I have one single chambered microbial fuel cell. I want to analyse the internal resistance. I have one digital multimeter. From literature I know that we can calculate internal resistance from voltage versus current graph. Can anybody will explain the procedure of how to plot voltage versus current graph, or if is there any technical diagram for that?
You do not need to plot it anymore.
I will assume that you have n voltage-current pairs. Get the ratio r1=v1/i1, r2=v2/i2, r3=v3/i3, ..., until you get everything. Then, get the average of r1, r2, ... The resulting value is your average internal resistance.
My approach would be to plot your v-i pairs as points on a graph with voltage on the horizontal axis and current on the vertical axis. If the points can be reasonably approximated by a straight line, then the slope of this line is your resistance. I wouldn't be surprised, though, if your points do not lie on the straight line, especially if your measurements include very small and very large currents. Then, the v-i relationship of your cell is nonlinear and you have to find a different model. Simple curve fitting (using MS Excel, for example) might do, but then you should also be able to explain the nonlinearity.
Hi buddy!
I am sure you have already found a solution based on the previous comments, but in case you want to see a different approach and double check your results, here it is:
The internal resistance is calculated by combining Ohm's and Kirchoff's law where Rint=(OCV / IL) -RL where OCV is the Voltage in open-circuit conditions, IL is the measured current under a known load and RL is the value of that load.
I hope that helps.
The formula is better explained in this paper:
Ieropoulos, I., Greenman, J., & Melhuish, C. (2008). Microbial fuel cells based on carbon veil electrodes : Stack configuration and scalability, (October 2007). doi:10.1002/er
Cheers,
George
No worries. If you need a copy of that paper let me know.
Cheers
Georges’ answer tells me that your model of a fuel cell is simply a series connection of a voltage source Voc and an internal resistance Rint. If this is indeed the case, than two simple measurements of an open-circuit voltage and a short-circuit current, Isc, would be sufficient to obtain the required internal resistance (Rint = Voc/Isc). If Rint is small (comparable to the input resistance of your ammeter) then measuring the short-circuit current will not be accurate. A better method would be to use a variable load resistor RL and measure voltage across it, VRL, for different values of RL. Since RL obeys the Ohm's Law, its current (if you need it) is simply IL = VRL/RL. Of coures, you do not need this current, because the internal resistance can be calculated directly from: Rint = RL*(Voc - VRL)/VRL (this formula is mathematically identical with the one given by George). My point is, however, that you should calculate Rint from several measurements, taken at different loads RL. Only if they give reasonably close values of Rint you can be sure that your initial model of the fuel cell is correct.
The equation V= OCV - I*R(internal) can be valuable if the measured voltage and current from variable resistors load are ploted on excel sheet. A negative slope is obtained with corresponding to internal resistance and intercept corresponding to OCV (open circiut voltage)
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