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The internal resistance forms a voltage divider with the load. If the voltage is 12V with no load and drops to 6V when the load is 1 ohm then the internal resistance is 1 ohm. But the voltage will not drop that low, maybe it drops to 10V so the missing 2V has a current of 10A and the internal resistance is 2V/10A= 0.2 ohms.
Batteries are complex beasts; they have impedance and not resistance. But we often talk about the real part of the impedance as the resistance and I guess you want to know or estimate the Re (Z) under reasonable conditions (DC; zero load) approximately.
We need to construct a model for the battery and the good old Randle's cell model is simple and convenient. We have something called Warburg Impedance (const phase element) that is characteristic of electrochemical cells.
There are charge transfer resistances at both the electrodes (that makes life hell) but you wanted the internal resistance, right? That is called Rs in the Randles cell model.
You do a EIS spectrum and make a Nyquist plot. The point where the semicircle touches the x-axis is the series resistance.
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