Building a "Baghdad Battery"

Step 5 Monitoring Performance of Cells

If you want to minimize complexity, use only an analog voltmeter and ammeter (0-100mA) and observe the values during charge/discharge - ignore most of the discussion below.

To produce charge and discharge curves for your cells you can either log by hand the readings on your meter, or invest in a DAQ or microcontroller to automate data logging. A spreadsheet or similar application is helpful for plotting data. I am rather fond of the Labjack products (seen in some of my photos) - I used their DAQ+software and opamp. DAQFactory is the software that comes with Labjack's measurement devices and it's a good alternative to Labview. Although I am using the scaled down version of DAQFactory that ships with Labjack's products, it can be upgraded to enable some very sophisticated process control applications, but at a fraction of Labview's cost. The Arduino (Atmega) and Parallax Propeller microcontrollers are good choices too, but will require additional setup time - I am using them for other projects where cost is more of a concern. Labjack + DAQFactory is nice for speed of setup, plus they have a great support staff, but so does Parallax, and arduino has a huge volunteer support community.

Besides a measuring device, consider:

• Your measuring device may already have high input impedance (>100Mohm), but if not, use an operational amplifier (opamp), otherwise cells with high internal resistance (such as the Baghdad Battery chemistry) will create problems in your measurements. Opamp's are also helpful to amplify the voltage drop across the current shunt resistor, which is low. Opamps allow differential measurements too, so it is easier to read the voltage drop across the shunt resistor.
• If not using an ammeter, a small resistor provides a voltage drop for calculating current - ideally around 1 ohm (for the voltage/current of the Baghdad cell) although mine was 0.33 ohm. Using ohm's law simply measure the voltage drop across the resistor (dV) and solve for current I=dV/R, in my case R=0.33 ohm, Watts are calculated as P=I*V
• A small resistor or other device to act as a load, I used a 330 ohm resistor for the Baghdad battery, which seemed to be an appropriate load for the cell. The resistor value will differ depending on the battery.
• A terminal block for wiring connections - I find the European style blocks (shown in the photos) easier to use, but stick to the 20A rated blocks since the larger block size is easier to work with and only costs a few cents more
• To charge the battery, a constant voltage source such as another battery (e.g. earth battery) or in my case a digital to analog converter (DAC) on the DAQ works fine for the Baghdad Battery chemistry, but keep in mind that to properly charge some cell chemistries you will need a constant current source or a mix of both at different time intervals. I generally charged at 1.5 V and limited current to 20 mA - again, similar to what an Earth Battery might provide.

If you are just getting started in electronics and some of the terminology seems obscure, I highly recommend Getting Started in Electronics by Forrest Mims.