Hand crank charging for a high school engineering project

Mark Goldwater

Joined Nov 28, 2016
2
Hello. For a school project, my group is making a rolling cart that uses power it generates from motors attached to the wheels to charge a cell phone. I was wondering if anybody could help by suggesting a way that I could store the charge from the rolling wheels so that, even when the cart is not rolling, the phone continues to charge. Is there a way I could do this without greatly increasing the resistance on the wheels but still generate a descent amount of power?

thanks!

MrChips

Joined Oct 2, 2009
30,518
We hope this is not an over-unity project:
Battery powers a motor to drive a cart > motion from wheels to charge the battery.

GopherT

Joined Nov 23, 2012
8,009
We hope this is not an over-unity project:
Battery powers a motor to drive a cart > motion from wheels to charge the battery.
Does everything have to be a challenge here? READ the post before assuming nefarious activity.

Hello. For a school project, my group is making a rolling cart that uses power it generates from motors attached to the wheels to charge a cell phone
He didn't say to power the cart itself so he doesn't have to keep pushing it! He even acknowledges that there would be a resistance on the wheels! Where do you get over-unity out of this poor HS student's thread?

GopherT

Joined Nov 23, 2012
8,009
Hello. For a school project, my group is making a roll cart that uses power it generates from motors attached to the wheels to charge a cell phone. I was wondering if anybody could help by suggesting a way that I could store the charge from the rolling wheels so that, even when the cart is not rolling, the phone continues to charge. Is there a way I could do this without greatly increasing the resistance on the wheels but still generate a descent amount of power?

thanks!
Mark, welcome to All about circuits.

When electricity is stored, the stored voltage will always be less than the input voltage (whether battery or capacitor as the storage device. Current available will be less as well, but, to keep looking at voltage that means you go from cart generator to battery (voltage drop) then back to cell phone (second voltage drop). This means you need a fairly high voltage from the source generator so you have enough to charge the iPhone.

Also, generator voltage depends on rpm. So, if you have a 12v motor with max rpm of 6000, then you will only generate 6v at about 3000 rpm and half that at 3000 rpm. Now, it is almost impossible to do 3000 rpm with a push cart unless you have a highly geared system.

You'll need to do some math and experimenting to get where you want. Just some mechanical things to think about before you get too far down this path.

WBahn

Joined Mar 31, 2012
29,885
Hello. For a school project, my group is making a rolling cart that uses power it generates from motors attached to the wheels to charge a cell phone. I was wondering if anybody could help by suggesting a way that I could store the charge from the rolling wheels so that, even when the cart is not rolling, the phone continues to charge. Is there a way I could do this without greatly increasing the resistance on the wheels but still generate a descent amount of power?

thanks!
It's an interesting project. You will probably find that it is more hassle than it is worth and has no economic viability, but it's a good school project even if what you primarily learn is why it is more hassle than it is worth.

One of the first things you should do is come up with an energy budget and see what falls out from that. How much energy does it take to charge a cell phone battery? How much time do you want to allow for this to happen in? How fast can you charge a cell phone battery? How much time is the cart going spend in motion? How much additional force will be needed to push the cart assuming all of the extra force makes it into the battery? How much does that number go up if you make reasonable estimates on the major system efficiencies?

If all of that makes the project look reasonable, then start considering energy storage technologies. You might consider capacitors, inductors, batteries, hydraulic accumulators, etc. Think outside the box and come up with as many as you can think of and then rule as many of them out as you can based on engineering reasoning. You will probably be left with only one viable option and that will be a battery on the cart that the wheels charge and that, in turn, is used to charge the cell-phone battery. This is akin to the batteries in emergency stairwell lights, so you might look into how those operate.

Bernard

Joined Aug 7, 2008
5,784
What is the generator output V ?
How is this V regulated to cell phone input V ?
On the net I found a " solar cell phone charger ", which had nothing to do with solar but which had cell phone charging input & 5 V, 1 A @ 48,000 mAh. output. On case is called POWER BANK. Might be possible to take generator output to a boost-buck convertor regulated down to 5 V into power bank.

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