Question about battery internal resistance measurement
A battery ESR test depends on vendor choice of test method. The most consistent is Hioki’s low-frequency AC-IR method. It cuts testing time down to 10 seconds while still maintaining measurement precision and stability.

The battery ESR determines the C rating for charge and discharge which varies widely upon dimensions and chemistry.   Thus a 1C battery has an ESR 10x bigger than a 10C battery.  if it has no rating, assume it is 1C with high ESR.

Although 1kHz ESR Test gives the lowest result 1Hz IR drop gives the same results as DC ESR = IR drop test results in 15 seconds without full charge and using only 1% of max rated current of the DC ESR Test.  

The equiv circuit is R1+R2//C1 so 1Hz Test gives same results as DC with ESR= R1+R2. But the 1kHz Test takes 1 second and ESR=R1 for production speed.

The equiv cct. Is more complex for testing mAh for C1//C2 with separate ESR’s for each C. This also describes the short term memory effect.