The battery internal resistance tester is a measuring instrument used to measure the internal resistance, voltage, and temperature of rechargeable batteries such as lead-acid batteries and lithium batteries to judge the health status of the battery. It can also be used as an instrument to measure the ESR parameters of electrolytic capacitors.
Load current flows from the battery (under test) through the meter leads en route to the load resistance (inside the meter) so the value of ‘internal resistance’ obtained will be the sum of lead resistance and battery internal resistance.
The resistance of modern lead acid and lithium-ion batteries stays flat through most of the service life. Better electrolyte additives have reduced internal corrosion issues that affect the resistance. This corrosion is also known as parasitic reactions on the electrolyte and electrodes.
So experiments to determine internal resistance should be confined to loads for which there is minimal or negligible discharge during measurement. There are more complex electrical models of battery chemistry and these will better approximate the battery’s time-dependent electrical output during discharge.
The overall battery resistance consists of ohmic resistance, as well as inductive and capacitive reactance. The diagram and electrical values differ for every battery. Measuring the battery by resistance is almost as old as the battery itself and several methods have developed over time, all of which are still in use.
Resistance does not reveal the state-of-health of a battery and often stays flat with use and aging. Cycle test on Li-ion batteries at 1C: What Is Impedance?