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Old 04-14-09, 01:08 PM   #1
robtown
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Power P7 flashlight with Dynamo hub?

I have a two cell (18650) P7 flashlight and a one cell P7 flashlight. Both throw great light, though the 2 cell is bulky for a bike light.

I'm currently getting a Sturmey Archer 6v 3watt hub wheel for a new bike. My question is - could a P7 be adapted for use with the dynamo? The 18650 are nominal 3.7v.

Would a dummy battery connected to the hub work? Could the 18650 be wired in parallel - and be kept charged up while moving while providing a stand light?

What's horribly wrong with my reasoning (I suspect the P7 draws too many watts)?
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Old 04-14-09, 02:08 PM   #2
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It's not that the P7 requires too many watts, it's that it requires too few volts and too many amps.

As for charging lithium cells from a dynohub, I guess it could be done, but not at the same time as powering lights.

A single 18650 can (theoretically) output 2.5A for one hour @ 3.7V - to recharge it from a 3 Watt source would require the full (high speed) output of the hub for maybe 4+ hours, depending on the losses in the charging circuit.
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Old 04-15-09, 02:45 AM   #3
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You can use a cap and regulator to reduce the voltage and increase the amperage available.
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Old 04-15-09, 05:16 AM   #4
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You can use a cap and regulator to reduce the voltage and increase the amperage available.
A regulator can reduce the voltage, but can't increase the current.

In any case it would be inefficient, it would be far better to use some LEDs in series.

The P7 is 4 LEDs in parallel.

The MC-E LED can be series or parallel, so would be a better choice.

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Old 04-15-09, 08:27 AM   #5
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you could use a switching regulator, and saying that glosses over a lot of complications. And that would be a lot of work for not much gain. The MC-E is a much better choice.
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Old 04-15-09, 10:02 AM   #6
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A regulator can reduce the voltage, but can't increase the current.

In any case it would be inefficient, it would be far better to use some LEDs in series.

The P7 is 4 LEDs in parallel.

The MC-E LED can be series or parallel, so would be a better choice.

Could you mod the P7 and rewire them in series? Or is is just easier to buy loose LEDs?
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Old 04-15-09, 07:00 PM   #7
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Could you mod the P7 and rewire them in series? Or is is just easier to buy loose LEDs?
They are hardwired in parallel. It seems like a bad decision since there are other apps that would probably be better served by series configurations. If you have a bonding machine going to series probably isn't that hard....

Seems like there are at least 3 companies that make multi-emitter LEDs that can be run in series, so Seoul is missing out. There are MC-E flashlights, not sure how hard it is to convert them to series configuration.
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