Self-rechargeable battery

[JRamirez]

I'm working on a project which involves a circuit board (leds)in a small enclosure (3"Hx3"Wx2"Deep). This would be mounted close to a mechanically driven shaft with variable rpm's(0 to 500 rpm). My question is if it's possible to mount a motor on this shaft to generate enough energy as to recharge the battery in the circuit? If it's a 4.5 volt circuit,What is the smallest motor/battery combination I can use? I appreciate any suggestions.

 

[ve7brz]

You can certainly use a small DC motor to recharge your batteries. Unfortunately the RPM figures you give are much too low to be useful. For a small motor to be used as a generator it needs to be turning at about 18000 to 20000 RPM. These figures become lower as the motor becomes larger so you'll need to define the size of the largest motor/gearbox combination you can accomodate and work from that. You'll not find a suitable small motor which will produce enough voltage at 250 RPM to 500 RPM.

 

[Foxfur]

Check out:

http://www.techlib.com/electronics/stepper.html

It has some info on using steppers as generators.
If you make a large pulley for the stepper and use the shaft to drive it, it might get you there. I'd think about using a charge pump and regulator as well.
Ff

 

[ve7brz]

Hi,

I like Foxfur's idea of using a charge pump to raise the voltage. However we still need to find a suitable generator (motor) to provide the power you require. Please tell us the current you require for this project.

 

[resqcapt19]

Small permanent magnet DC motors like those used in toys will produce current at much lower RPMs than 18,000. Many of these small motors run under 500 RPM.
don

 

[ve7brz]

Hi Don,

That's exactly what I had in mind. I'm looking at a particular motor but the current requirement may be too much for it.

 

[Foxfur]

How about using an old bicycle generator (the one you'd flip down to touch the rear tire sidewall when the sun went down)? With one of these and perhaps an LM317T and some caps, methinks you might have a system. Rewinding the generator to suit your I & V needs could eliminate the needs for a voltage regulator providing the shaft rotational speed is kept constant. You might consider a diode on the + output so that your tiny genset does not try to act as a prime mover when the shaft stops rotating (insert puff of smoke here...).

Check this out...
Take an off the shelf bike gen/light that uses rechargeable batteries.
Hack hack, chop chop, modify the drive system to accept it.
Enjoy!
Here's a link to a new design of a 'sidewall' generator as they call them these days. 4.5watts, 6volts, not bad...

http://www.dynosys-ag.ch/lightspine.html

I think this will work (I want to say "Perfectly" but won't),
Foxfur

 

[JRamirez]

Thanks for your replys. We are experimenting with a small motor (toy motor) and it seems to be generating enough current (25-30 ma, 4.5V) to energize our led circuit. We looked at the bicycle dynamo as an option but it takes too much torque to turn. Now we are looking for a way to store and regulate that current/voltage to achieve a closed loop.