Working Battery
I was watching the King Of Random’s channel again last night and I found his video on how to make a three penny battery. I’m an electrical engineer, but I’ve never made my own battery even as a kid, so I thought this looked like an interesting project for this afternoon.

This project works because pennies are actually made from zinc with a very thin layer of copper of the outside (at least new pennies). Scrape away the copper and you expose the zinc. When two dis-similar metals are separated by an electrolyte, a liquid that can carry a charge, it forms a potential across the two metals. Stack a bunch of these “cells” together and you have a battery.

Penny holder for sanding copper off one side

To scrape the copper of the tail side of the coin I built a quick jig. Pennies are very close to 3/4″ wide, so I used a 3/4″ Forstner bit to drill shallow hole (just shy of the depth of the penny) in a small block of maple.

Zinc interior of penny exposed

After rubbing the back of the penny against a sheet of stationary 100 grit sandpaper for about 30 seconds, I got a nice shiny zinc face. Just be careful, after all that rubbing, the penny will be quite hot.

Soaking the cardboard electrolye holders in vinegar

For the electrolyte, I used white vinegar. You can’t just put the vinegar between the pennies, you need a material to hold the vinegar and keep it from evaporating. So I cut out some discs slightly smaller than the pennies and soaked them in vinegar for a few seconds.

Once I had one disc and two pennies, I put the cell together and tested the voltage. It read about 0.7V, that was pretty good for my first try. As I added more cells to the battery the voltage wasn’t going up in steps of 0.7V, I was getting something less each time. I still haven’t figured out what is going on.

I stacked enough cells together to get between 3.5V and 4V then I tried putting an LED across the electrodes, sure enough there was enough voltage to light it up. Let me back up a bit. It makes a huge different how you stack the cells. To increase the voltage you want to connect the positive side of one cell to the negative side of another. with the dual sided pennies we can cheat a bit because there is already an connection between the cells it you just stack it right.

So take one of your dual sided pennies put it copper side down, then place a piece of soaked cardboard on top of that. Now stack a another dual sided penny copper side down — you have one cell. But if you stack another piece of cardboard and another dual sided penny copper side down, you have two cells in series. All you need to do to add more cells is stack more in this fashion.

LED illuminated in the Dark

Things I still want to try/figure out:

  • Why am I not getting the same voltage for every cell?
  • Would thicker or thinner cardboard be better? Or maybe some other material.
  • Is vinegar the best electrolyte? How about apple juice? Salt water?
  • How much current can I get out of one cell?
  • How long will a cell last? can I replace the electrolyte and get the cell working once it dies, or do I have to scrape the tarnish of the electrodes too?

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