traditionally referred to as a "dynamo". So bear with me, you electrical engineering folks.)

A dynamo is basically an electric motor working in reverse. In an electric motor, you run electricity through wires to push magnets attached to a wheel, turning the wheel. In a dynamo, you turn a wheel which moves magnets around, pushing electricity through wires. So the faster you turn the wheel, the more electricity you get. Bike dynamos come in two major types: The sidewall dynamo and the hub dynamo.

Sidewall dynamos:

A sidewall dynamo is a device shaped like a miniature wine bottle, with a rough-edged wheel attached where the cork would be. The idea is that you anchor this thing to one of the struts on your bike along the inside edge of a wheel, positioning it so that the little wheel on the dynamo is touching the sidewall of the tire. As your bike wheel turns, the little wheel of the dynamo rolls along the endless circle formed by the rim, turning magnets inside the wine bottle, generating DC power. A spring in the anchoring mechanism keeps the dynamo pressed to the rim even if the rim is warped. (Every bike wheel on the road today is at least a little bit warped).

The major drawback to a sidewall dynamo (or "bottle dynamo" as it’s sometimes called) is the friction involved. To deal with the warping in the rim you need a spring applying slight but constant pressure, anchoring the dynamo wheel to the tire, so the wheel gets enough purchase to be turned by friction. That friction is just wasted energy, and riding around with it is like riding around with a tiny handbrake on your bike, working constantly against you, giving you nothing. Because of this problem, sidewall dynamos are usually built with a cable or a lever attached so you can pull them completely away from the wheel when you don’t need electricity.

Reading this description you may get the idea that a sidewall dynamo adds a lot of extra resistance to a bike, making the bike much harder to pedal – and it’s true, there are some really crappy dynamos out there that are only bearable for short distances – but in general the increase in effort is equivalent to the difference between pedaling a bike in shorts vs. pedaling in pants. You wouldn’t want to do it all day, and I personally think it would be ridiculous to use one for a cross-country tour, but if it’s the only thing keeping your headlamp alive on a dark night as you pedal home from Karaoke night at the King of Clubs, then better to have it than not!

(Incidentally, a company called bike2power has created an inexpensive iPhone charger centered around a bottle dynamo. While I would not use a bottle dynamo, the website claims their charging apparatus can also be connected to a hub dynamo (see next section). On the other hand, they “recommend” using their own dynamo, which makes me worry about over-voltage and warranty issues.)

Hub dynamos:

The other type of bicycle dynamo is a hub dynamo. You don’t see these much in the U.S., but they’re all over Europe, especially Germany, where there are even widely respected laws regulating their quality and usage. In these new energy conscious times I expect hub dynamos to become more common here in the States.

A hub dynamo is a thick cylindrical device that you swap for the axle in one of the wheels of your bike. Inside it are a couple dozen slender magnets arranged in what’s called a "claw-pole" design. The magnets are moved around each other inside the hub by the rotation of the bike wheel, generating electricity with very little friction.

Very little? Are you sure? Because I have a bottle dynamo and I can totally tell…

Yes, I’m sure. The difference between a bottle dynamo and a hub dynamo is striking. The drag from a good hub dynamo is so slight that you will have a hard time telling it’s on. (You want to add real drag to your bike? Switch from road tires to mountain-bike tires. There’s drag, and then there’s drag. Ugh!)

To visualize how it works, hold your hands in front of you and make claws with your fingers and thumbs. Draw the tips of the fingers on your right hand inward just a bit more than the fingers on your left hand, so you can fit your right claw inside your left claw. Now rotate your claws in opposite directions. This is what goes on inside the hub. Your fingers are the magnets, passing along each other, creating a magnetic field. Since each group of magnets (left claw and right claw) moves closer to the other group, then farther apart, over and over as they circle around, they pull electricity in cycles through the wires in the hub. Forward, then back. So power that comes out of a hub dynamo is AC power.

To power a light or charge batteries you need DC power. So if you’re going to use the electricity generated by a hub dynamo, you need to convert it to DC. This is generally not a problem and can be accomplished with a simple electrical circuit known as a "full-wave bridge rectifier".

Building a circuit? That’s more than basic soldering…
Well, a bridge rectifier is a really simple circuit. It’s four little components wired together in a diamond shape. But if you use the same materials I use, the TuneCharger board has a bridge rectifier built in. So no worries.

Since I plan to ride my bike a lot, and power a lot of electrical devices – not just headlights – with the power I generate, I’m going to go with a hub dynamo. It’s more expensive, but it’s much more mechanically efficient, and the amount of power I can generate in a day’s ride is a big concern to me. I want to be able to spend a day or more exploring whatever spot I ride to, and keep my gadgets charged the whole time.

Now I need to choose a hub. I’m going to gloss over a lot of mouse-clicking and page-reading here, and simply say that all the internet literature I’ve read points to the Schmidt SON hub as the best. There are five variants, only one of which matches my requirements:

  • The Schmidt SON20 (Wrong size for my wheel)
  • The Schmidt SON20R (Wrong size for my wheel)
  • The Schmidt SONXS100 (Too light for my bike weight)
  • The Schmidt SON28S (For bikes with disc brakes)
  • The Schmidt SON28 (For mountain-bike sized wheels, like mine)

So, by process of elimination, the SON28 is the one for me. Now let’s see about putting it on a wheel.

To The Wheel

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