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I want to create bike-powered alternator system to recharge 12 V batteries. People can only do about 300 W mex.

To turn an alternator with higher torque requirements, especially when the battery it is connected to has a low charge, won't work. Can I "trick" the alternator to be in a range that can be peddled by having a separate circuit branch between ground and the voltage regulator input terminal that would give me the ability put in a voltage independent of the battery? In this way, I can adjust the alternator load requirements to match the maximum peddle power available.

I thought I could put a 12 V and 6 V battery in series with a rheostat connected to the ends and adjust the voltage output to the alternator.

I wonder if a second option would be to have different voltage diodes controlling a transister to give me different voltage outputs in 11.5 V - 15 V range. I plan to have a voltage, current, current power and total power monitor on the line to give me the feedback I need to make this work.

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    \$\begingroup\$ 300W is a power, torque requirement != power! \$\endgroup\$ – Marcus Müller Apr 8 '17 at 20:21
  • \$\begingroup\$ What is your actual goal. Don't use a rheostat if you can help it. You don't want to force people to pedal, just to heat up a rheostat. Maybe you can just put a DC-DC converter between the battery and alternator. But do you want to force everyone to put out 300 Watts, or do you want to let them set the power output they are comfortable with or ??? It is important to think about how this will feel to the person pedaling, and whether you want to hit 300 W at a specific cadence or allow them to choose a comfortable cadence, then adjust output power until the resistance is also comfortable. \$\endgroup\$ – mkeith Apr 8 '17 at 20:35
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    \$\begingroup\$ I think if you add a DC-DC converter between the alternator and battery, you can easily trick the alternator. You would want to be able to control the output current of the DC-DC. When the alternator output voltage climbs to some high value, the alternator will stop supplying current, and will have low mechanical resistance. But if the DC-DC converter starts to draw current from the alternator, then the alternator will need to supply some current, and the mechanical resistance will increase. By setting the output current, you can control the mechanical resistance of the alternator. \$\endgroup\$ – mkeith Apr 8 '17 at 20:39
  • \$\begingroup\$ peddle /ˈpɛd(ə)l/ verb try to sell (something, especially small goods) by going from place to place. pedal /ˈpɛd(ə)l/ noun each of a pair of foot-operated levers used for powering a bicycle or other vehicle propelled by the legs.One could peddle by pedal on a bike mounted shop. \$\endgroup\$ – Transistor Jul 5 at 6:12
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Alternators are given an excitation current, which controls the torque versus speed. There's no benefit to be had in changing the batteries' load (just trickle a little more or a little less excitation current to control the speed-versus-torque curve).

A normal bicycle cadence is something like 1 revolution of the pedals per second. So, consider biasing the alternator so that low speeds (say, one third revolution per second) offer low power (30W, for instance) and highest speeds (which are associated with best blood circulation in the leg muscles) give 300W output, at about 1.3 rps.

A racer might top 2 rps, so an adjustment range for different riders is appropriate.

Output current of an alternator at the various speeds can be measured at a variety of excitation currents, and an excitation current versus average cadence can be implemented to make any desired curve. But, you cannot just DO this theoretically, someone has to get on the bike and decide what's comfortable and productive.

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