Recharging a Battery With a Hand Crank?

[bmhiggs]

Could a normal rechargeable battery (meaning the kind you plug into a usb port or electrical outlet) be recharged with a hand crank generator (assuming it was generating a sufficient voltage)? Would there need to be some kind of converter to make it possible, or just connect some +/- leads?

I'm a high school physics teacher with some inventive students and I'm trying to give them some direction. I was looking at some rechargeable products on Amazon, but it wasn't clear to me whether they were actually rechargeable batteries or just high-capacity capacitors.
Thanks!

 

[Baluncore]

Welcome to PF.

Would there need to be some kind of converter to make it possible, or just connect some +/- leads?

That will depend on the choice of battery and generator.

For small amounts of energy, electrical capacitors are more efficient than chemical batteries.

 

[bmhiggs]

Welcome to PF.That will depend on the choice of battery and generator.

For small amounts of energy, electrical capacitors are more efficient than chemical batteries.

Can you give some more info there? I'm assuming a small DC generator (basic hand crank kind).

 

[Baluncore]

There are too many possible choices.
You have a choice of generators.
You have a choice of energy storage technologies.
You must run a dating agency, to connect a couple that can live together.

 

[berkeman]

Welcome to PF.

Could a normal rechargeable battery (meaning the kind you plug into a usb port or electrical outlet) be recharged with a hand crank generator

You don't get much energy out of a hand cranked generator, maybe a few watts. By comparison, a reasonably fit person can generate around 100W with a generator that is driven by a bicycle chain. Normal folks can only pedal at 100W for short bursts, but the pro racers can put out over 100W for long durations.

USB ports are 5V at 1A max for standard ports (I think), so that's 5W which you could do with a hand-driven crank mechanism for moderate durations. A common cellphone battery is 3-4Ahr in energy storage (at at the equivalent of 5V), so to recharge a 4Ahr battery with a standard USB 1A connection takes 4 hours. That's a long time to be cranking away...

https://www.androidauthority.com/smartphone-battery-size-poll-results-1221015/

 

[bmhiggs]

Nice - I'm more wondering about just the physical connections though. For clarification, the invention under discussion would just be for a handheld flashlight (I'll add that to the original post.) So power and efficiency aren't as important. Could you just solder the lead wires of the generator to a battery casing? Or would you need some component to met that voltage actually charge the battery?

 

[berkeman]

To make the flashlight recharger as simple as possible, you will be sacraficing efficiency a fair amount, but that could be part of your discussion with your students. You would put a Schottky (low voltage drop) diode in series with the DC output of the generator, so that positive charging current will only flow into the battery while the generator output voltage is greater than the battery voltage.

And you would need a Zener diode in parallel with the battery as a voltage limiter so that the generator's unregulated DC output voltage does not rise too high and damage the battery. But of course there can be a lot of lost energy in just using a Zener diode as a voltage regulator.

The next step up in efficiency is to use a Buck-Boost topology DC-DC converter circuit between the generator and the battery. That circuit is able to generate the regulated output charging voltage whether the input voltage is below or above the output voltage.

 

[bmhiggs]

Thanks!! That helps!

 

[Tom.G]

You might get some more ideas/info from this search:

https://www.google.com/search?hl=en&source=hp&biw=&bih=&q=shake+to+charge+flashlight

Cheers,
Tom

 

[Rive]

the invention under discussion would just be for a handheld flashlight

There are two sides for that: the light and the generator. Mind the 'light' part, that'll save you a lot af workout later. You will need a LED and a constant current driver with wide input voltage range. That'll allow you to use a capacitor instead of a battery. And to charge a cap, you need far less electronics than for any battery.

 

[Tom.G]

The mini LED flashlights sold here for about USD $2 use three AAA size batteries (0.43 x 1.7in., 1 x 4.3cm.) and a series resistor.

 

[Rive]

For battery that's barely acceptable, but with a cap (which would be a better match for a low cost generator) the voltage swing is just too big.

 

[DrClaude]

I have one of these:

and while the manufacturer says that 1 minute of cranking can give you 30 minutes of light, I have tired my arm long before I can get a significant amount of power.

 

[Dullard]

From my POV, this is a great project for kids. My 6th grade Science teacher had a hand-cranked lamp. I still remember (and respect) the amount of human work required to produce a trivial amount of electrical power. $.07 per KWH (or whatever) never seemed expensive (to me) after that experience.