I am doing an experiment where I measure the internal resistance of two battery cells. I've measured the internal resistance of both of the cells, and when they were in series. The series internal resistance was the resistances summed. However, when I connected them in parallel, the result was unexpected.

By connecting two battery cells in parallel you should be able to calculate the internal resistance to the equivalent battery if you know the internal resistance of both of the battery cells by $\frac{1}{R}=\frac{1}{r_1}+\frac{1}{r_2}$? Am I wrong here? Because the internal resistance I measured was higher than the single battery cells' resistances.