Meaning
Total opposition to the flow of electric current within an individual electrochemical cell dictates both the energy efficiency and the thermal behavior of the battery under load. The value of cell internal resistance determines how much energy is converted into waste heat rather than delivered to the external circuit. It consists of both electronic resistance from the tabs, current collectors, and active materials, and ionic resistance from the electrolyte and separator.
Sourcing engineers monitor this parameter closely because it directly affects the fast-charging capability of the battery. This parameter serves as a primary metric for evaluating cell health and manufacturing quality.
Electrochemical Mechanism
Chemical transport within the electrolyte and the kinetics of charge transfer at the electrode interfaces govern the dynamic part of this resistance. As current passes through the cell, ions must migrate through the porous separator, a process that is highly dependent on temperature. Lower temperatures increase the viscosity of the electrolyte, which raises the internal resistance and limits power delivery.
The electronic component remains relatively stable, but the ionic component changes dynamically with the state of charge and temperature.
Measurement and Testing
Two primary methods exist to determine this value, namely alternating current impedance testing at one kilohertz and direct current pulse testing. The alternating current method captures the ohmic resistance, while the direct current pulse method includes the polarization resistance. Pack designers require both values to build accurate thermal and electrical models of the battery pack.
Discrepancies between the two measurements occur because they capture different physical processes within the cell. Testing must be performed at a standardized temperature to ensure comparable results across different cell batches.
Degradation Effects
Repetitive cycling of the battery causes chemical degradation, which leads to a gradual increase in this resistance value. The growth of the solid electrolyte interphase layer on the anode consumes active lithium and increases the ionic barrier. This degradation reduces the maximum power output of the battery and increases the cooling load on the thermal management system.
Sourcing contracts often define the end of life for a cell as the point when its internal resistance doubles from its initial nominal value.
