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
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 . 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
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) . 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 . 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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