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I have the following setup:

  • Shimano DH-3N72 hub dynamo
  • cycle2charge.de USB charger
  • LED headlight and taillight, both capacitor-backed

The headlight and USB charger are both directly attached to the dynamo. The taillight is attached to the headlight so that the switch on the headlight switches both off.

With the lights on, the USB charger hardly gets any power. My phone will show that it's charging only after some time (presumably once the capacitors of the lights are fully charged) and only if I'm going fast. If I switch off the light, it takes a much lower speed for the phone to start charging.

I do understand that the lights reduce the amount of energy available to the charger (also the manual of the charger says so), but I am surprised that the effect is this significant even with LED lights. Is that really due to the power consumption of the LEDs themselves, or is this effect due to inefficient circuitry or some kind of voltage regulator? (My hub dynamo came with a regulator device, which I didn't install. It is meant to be hooked up in parallel with the lights and is apparently some kind of ballast. In a setup with two incandescent lamps, if one of them fails, it is supposed to prevent the other from burning out.)

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A hub dynamo is typically rated at 6V/500mA, and most lights will use all of that. Modern LEDs give you a lot more light from that power, but it's still nowhere near as much as the high powered battery lights you can buy (so if the manufacturers could get more power into the lights they would probably offer it as an option).

So the extra power you're getting into your phone comes from the "sometimes you can get more power" part of the dynamo specification, not the guaranteed part. In reality most dynamos will go well over 6V once you're moving at a reasonable speed, and the regulator you haven't installed is mostly to prevent incandescent lamps from burning out when that happens. With LEDs a better regulator has to be built into the lights because LEDs are more sensitive to the exact voltage/current they get.

This answer gives more links to information about how much power you can get.

  • Agree with Moz - there's only so much power available. Consider not charging your phone with a bike, or using a USB powerbank as an energy storage device inline and charge the phone through that. – Criggie Dec 22 '15 at 2:26

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