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Using a dynamo to charge your FP

I don’t think that would help, since inductive loading is less efficient than using an USB cable.

Maybe these devices from B+M can provide a solution:
http://en.bumm.de/produkte/e-werk-usb-werk.html
The USB-WERK has a small integrated cache battery, the E-WERK can be combined with a 1400 mAh cache battery.

Didn’t test them myself, though.

It’s already some time since you asked but still: I’ve been using several versions of the Forumslader (some older versions and more current ones) with my wife’s Fairphone 1 and my Fairphone 2. All of these versions have in common that they provide around 5 V voltage and up to 3 A current at the USB outlet. When checking the energy consumption of both phones using the Forumslader app you can see that they consume about 1 A resulting in 5 W charging power. That means they are charging as fast as they can.
The Forumslader is easily able to power (and even charge) a FP2 while navigating e.g. with Osmand. Depending on the speed and the exact hub dynamo model the Forumslader can generate more than 10 W. The power that is not consumed by the phone (or any other consumer) is stored in a buffer battery which will continue charging the phone while standing still (battery is sufficiently big to charge a complete phone). I didn’t have any issues with this combination yet, nor do I expect any. The latest versions come with a Bluetooth LE module which my Forumsladers don’t have. So that would still be something to test but it should work with the FP2 and maybe also with the FP1 once the Android update is ready.

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Some more testing of the bluetooth LE Forumslader:
Yesterday I drove 65 km with an average speed of 28,5 km/h, producing renewable energy. At the beginning the buffer battery was charged around 90%, the FP2 around 65%. At the first 20 km the dynamo hub charged with 9 - 10 W. The phone was switched on the hole time, but without running GPS. After the buffer battery was filled to 100% the charging power went down to 4 - 5 W, than switched off to 0,0 W, than again on to 4 - 5 W and so on. In that state the charging currant for the phone (charged to 100%) stayed around 200 mA, the screen on, the phone running the Forumslader app, showing speed, charging power, currants, voltage and some more.

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That fits my experiences very well. For maximum power you traveled at the best speed. Using a SP dynamo would give you a maximum charging power of 13W at approximately your speed.
I like the power/100km display in the app, too, because it shows that when going slower (around 22kph) on a given distance you might produce more energy than when going faster.
Lately we’ve traveled through the Slovenian Alps by bike, we had a Forumslader connected to an Alfine hub dynamo and our FP1 and FP2. As long as we did around 50km per day the power was sufficient for both phones. Since that became too exhausting (next to the Forumslader I pedaled camping and cooking gear and a child trailer + passenger up to 14% uphill) we had to fall back to some wall sockets from time to time. But that’s acceptable.

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Same as https://www.youtube.com/watch?v=DlW5QpVN-Qg
even if it’s in italian, it is possible to see to work

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