Building a "Baghdad Battery"

Step 6 Performance of this "Baghdad Battery" replica

Alright, now that you have your cell built and some means of monitoring voltage and current of the cell, you will want to generate some charge and discharge curves to characterize the cell.

The photos below should tell most of the story, but in summary:

• charge at 1.1 to 1.5 V and a few milliamps of current
• discharge with a 330 ohm load
• repeat 1-2 times if you like, your cell should be providing about 7 mA-h of capacity. Then let sit for 3 days and you will see the 5x increase in capacity I note below

One very relevant aspect of this chemistry that I almost overlooked, and you will want to emulate: If you let the cell sit for 3 days after an initial charge/discharge cycle you end up with a rechargeable battery that has 5x the capacity - quite a remarkable increase in capacity. So, when I first charged/discharged the cell I thought I had created a replica roughly equivalent to the acid chemistries previously attempted, except with the added benefit of it now being rechargeable. After a 3 day rest though, I had a cell with 5x the capacity of the acid chemistries - my cells now averaged 33 mA-h capacity on a single charge when made according to the dimensions of the artifacts.

This cell likes to take a charge slowly and give it up slowly - it will take three days to discharge with the 330 ohm load, and 1-2 days to charge, I actually never fully charged or discharged the cell since I did not have the patience, and have several other experiments that are in neglect at the moment. The cell seems ideally suited to take a charge from the voltage/current characteristics of telluric currents present in the Earth, or earth battery as some have labeled it. I tested other voltages and currents and they do not seem to work as well. The slow release of current seen in this cell would likely work well to run a homopolar motor but since I have not created a well enough balanced piece of copper or bronze wire yet, I have been unable to fully test this theory. Also, if you attempt to discharge the cell too rapidly (too large of a load) it seems to ruin the cell.

One additional curiosity of the alkaline cell chemistry is that either the Fe or Cu can serve as the anode, although with Fe as the anode it is a better cell. With an Fe anode the electrolyte turns blue upon charge (indicative of Cu ions), and becomes clear on discharge. With Cu as the anode the solution is clear on charge and discharge and the metals actually appear polished.

It should be noted that all of my experiments (and cell performance curves) were done without careful attention to ambient temperature, usually around 55-65 deg. F. We keep our house relatively cool and try to minimize furnace use. Temperature is the most important environmental factor when evaluating electrochemistry, and 80-90 deg. F would likely produce better results, and provide a more accurate recreation of the 'Baghdad Battery' since temperatures were likely much warmer in Mesopotamia. I tested the temperature dependence of the chemistry by allowing a minor ~5 deg rise in temperature and observed a 10% increase in rate of charge current and release of the current on discharge. The increase in charge/discharge rate is a function of what is commonly called internal resistance. Most battery chemistries have a lower internal resistance as temperature increases. As most batteries are cycled, their internal resistance will increase with time - in addition to self discharge rate, and a decline in capacity. I am developing a reaction chamber to control environmental parameters and further automate my other electrochemistry experiments - this will allow a more precise characterization, but the results I've presented will have to suffice for now.