Characterization and Modeling of the Voltage Dependency of Capacitance and Impedance Frequency Characteristics of Packed EDLCs

Frequency characteristics and their dependence on charge voltage for a packed electric double layer capacitor (EDLC), which consists of a series of connected EDLC cells, are characterized and modeled based on small ac signal analysis. The results indicate that rated capacitance of a packed EDLC is valid only at frequencies lower than 0.01 Hz at the rated charge voltage but the capacitance is lower for frequencies higher than 0.01 Hz and for lower charge voltages. A conventional simple RC-equivalent circuit, consisting of a capacitor and a resistance in series, is inadequate for expressing EDLC frequency characteristics; therefore, a second-order RC equivalent circuit is used as a model for a packed EDLC. The charge voltage dependency in the frequency characteristics of packed EDLC is evaluated, based on this equivalent circuit. The parameters of the equivalent circuit and their charge voltage dependencies are evaluated from the results. The proposed packed EDLC model is validated by charge and discharge operations in an experimental circuit. The results show that the model can accurately assess the charge stored in a packed EDLC.

I. Introduction

The electric double-layer capacitor (EDLC, also known as a super capacitor) is being utilized in high-power, high-voltage, and high-energy storage as an alternative to, or a compensator for batteries [1]–[10]. Because the charge and discharge operations in an EDLC are not accompanied by chemical reactions, it affords quick response, high input and output currents, is maintenance free, and has a longer operational life [11]–[13]. Although the capacitance of an EDLC is higher than conventional electrolytic capacitors, the rated voltage of a unit cell must be low (1.0 V for aqueous electrolytes and 3.0 V for organic electrolytes) to prevent an electrolytic process at the electric double layer. Therefore, capacitor cells must be connected in series and used in a packed configuration to obtain sufficiently high output voltage for practical circuit applications.