Internal resistance of a solar cell; strange results?

[cbs27]

Homework Statement

Basically, my task is to find out the internal resistance of a solar cell. I set-up a circuit as seen here (http : / / mt14 . quickshareit . com / share / picture666d18 . png), and measured the Current and Voltage on different settings of resistance in 5Ohm intervals from 0 Ohms to 50 Ohms. Now that I have calculated the resistance from the measured voltage and current, the resistance remains higher that the controlled resistance (on the variable resistor), however at 35 Ohms of controlled resistance, the calculated resistance drops below the controlled resistance. The data can be found here (http : / / mt15 . quickshareit . com / share / picture58258f . png). Is this normal behaviour? Also, what am I to make of these results?

Homework Equations

R = V/I
E = Ir + V
E = V + v
V = (-r) I + E

The Attempt at a Solution

Above, in the attached images. Please note that due to the 15 post requirement before posting links, I have had to place spaces before and after each dot and slash.

 

[Redbelly98]

The internal resistance is best estimated from the slope of a graph of I vs. V, at the high-V end of the curve. That will at any rate put an upper bound on the resistance, which could conceivably be less than what is calculated this way.

It would have been useful to also do an open-circuit (R=infinity) voltage measurement.

 

[Moetasim]

I would first examine the experimental set-up and make sure that all components were functioning properly and that there were no errors in the measurements. I would also check to see if there were any external factors that could be affecting the results, such as changes in sunlight or temperature.

Next, I would analyze the data and try to find a pattern or explanation for the strange results. It is possible that there is a non-linear relationship between the voltage and current, which could explain why the calculated resistance is higher than the controlled resistance at certain points.

I would also look into the internal structure and properties of the solar cell itself, as these could also affect the results. For example, if the solar cell has a high internal resistance, it could be causing the strange results.

Overall, it is not uncommon for scientific experiments to yield unexpected or unusual results. I would continue to investigate and analyze the data in order to better understand the behavior of the solar cell and potentially make improvements to the experimental set-up for more accurate results.