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Figure 38 Common failure mode scenario for Type II electrochemical capacitors
Overvoltage in
one cell in string
Electrolyte
decomposition
Stable operation on
oxygen cycle
Venting of oxygen
gas
Consumption of
electrolyte
Open circuit
Figure 39 Common failure mode scenario for Type III electrochemical capacitors
Cell Life Predicition
Weibull analysis is a proven approach for predicting capacitor life. The experimental
conditions and number of capacitors to test are selected with an experimental design. The
results of life testing and reliability for a 0.1 F, 5.5- NEC electrochemical capacitor are
shown in Figure 15. These are multi-cell modules using type I design. Capacitance loss
occurs faster at the elevated temperatures.
Figure 40 Life performance – capacitance loss as a function of time and temperature (NEC
symmetric, aqueous design type)
The straight-line behavior in the Weibull plot of Figure 41 indicates that this model does
represent cumulative failures. The slope of the 50 C and 70 C test data are the same,
indicating that capacitors at these temperatures have the same failure modes. The 85 C
line has a different slope, indicating introduction of a different cell failure mode. The 50
and 70 C test data can be used to derive a temperature acceleration factor for life
prediction.
Electrochemical Capacitors 67