Unexpected dielectric behavior in aluminum wet electrolytic capacitors

It is well known that a capacitor can recover part of the charge due to previous polarization after it was quickly shorted. Stretched exponential or power law currents referred to as Kohlrausch dielectric relaxation and described as empirical Curie-von Schweidler law have often been observed. To date, experimental studies on dielectric relaxation have been carried out with small capacitances and for a short time. The present paper shows the results of an experiment conducted with 3300 μF aluminum wet electrolytic capacitors for 95 days at a constant temperature (25°C). After some time had passed where we could distinguish the imprint of stretched power or exponential decay, in some of the tested capacitors the voltage did not decrease and even rose. In a few capacitors, a decay was not detected from the beginning to the end of storage. Measurements conducted with different dissipative loads demonstrated the linear dependence of this voltage on the resistance. Further measurements from 20 to 40°C demonstrated that this current is temperature dependent. The ability of a capacitor to show an increase of charge even if shunted with a dissipative load was found to be correlated with its leakage current. Logistic models to predict this behavior are presented.