Calculating the internal resistance
The measurement results you obtained are not enough to determine the internal resistance.  







simulate this circuit &ndash; Schematic created using CircuitLab

You measured the voltage across BATT and Rinternal, and the current through them, (green coloured variables below), but it still gives you 1 equation with 2 (red coloured) unknown variabless (which has no unique solution)

$$ \color{green}{V_{load}} =  \color{red}{V_{BATT}} -  \color{green}{I_{load}} \color{red}{R_{load}}$$

You need to measure the battery voltage 


without load (\$V_{open}\$). With (hardly) no current, there is no voltage drop across \$R_{internal}\$ and therefore \$V_{open}=V_{batt}\$
with the resistance load (\$V_{load}\$) and the load current (\$I_{load}\$) 


With those, you can determine the internal resistance.  

$$ R_{internal} = \frac{ V_{open} - V_{load} }{I_{load} } $$