Comparison of Methods for Calculating the Capacitance and Internal Resistance of Electrical Double-Layer Capacitors

When evaluating the characteristics of electrical double-layer capacitors (EDLCs), also called supercapacitors (SCs), calculating the capacitance and internal resistance is often necessary. The variety of approaches available for determining these parameters can confuse the researcher and lead to erroneous results. To highlight the features of the practical application of the principles under consideration, in this paper, the authors compare 6 methods for calculating the capacitance and internal resistance of EDLCs. On the basis of the results of computer modelling of electrical equivalent circuits, the linear approximation method (LAM) and energy conversion method (ECM) in galvanostatic charge / discharge (GCD) mode demonstrate high accuracy of capacitance calculations subject to the conditions set out in the IEC 62391 standard. However, in the presence of noticeable distortions of the GCD curves, the most effective is the ECM based on the analysis of the entire discharge section. The low-frequency alternating current method (ACM) and cyclic voltammetry (CV) should be used with caution when the internal resistance of EDLCs is high since a significant error in the capacitance estimate is possible. It is also shown that the internal resistance, estimated by the magnitude of the voltage drop at the moment of switching from the charging mode to the discharging mode, can differ by tens of times from the resistance calculated by approximating a limited linear section of the discharge curve using the least squares method.