The discharge graph can be simplified to two numbers by using a vertical cutline of the graph taken at some point during the discharge curves. In this example, the output characteristic past the 1/2 way point was used:
Originally Posted by UnknownVT
The slope of the line in the figure below is equal to the internal resistance "Ro" and the unloaded operating point by "Vo". Lower internal resistance will result in tighter spacing of the discharge curves for different current loads.
Batteries are very close to "ideal voltage sources" and can be represented by an equivalent circuit. The drop in output voltage (Vt) when loaded is determine by the "output series resistance" or "internal resistance" called R_o . You would a great electrical engineer gauged by your thought process. For people without access to anything more than a DMM, adding a low value load resistor (2 Watt rating for 1 ohm) to your toolbox would allow you to constuct a poor man's ZTS tester.
With just the unloaded voltage (Vo) and loaded voltages and knowledge of the load resistor value the "intenal resistance" can be estimated. The lower it is, the better. Once known, you can use it to predict the output voltage for different loads that draw more current (or less).
The series resistance is given by :
But I_load is known exactly due to the relationship between the "load resistor" value and loaded voltage:
R_int= (Vo - Vload)/ I_load [Equation 1]
Combining Equation 1 and 2:
Vt/R_load = Vload/R_load [Equation 2]
Example the eneloop:
R_int = (Vo-V_load)/(I_load)
= (Vo/Vload - 1) * R_load
Rint=(1.440/1.388-1) * 1 ohm =0.037 ohms (correction pointed out by Cemoi)