Hand crank flashlight to bike generator?

Question

can i take apart one of those hand crank LED flashlights and take out the part that is being cranked to produce the light. and some how put it on my bike wheel and when i pedal the bike it spins and recharges a 12v marine battery? any suggestions??? thanks

Answer 1

Max output capacity of those generators is only a few dozen milliamps, if that - if your battery is about the same size as a typical car battery, then it's capacity would be about 100 amp-hours.  Therefore it would take dozens to hundreds of hours to recharge it.

An automotive alternator and regulator is much more appropriate.  About the right gear ratio for the alternator pully might be the diameter of a 10-speed's rear tire to the diameter of the alternator's pulley, so a belt large enough to fit both plus an idler pulley to keep it tight on the alternator pulley might get you there, and the gears of the bike can fine-tune it to your physical capacity.  One way to calculate how much you'll have to pedal -

1) Assume your leg's max pedal output for an extended time to be say 1/5 horsepower if you're in pretty good shape, maybe 1/3 if you're Lance Armstrong

2) Assume bike/alternator/regulator/battery chemistry net efficiency to be about 50% (sorry, that's about the best considering total losses)

3) Assume battery depletion to be 75%, leaving it only 25% charged (this is as far as you should go even with a deep-cycle marine battery)

4) Then minimum required energy in watts into the battery would be the amp-hour rating of the battery times the 75% percent figure times 3600 (seconds per hour) times charging voltage of 13.5V divided by the efficiency losses of 50%

Ex - 100 amp-hour battery * 75% * 3600 / 0.5 = 540000 watt-seconds, aka joules

Divide that by the constant 745 to get horsepower-seconds, which would be 725.  If you're going to kick at 1/5hp, then the time you'll need to pedal your azz off would be

725/(1/5) = 725 * 5 = 3625 seconds, or slightly more than 1 hour.

...I think I got that all right....

...or, you could just jumper the battery to your car's battery while the engine is idling  for about 20 minutes....

Answer 2

Thats a lot of work, and you won't get much power out of it. And the voltage may be too low to charge a 12v battery.

You can get small generators that clamp to your frame and have a wheel that rubs against the tire and spins. There all the mechanical work is done for you. But I don't know if you can get one that puts out enough voltage (at least 15v) to charge a 12 volt battery.

What does the battery do? If you are using it to power the bike, then you are trying to build a perpetual motion machine, and it won't work. You will always lose power, and the bottom line is you will expand a lot of energy (you pedaling) for nothing.

analogy:

Start out with a £5 note. Go to a bank and change it for five £1 coins (minus a service fee). Go to a different bank. Change your coins for a £5 note, minus another service fee. Repeat until you have several million pounds!

Answer 3

This may not work without some serious modifications. To charge the 12V battery, I think you'll need 13 or 14 volts. The hand crank is probably only designed to charge a 3 - 6 volt battery.

Try measuring the voltage across the generator while you turn it, see how much you can achieve. It's probably regulated on the circuit board down to 6 or 7 volts, so you may get much higher when you crank it fast.