A more powerful battery

Earlier we tried to make a copper-aluminum battery out of pennies, foil, and old film canisters.  Long story short it didn't work.  So we quit fooling around and made a more robust setup. 

First we cut 7" sections of 3/4" PVC pipe and cemented endcaps on.

The we cut 7 1/2 " sections of 1/2" copper pipe and drilled a few small holes along the length.

Then 1/4" aluminum shafts were cut to 8" lengths.

The idea is the copper tube will extend out of the PVC pipe and the aluminum will extend slightly beyond that.

It's critical that the copper and aluminum do not come into direct contact in the cell.  So we used rubber bands to provide a spacing along the length of the tubes. 

Here is an assembled unit.

The cells need to be upright so when we add vinegar it doesn't spill out.  So we put together a wood frame to hold the battery.  (M did a lot of sanding on this.) 

We used cider vinegar poured in with a funnel. 

One cell gave almost half a volt, 0.473 V.

With the first one working we quickly set up the rest of the cells. 

There we go.  2.85 volts across a battery of six cells.  Now, can we light up an LED?

Success!  The LED on the left is bright pink from the battery voltage.  The dark red one on the right is identical but not hooked up for comparison. 

How does it work?  (I am a bit rusty, no pun intended, with my chemistry so lets see if I can get this right and not mix up the directions.)   The copper is oxidized ("rusted") as it enters the solution.

Cu -> Cu[+2] + 2e[-] 

Each atom of copper that enters the solution frees up two electrons that can flow through the circuit. 

An excess of hydrogen ions (H[+]) are present in the acidic vinegar solution.  At the aluminum end electrons flow in and react with the hydrogen ions to produce hydrogen gas (hydrogen is reduced). 

2H[+] + 2e[-] -> H2

The cells are well vented with open tops so the hydrogen does not accumulate to dangerous levels that could ignite. 

The resulting redox reaction (reduction-oxidation) drives electrons through the circuit connection.

The expected voltage can be estimated from the metal's anodic index, this is a measure of the voltage between various metals and gold as a reference.  Copper has an index of -0.35 V, aluminum has an index of -0.9 V.  The difference between them is 0.55 V, more or less the half volt we got with cider vinegar as the electrolyte (ion solution).