This has been posted already, my bad, it's here : https://lemmy.ml/post/224388
Here is a short summary I (OP) made :
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This one is a static exercise bicycle with no gears.
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You also have to build a board to convert current / voltage on the output if you want to charge batteries or power electronic devices that are sensitive to power changes
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Power output overview : "The power output (W) of a bike generator corresponds to the voltage (V) multiplied by the current (A). We obtained roughly 100 watts (12V, 8-9A) of power during a short and heavy workout. During a moderate effort – which we can sustain for a longer time – power production is between 45 and 75 watts. The power output not only depends on the bike but also on the person who operates it. Athletes could produce more power, while couch potatoes would (initially!) generate less."
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The simple dynamo is under the wheel. So, much friction. A better system could waste less energy ?