Technical Papers

Electrochemical Capacitor Power Performance: Stability as a Function of Temperature, Voltage, and Operating Time
presented at the International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices, December 1993
John R. Miller

 

Introduction

Double layer capacitors (DLCs) are power sources that generally do not require maintenance. Sealed DLCs show essentially unlimited charge/discharge cycle life-only small performance changes have been observed after 800,000 charge/discharge cycles at 85 C [1]. Long operational life is typically seen in room-temperature memory backup applications. Reliable operation at elevated temperatures has been reported for times longer than 20,000 hours [2].

Nonetheless, DLCs do experience property changes during use, particularly when operated at high temperatures. For example, their capacitance as measured using DOD-C-29501 test procedures usually declines monotonically with tirne. And their equivalent series resistance (ESR) often increases. Chemical reactions that create gas have also been reported in carbon-based DLCs, another indication of possible ongoing performance changes [3,4].

DLCs are now being considered for high-rate applications like the power system of an electric vehicle (EV) [5]. Their use is suggested as a convenient means to load-level the power requirements of the system. Improved vehicle performance, longer battery life, and reduced life-of-system costs are projected.

Unfortunately, little information is available on the stability of DLC power perforrnance. Clark and Baca recently reported on the power performance stability of a carbon-based DLC at a fixed power level [6]. Their test data on activated life, defined as the discharge time of a 0.16 F system with a 7 W load, spans more than 15,000 hours. A 40% decline in activated life was observed under some conditions. They concluded that additional information was needed to understand the observed performance changes.

Power performance stability is perhaps of equal importance to energy storage stability in DLC/battery applications like EVs. Capacitors are incorporated in the design primarily because of their high-power performance, not their energy storage capability. But stability of both is important in order to maintain acceptable functional performance in this type application.

Consequently, a significant decrease in DLC power performance over time would severely reduce overall system performance, possibly even negating some of the benefits offered by the com bmed system.

The objective of this study was to determine the power performance stability of a carbon-based DLC. Stability of other properties was also examined because of its importance to the overall system design. Presented results show the effects that temperature, operating voltage, and operating time have on DLC performance stability.

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