dynamo Hand crank battery charging

[myzter]

Going camping and I want to bring my electric scooter. Im using A123 M1's for my battery pack. Im also using several small Evergreen solar cells mounted on my scooter.

So, Ive got my riding wiring figured out as:

A diode in series with the output for my solar cells will keep power from going into the solar cells from the batteries.

Now when the scooter is stopped, the solar cells charge up my batteries. I have wired an extra positive and negative set of leads, with the idea of using a small hand crank dynamo to charge my batteries just a little bit faster. This dynamo will also have a diode in series with the output.

This is where Im stuck, I need to add some resistor/s in series with the ouput of the hand crank dynamo to prevent a flat battery from overloading the dynamo.

How would I choose the resistor I need, or the value of this would depend on the AH rating of the battery ??

 

[grindz145]

PV cells make very good battery chargers, but unfortunately dynamos don't really... You would need some type of regulator to keep the voltage in a usable range for your cellstack. This is really not a trivial solution I'm afriad. You might want to try to measure the output voltage with a multimeter as you crank first to get an idea how close you are. All in all you will need to do alot of cranking

 

[dogman dan]

I bought a hand cranked radio once, About 5 minuites of strenuous cranking woud run the radio about 10 minuites. That's a little radio that runs for days on a few AA's. Good for emergencies, but you'd have no time to ride if you tried to hand crank enough power to charge an ebike.

 

[amberwolf]

Well, if that dynamo was a bicycle light generator type, mounted on the wheel of a stationary bike, you could probably generate a reasonable amount of power if you did nothing but sit there and pedal it all day.

Personally, if I were going to be pedalling all day, I'd rather be riding around enjoying different scenery, though.

Realistically a hand-cranked dynamo usually doesn't generate much total power--your arm isn't nearly as capable as your legs are, generally. I dunno about you, but I could probably hand crank one of those things for 20-30 minutes and then my arm would fall off with a thud. and I would not be cranking very fast for more than a few minutes at best. I doubt I'd put out 50W, call it 25Wh.

You might get 100-150W, for an hour or two, be generous and call it 300Wh out of pedalling something, however. But the generator isn't 100% efficient, perhaps, including drivetrain losses, 50%-80%. Then you lose power in the conversion/regulation process to upconvert and/or downconvert the generator power into power levels acceptable for charging the battery pack. That could leave you with only 70-80% of the original 50-80%. Lets be generous again and say you get 80% at both stages, so you now get maybe 64% of your original 300Wh, or 192Wh.

How big is your battery pack again? A few KWh? Not a lot of charge from human power that way.

It could be done; made more efficient, but you're not going to enjoy just sitting there cranking or pedalling, right? I mean, you're there to enjoy the camping--go enjoy it!



EDIT: If you really want to try doing it, well, first you need to know what voltage the dynamo generates. If it is not high enough, it will not charge your batteries even without a diode or resistor in series with it. The voltage probably will not be very high unless you crank it fairly fast, too (assuming it is not regulated). What voltage does your wall charger put out to charge your pack? You'll need to reach that voltage with the dynamo to do it.

If the dynamo is much higher than that pack voltage, you'd need more than just a resistor to limit the voltage to the pack from the dynamo. If you just use a resistor, then at high charging voltages there might be high currents (which I doubt you could generate), the resistor would have a high voltage drop across it, wasting most of the generated power as heat. At low charging voltages (very close to the pack voltage) then the resistor might not effectively let enough current in to charge the pack much anyway.

You'd need to basically build a buck/boost converter (charger) with the dynamo as input, to effectively use it to charge the batteries. To me, that would not be worth it unless I was really desperate for power.

If you have a regular ICE vehicle that you are transporting yourself and everything else with you up there with, you would have a much easier time if you used a lighter-socket inverter to power your regular wall charger, and then just periodically run the engine to recharge the vehicle's battery (or run it while charging the scooter). It's not very efficient, but it would be more effective than the dynamo, almost certainly. And I daresay more fun, in that it would leave you time to enjoy your trip.