Meaning
Total internal resistance encountered by alternating or direct current flowing through an electrochemical cell represents a major determinant of power delivery efficiency. Measurement of equivalent series resistance combines electronic resistance from current collectors, tabs and active material coatings with ionic resistance through electrolyte solutions and separator pores. High resistance values cause measurable internal voltage drops during high-power discharge pulses.
The metric applies to individual cells, sub-modules, modules and assembled battery packs under specific temperature and state of charge conditions.
Ohmic Resistance
Metallic bus bars, joining welds, foil current collectors and active material contact interfaces constitute the purely electronic portion of internal cell impedance. Electrolyte conductivity, tortuosity of the porous separator and charge transfer resistance at particle surfaces make up the ionic portion. Increases in equivalent series resistance signal degradation mechanisms like electrolyte oxidation or tab weld fracturing.
Thermal Impact
Joule heating generated inside a cell scales directly with the square of operating current multiplied by internal resistance values. High heat generation during rapid charging accelerates chemical degradation pathways and risks thermal runaway conditions. Low ambient temperatures reduce ionic mobility, causing equivalent series resistance to rise sharply and limit low-temperature power output.
Measurement Standard
Electrochemical impedance spectroscopy isolates high-frequency ohmic resistance from low-frequency charge transfer resistance. Alternative direct current pulse methods apply ten-second current steps to calculate internal resistance based on immediate voltage responses. Quality control standards specify maximum acceptable equivalent series resistance values for cell screening prior to pack integration.
