Charging from bike dynamos has been one of the most unsatisfactory of all the broken promises of technology in cycling; it is not too strong to say it is an expensive joke. There is no charger operating off a hub dynamo that will recharge an iPad satisfactorily, period.
Andre,
I've had great success using a hub dynamo to charge my on-tour gadgets, and found it far from an expensive joke. It has in fact been both a convenience and a necessity for me, as it has resulted in being able to keep my gadgets charged for an extended period when away from mains power.
By matching device demands to available power, my experience with bike-powered charging has been consistent, reliable, and trouble-free.
With my on-bike charging systems and a suitable buffer battery, I can and do power and/or charge...
• AA and AAA batteries to power my LED headlamps, weather radio, GPS, a small electric toothbrush intended for travelers, and the batteries for my folding keyboard.
• My GPS directly, thus preventing drain of the AA cells that power it and allowing me to track my progress over some 16+ hours of daily travel.
• My electric shaver.
• The accumulator batteries on my solar panels if there is rain or insufficient sunshine for solar charging.
• My buffer battery when it is not being used as a buffer and -- depending on load -- sometimes simultaneously. The buffer battery overcomes the problem of interrupted power or charging during stops or periods of low speed.
• My smartphone, which serves as telephone, email, SMS messager, web browser, electronic journal, e-book reader, backup GPS and compass, map source, Skype communicator, ftp uploader, video and still digital cameras, word processor, photo/video editor, and more. In turn, my bicycle-charged smartphone can and does power a solid state drive, a multi-Terrabyte hard drive, and a portable DVD player/burner.
I doubled my bicycle charging ability by equipping a trailer with a dynohub and charging system of its own. It will even charge overnight while I sleep if I invert the trailer and clip vanes to the spokes to catch the desert night winds and so spin the wheel to generate electricity as if I were riding.
Being able to do all this when away from mains charging power for an extended period has been a huge advance for my touring convenience and safety in remote areas, and something I have found I can can count on. It has saved me kilos in spare batteries that no longer have to be carried.
Bicycle charging systems are expensive, but I really hope people won't dismiss the idea of bicycle charging as impractical or a joke. I continue to recover part of my costs by using replaceable rechargeable batteries instead of disposables or embedded rechargeables that require the entire device be replaced when the battery will no longer accept a charge. I use my charging systems successfully every day I ride, easily and with no real fuss or bother. Once set up for my needs, the systems I have simply work so long as I continue to ride. I do also use solar power and accumulators to augment my power generation for days when I am immobile or slow and at night, but the bike systems work fine. It is terrific that a number of suitable dynamos and charging systems are now readily available to make this possible for anyone, so long as device demands are matched to the power available.
My success has been the result of carefully matching my gadgets to the available power source. An iPad won't charge directly, but I knew this and chose another device instead (my smartphone, which has proven to meet my needs fully). An iPad can be indirectly charged in part and its power extended through my buffer battery, which is bicycle-charged. There are other ways to charge or power an iPad indirectly by bicycle dynamo and charging system.
As for pedelecs/e-bikes using dynamos, I saw a number of them so equipped in Austria last summer. Powering lights and chargers with a dynohub minimizes parasitic drain on the battery for those actually *touring* with pedelecs/e-bikes, allowing more of the battery to be used for propulsion and longer range between charges. When I stopped and talked with several of the users, they were very clear it was common practice for touring longer distances with electric-assist and needing to maximize the battery's charge for propulsion to ensure they made it to the night's lodging for example, something very uncommonly employed when commuting. High drain devices were powered by the dynohub instead of running the battery down more quickly. Apparently, less energy was lost to increased drag than would be lost by powering gadgets directly. They said it didn't make a huge difference in battery live but made enough to be useful.
I have to politely disagree about the usefulness of dynohubs with higher output. Because the output is load-dependent, 6v systems commonly deliver increased output under increased load. One example is running two incandescent light sources in series, as long used in randonneuring; I've done so myself with great success for many long nighttime runs before LED lighting came on the scene. The cost comes in the form of higher resistance at higher speeds.
All the best,
Dan.
