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So, I was doing some reviewing over on bicycles stack exchange, and I realized the Cygolite Metro line of reusable chargers would fit my budget - AND be USB rechargeable!

Alas, I've been told constant charging is bad.

The idea was to hook a dynamo up to a battery and have the battery in turn charge it... I guess I can just plug unplug it manually. I was gonna waterproof the battery, probably with some kind of project enclosure. I suppose I could use something like this to save me some build time.

I expect my load to be 7 to 10 watts. Does any one have a sense of how much resistance that would add to my ride? (Hypothetically, let's say I build my own.)

Also, I do giant hills. If I do roll my own system, what can I do to make sure I don't surge too much power into the battery coasting down a hill?

Will generators generally make fixed volatages with varying amperages, usually? (Ie, is that how power fluctuates?)

I survived the first half of Engineering Circuits one (drop related drop.)

I'm a bicycle commuter.

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  • \$\begingroup\$ The average human in decent shape can put out 150-200W for long periods of time. (I can sustain about 250W on my rowing machine for 8 minutes at a time, but that's pretty intense.) A 10W lighting load will be barely noticable. \$\endgroup\$ – Dave Tweed Jun 26 '14 at 1:56
  • \$\begingroup\$ @DaveTweed: the average human also has a remarkably sophisticated motion control system, and can definitely notice 10W. However, that won't be a deal breaker. \$\endgroup\$ – whatsisname Jun 26 '14 at 2:39
  • \$\begingroup\$ Some 20-30 years ago that was the common solution around where I live. We had this small generator near the front wheel which had two positions: its small rubber wheel touching the front wheel and not touching. When touching and the bicycle moving, the front lightbulb would be lit, adrquate to light up the road ahead nicely. \$\endgroup\$ – Szidor Apr 5 '16 at 11:29
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A voltage regulator (like the 7805) will make sure there will be a defined voltage on the output. R1 can be left out.

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The idea was to hook a dynamo up to a battery

I'm assuming you're talking about a hub dynamo here. Most hub dynamos (Shimano, SRAM, SP) are 6V, 3W, so you can estimate a best case charging time, and compare that with your commute time.

I'm a bicycle commuter.

You will need to look at how much (continuous) charging current the light will draw - chances are, the dynamo will not be able to provide it as your speed varies. You will need to effectively limit the power drawn from the dyno and feed a "cache battery", which will in turn charge your light. However, given the inefficiencies involved you will be better off just charging the light before you start riding, or you could get a light that runs directly off the dynamo (like the IQ-Fly or Lumotec-Lyt).

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In regards to resistance:

A SON28 GenHub has almost no appreciable resistance while riding. If you pick up the front wheel and turn it slowly you can feel the bump.bump.bump of the windings and magnets, if you put the bike down and roll it forward slowly you can't. So adding your body weight to it and pedling you won't notice it.

In regards to the concept and idea:

What you are planning is completely doable. I use a SON28 front hub connected to a home made rectifier/regulator circuit (I joined here today specifically to try and refine it for this season). That in turn provides USB power to a buffer battery , a cheap Amazon unit with (2)18650 Chinese LiPos in it, then to my tablet or anything else I have to charge. Yes constant charging is bad, but after 15,000 mi in 2 years it still works. If I have to change it that infrequently , that's an acceptable cost/benefit for me to abuse it and replace it. I did a youtube video explaning the entire charging system here ( I hope posting youtube links is OK here, I am new here so not sure what's kosher or not yet).

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