Thursday, March 10, 2016

Chemistry for Preppers: a DIY Battery

Most of you have seen the grade school science project where a kid pushes a couple of coins into a potato or lemon and measures the voltage produced. The potato/lemon is acting as the electrolyte in a simple voltaic cell: the electricity is coming from the interaction of the metals (Cu in a penny, Ni in a nickel) in the coins. Modern coins aren't made of the same metals as they once were, so you'll need to find a penny dated before 1982 if you want 95% copper instead of copper-coated zinc.

Electrode Potential
The voltages produced are calculated using the electrode potential* for the reaction that takes place at both the anode (-) and cathode (+). The reaction at each electrode is called a “half-reaction” because it doesn't occur by itself; there has to be a complimentary reaction at the other electrode to produce electricity. Half-reaction equations are completely reversible, but you have to reverse the sign on the voltage if you run it backwards. Ignore the temperature, pressure, and concentration specifications since they are mainly for use in a lab; what we're looking for is the math to figure out how much voltage an improvised cell will produce. That will allow us to work out how many cells we'll need to string together (in series) to produce the voltage we desire.

A Copper/Zinc Cell
I chose this type as an example because I am testing a very simple DIY battery for charging a cell phone or radio. Zn has a lower (more negative) potential, so it will oxidize before the Cu will. That means the Zn will lose electrons (oxidize) and become the negative post (anode) of our cell.

At the anode, the half-reaction is Zn → Zn2+ + 2e- with a potential of +0.763 Volts.
(reverse the voltage since we reversed the reaction shown in the table)

And at the cathode, the half-reaction is Cu2+ + 2e- → Cu with a potential of +0.337 Volts.

0.763 + 0.337 = 1.10 Volts per cell

For my DIY cell I'm taking a short piece (~3 inches long) of ½ inch copper pipe for my anode, sealing one end to make it water-tight and filling it with a weak acid (electrolyte). I chose ½ inch pipe because it is still found in a lot of houses and can be purchased at any home improvement store. Around here, a 10' stick (enough for ~40 cells) costs about $8.00. That's $0.20 per cell. The cathode is a zinc-plated roofing nail stuck through a cork or stopper that is inserted into the piece of copper pipe. So far I have produced about 1.0 Volts, so I know I'll need to tie 5 or 6 of them together in series (a battery) to get the voltage I need for a USB connector (5 VDC). I'm trying to find a small enough load for it so I will be able to measure the milliamps produced and get an idea of how long it will last in use before needing to be refreshed. Once I know the amperage, it's just a matter of linking several batteries together (in parallel) to get a useful amount of electricity out of it. It will look like something cobbled together, but I'm less interested in how it looks than how well it works.

As I get more data and time to play with this idea, I will post updates. I already have several ideas for how to improve the output, but I want to get a basic cell working before I go off on tangents. There may even be pictures if I remember to take them while I'm playing with things.

* Electrode potential is also an indicator of how “active” an element (usually a metal) is. The more negative the potential, the easier it is for that material to oxidize. Placing two metals with large differences in electrode potential in contact with each other will cause the one with the more negative potential to corrode. This is why steel bolts are such a pain to take out of aluminum engine blocks.

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