Human-Powered Live-Blogging On A Bike In France
March 6, 2007 9:00 PM   Subscribe

Calling all electrical engineers, cyclists and gadget freaks: How can I modify my SON dynamo bicycle wheel hub so that I can use it to power a variety of peripheral devices (cell phone, iPod, GPS, etc.)?

I am a randonneur. I ride my bicycle very long distances (this year I plan to complete the Paris-Brest-Paris, a 1200km event held once every four years in France in August). And I get very bored sometimes. I carry a cell phone with me for emergencies, a tiny digital camera, as well as an iPod full of audio books and music that help me get through the long miles. I also would like to add a GPS-enabled Mobile PC device/Blackberry. Ideally, I would like to live-blog the ride, with text, audio, and photos from along the route, without having to carry a ton of spare batteries. Most of the devices are rechargeable, but recharging them would require stopping long enough to plug them in to an outlet, somewhat counter to the notion of staying motion. We are talking 90 hours on a bike here, so the ten or so hours you'd normally get from these devices won't cut it. Carrying extra batteries is not an option either since most devices have their own internal rechargeable batteries.

The front hub on my bicycle is a SON dynamo hub. These hubs are traditionally used to power a headlight so that randonneurs can ride at night without having to rely on battery powered lights. The hubs generate approximately 6V of current when spinning, and have two outputs: one for the main headlight and one for an optional second light.

My question is this: is it possible to somehow use the second output to serve as a power source for peripheral devices of varying voltages? How would this be achieved? Are there off-the-shelf parts available for use in putting such a system together? Is a 6V dynamo sufficient to power most devices (ie, my Powerbook input is 24.5V, so powering something as big as a laptop is out, right?) I am assuming it would require an intermediate energy storage device (ie, a specialized battery) to go between the device and the dynamo (since the current is not present when the bike is at rest), but what about the varying voltages of all the separate devices (anywhere from 1.5 V to 5.5V, not to mention all of the different connector types).

I did find a system called the GeoBici that makes reference to a specialized alternator used to power an iPaq Pocket PC with GPS, but no specifics were given, nor can one purchase said system.

Other issues: a lot of extra weight and overall bulkiness would make this not fun. I also don't want to jeopardize the function of the headlight, which is actually the most important part of this whole system.

Finally, I can tell you that there are hundreds, perhaps thousands of users of SON hubs who would like to see products developed along this line, so all you entrepreneurs out there might want to get a'crackin'.
posted by piedrasyluz to Technology (14 answers total) 2 users marked this as a favorite
Your basic system block diagram looks like this:

Hub ---> voltage regulator ---> battery charger ---> battery --> peripherals.

Your laptop may be out, but you can get low powered, web capable devices like smartphones that may allow you to blog.

If you chose your devices carefully, you could, for example, have an entirely 5v system. This would save you 99% of the engineering hassle that seems so daunting now.
posted by fake at 9:52 PM on March 6, 2007

I don't know how to build what you are looking for, but I did notice these two points. It will produce more than 6 volts at high speed and the volts you will make are AC. You will need DC to charge your gadgets.

From the above site Son Dynamo.

And also be aware that the SON's output is AC, not DC.

Without exception, every headlight I sell for use with either a hub dynamo or sidewall dynamo incorporates circuitry to prevent the overvoltage of the bulb.
posted by lee at 9:55 PM on March 6, 2007

You say it produces "6V of current". Voltage is a measure of EMF; current is measured in amperes, or in this case more likely milliamperes, and we'd need to know that number, too. Unfortunately, it's certain that the current isn't a constant. It's certain that it's a function of your rolling speed.

I think it's virtually certain that your dynamo is an alternator, which means it's producing AC, frequency dependent on how fast you're riding. AC at 6 V will power a headlight just fine, and producing AC is more reliable.

Your other devices you're talking about need DC. If enough current is available and the voltage is right, you'd need a bridge rectifier and a couple of honking capacitors to eliminate ripple, plus a voltage regulator.

To produce 24.5 volts from 6 volts you don't use a battery, you use what's called a "DC-to-DC converter". (Because of the wildly-varying frequency of the AC, you wouldn't want to try to use a transformer. Besides which, transformers are heavy.)

My biggest concern would be the voltage. You say it's 6V, but it's not that simple. I'm sure it bounces up and down quite a lot, as a function of your speed and the electrical load, and that might render it useless for recharging devices like you want to.

Unfortunately, that kind of electrical source sounds like just about the worst choice possible for powering electronics.

[I want to make sure you understand something: the more current you draw, the more turning resistance the hub will have. If you're powering a headlight and charging some device, then it will feel like you're riding uphill even if the ground is level. Nothing is free when it comes to energy, and the electric power in this case is coming indirectly from your leg muscles. Presumably you know that, but I just wanted to make sure.]

On preview: OK, it is an alternator, and when they say it's 6V, what they mean is that they've included a 6 volt regulator to prevent the headlight from blowing up. That means the voltage can be lower than 6V, but not higher. It also means it's going to vary a lot, which is not good.
posted by Steven C. Den Beste at 10:02 PM on March 6, 2007

(Actually, they're using a couple of regulators in series with a couple of diodes to limit the AC voltage. PICK pick pick...)
posted by Steven C. Den Beste at 10:04 PM on March 6, 2007

It looks like, if you use a SON28, you've got a nominal 6 volt, 3 watt tail light power source, above 5.5 mph. That's not much power availability, off the headlight nacelle, and worse, it is nominal 6 volt, variable frequency, variable voltage power, which greatly restricts your choice of parts for voltage transformation and DC conversion. You could use a bridge rectifier and a DC-to-DC inverter to convert this to something like the voltage needed to charge a Nokia 6400 series phone, but it will be a slow, slow process of charging the phone battery, if you ever let it go flat, or ride out in the country, where the phone automatically ramps up it's power output to full 600 mw output, where your SON hub, with rectifier and DC conversion inefficiency and phone losses, is just about enough to keep your phone going by pedaling. And your phone might not even allow charging from such a low capacity source, if it thinks your rig's output isn't capable of supporting the phone to the degree a wallwart will. My Nokia 6410 wallwart outputs 3.7 VDC at 340 ma, if that's any help.

You might get a PDA or PDA/phone combo like a Treo to work off a rig like this, if the PDA has similar input requirements. Quite frankly, I doubt most PDA's are going to have as minimal a power budget as any cellphone, because of their screens and backlights. Most cellphone designers would kill for the power budgets of even the most stingy PDA form factors, but you might find that 3 hours of pedaling would give you 20 minutes of PDA time, if you weren't using a high power accessory like a WiFi card or its builtin equivalent. WiFi cards are notoriously inefficient, powerwise. If you want to even think about blogging via WiFi at stops, get a 12 V sidewall dynamo, as a start, in addition to your SON hub, and prepare to spend most of your ride pushing your SON hub at night for lighting, and your sidewall dynamo, day and night, for communications recharging.

Forget any modern laptop, entirely. Your generator sources are entirely insufficient to either charge or power any laptop built today, in any reasonably portable scenario.
posted by paulsc at 10:04 PM on March 6, 2007

Geeze, what I shoulda said...

If it's 6V AC, then if you run it through a bridge rectifier and use caps to eliminate ripple, you don't get 6V DC. (Unless the 6V AC rating is RMS, not peak.)
posted by Steven C. Den Beste at 10:10 PM on March 6, 2007

maybe you could get a solar panel?
posted by tev at 10:12 PM on March 6, 2007

SCDB: yes, I'm definitely aware that the more current I draw, the more difficult it will be to pedal. That's part of the trade-off of using the SON Hub to power a headlight as opposed to using a battery-powered system in the first place. Unfortunately, during the day, even though the light can be switched off to preserve the halogen bulb, I will still be pushing the hub, but not using any of the energy it generates (some models do charge a battery that powers a back-up LED headlight, but not the one I purchased).

I am very curious to know how the GeoBici solved the problems you mention, because they specifically state using a special kind of alternator on the front hub of the bicycle to power both a GPS device and an HP iPaq with wireless connectivity. And all that on a bike that most tourists won't ride faster than 8-10mph.
posted by piedrasyluz at 11:31 PM on March 6, 2007

3W is probably enough juice to recharge a PDA: I just hooked my HTC Apache (a full-blown Windows Mobile PDA that you could absolutely use for blogging, though no GPS) up to its wall wart through a Kill-A-Watt ammeter, and it registers .01 amps, or a little over 1 watt, including transformer/rectifier loss.

If you can smooth out the power enough for such a device to deem it acceptable (they can be picky about "authorized" chargers), you should be able to keep it juiced up, especially if you're frugal (i.e. keep the backlight off as much as possible, leave it in "flight mode" whenever you're not actively communicating, etc.)

If the extra drag is a problem, maybe you could rig up a switch and manually turn off the charger when you're going uphill. And if you want music, I'd suggest leaving the ipod at home and dropping $50 on one of the many available flash-based mp3 players that run for hours and hours on a single AAA battery.

On preview: piedrasyluz, you're actually encountering less resistance when the bulb is off. Sure, you're still spinning the dead weight of the alternator, but when the bulb (or any load) is on you're also pushing energy into the circuit, which you'll feel as electromagnetic resistance to the spin of the wheel. I'm not sure how noticeable this actually is when added to tire rolling resistance, drive train inefficiency etc.
posted by contraption at 12:03 AM on March 7, 2007

Is avoiding battery powered lights a weight concern? If not, perhaps a solar panel on a lightweight rear rack would be a better bet.
posted by Good Brain at 12:20 AM on March 7, 2007

There is quite a substantial group of cyclists here in Germany that faced the same problem, I think most of them also working with the SON.

They have created a website devoted to presenting their solutions:

"Rad-Forum" referring to the "Cycle-Forum" where the originating thread started, "Lader" being short for "Charger".

Unfortunately, it is all in German, but looking at the pictures and plans might help/inspire. Contact me if you have any specific question regarding translation...
posted by waldlaeufer at 12:46 AM on March 7, 2007

What you'd want to do. Note, warranty will die fast.

1) Remove the voltage regulator. Possible issue -- this means as it turns faster, the drag will increase. But you want more power, and this will allow the voltage to climb. Note that hooking things directly up to the SON now will probably result in them dying, so you'll then need...

2) ... a Charge controller. What you'll want to do is use the output of the SON (and possibly other outputs, say, solar panels on the rack trunk) to input into a power supply that charges a battery.

The point of this is to protect the battery from overcharge. The win of this is you go from the random inputs of the dynamo to a lovely, flat DC.

You can use a large battery if you want/need significant power times at night (though you'll almost certainly need to charge that off bike, unless you are very careful with the power budget) or a small battery that just covers runtime when you're going slow. But the battery is important, because it makes the next step vastly easier.

3) Power Unit: From the battery, we get converts to change the voltage to what we really need. Buck/Boost converter ICs have gotten cheap, and are very efficient. Linear regulators are even cheaper, but they're not as efficient, and they can only step down (and unless you deal with a low dropout regulator, you can't use a 9V linear regulator on a 12V battery -- the dropout's too large. You can us a 6V easily.)

It is more than possible to boost power from 6V to 24V. However, there's no free lunch -- if you boost 6V 1A of power to 24V, you'll only get .25A of current -- at 100% efficiency. In reality, you'll get much less.

I just don't know if you'll get the total power you need to charge a Powerbook out of a SON dynamo. Assuming you need 50W, at 6V, you need on the order of 10A. If the voltage rises without the regulator enough, that may change.

But this is another reason for the battery. We can charge the battery over time, storing that power, then use it at a higher rate later. So, you may not be able to run the Powerbook 24x7, but you might be able to run it a couple of hours at the end of the day, as well as run your lights at night.

And by the way. The point of P-B-P is P-B-P. Crank on, and good luck!
posted by eriko at 5:11 AM on March 7, 2007

I know a long distance kayaker who used a solar device to power his GPS, cell phone, etc. I don't know the specifics, but if you e-mail me, I could put you in touch with him. See email in my profile.
posted by desjardins at 5:56 AM on March 7, 2007

For solar solutions, check out solio, ctsolar, or sunlinq
posted by indigo4963 at 8:36 AM on March 7, 2007

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