An analysis of electrochemical capacitors

In most electrochemical capacitor applications it is necessary to series a set of capacitors (cells) to obtain a single capacitor capable of operating at a desired voltage. Performance modeling of these devices has proven difficult because of their nonideal behavior and the need to series a large number of equivalent circuits. In this paper open literature data is analyzed using the equations derived herein. The analysis considers two different load situations, constant resistance and constant-power, for an arbitrary number of series-connected cells. Data from a device built by the Evans Electric Company,(1) and a prototype device are analyzed. In general, excellent agreement is obtained between the model results and the reported data. This agreement allows delineation of capacitor characteristics, at the cell level, that heretofore has not been available. However, it was found that reported data are generally not adequate to allow full capacitor characterization. In part this is due to the requirements of the derived equations, but often it is due to reported data that do not fully describe the performed experiment. In doing this analysis a hierarchy of equivalent circuits was used to provide a good data fit so as to determine capacitor cell parameters. The cell model proposed by Conway(2) and a transmission-line model were used. The dispersion of capacitance values, due to various manufacturing variables, was also modeled using a simple Beta distribution.