Internal resistance of a cell varying with power drawn

[CharanV]

If we continue to draw a specific amount of power from a cell (re-chargable), how will the internal resistance of the cell vary? Is there any particular mathematical relation between them? Please help me understand

 

[anorlunda]

Here is a sample for NIMH rechargable batteries.

It doesn't look like any simple mathematical relationship to me. Strictly empirical.

 

[CWatters]

I used to fly electric powered model aircraft in competitions and we would improve performance by warming up the NiMh cells. I'm not sure if increasing the temperature increased the voltage or reduced the internal resistance as the effect is similar.