Internal resistance is one of a few key characteristics that define a lithium ion cell’s performance. A cell’s power density, dissipation, efficiency, and state of health (SoH) all depend on its internal resistance. However, a cell’s internal resistance is anything but a single, unvarying value. It has a complex frequency-dependent nature that relates to specific electronic and ionic attributes within the cell. Further, it is dependent on the cell’s temperature, state of charge (SoC), size, chemical makeup, construction, and age. The state of the cell and test conditions need to be explicitly set to have results that are relevant and comparable.
The most common methodologies for measuring a cell’s internal resistance include electrochemical impedance spectroscopy (EIS), alternating current internal resistance (AC-IR), and direct current internal resistance (DC-IR). Each of these are uniquely different making them particularly useful in certain applications while not being well suited for others.
Internal resistance is one of a few key characteristics that define a lithium ion cell’s performance and hence that is why it is tested extensively both in development and manufacturing. A cell’s internal resistance is not a fixed value; it increases with lower frequency or longer load duration due to additional resistive elements due to electrochemical and ionic mechanisms in the cell. Furthermore, it also varies with temperature, state of charge (SoC), size, chemical makeup, and age.