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.