AC Internal Resistance

Measurement Protocol

Executing this test requires the application of a low amplitude alternating current across the terminals of the cell. The resulting voltage response is measured to calculate the complex impedance through Ohm’s law. Because the frequency is high, the capacitive reactances of the double layers act as short circuits, bypassing the charge transfer resistances.

This separation ensures that the output reflects the physical conductivity of the internal components rather than the slower electrochemical kinetics. Handheld meters or automated testing channels apply this method in production lines where rapid validation is mandatory. Temperature control is necessary because electrolyte viscosity and ion mobility alter the resulting values.

Sorting Strategy

Manufacturers utilize the collected values to categorize cells into narrow bins before pack construction. This process prevents the integration of mismatched cells that would otherwise lead to localized thermal strain or uneven voltage distribution during pack operation. The measured ac internal resistance acts as a primary filter to detect defects like poor weld quality or inadequate electrolyte wetting.

When a cell falls outside the specified resistance band, the system automatically rejects it from the premium assembly stream. This sorting minimizes the risk of early pack failure and ensures uniform current distribution across parallel branches.

Degradation Mechanism

Over the operating lifespan of the battery, this value increases due to mechanical and chemical degradation. Continuous cycling leads to the decomposition of the electrolyte, which depletes the active lithium ions and increases separator impedance. Additionally, the growth of resistive surface films on the electrodes restricts ion transport.

Monitoring the upward trend of this resistance helps estimate the remaining useful life of the device. When the value doubles from its nominal starting point, the cell typically approaches its end of life condition for high power applications.