The effective insulator (dielectric) in an electrolytic capacitor is a
layer of metal oxide that forms on the positive electrode of the
capacitor. When a capacitor has not been used for a long time, this
oxide layer diminishes and we eventually have bare metal plates in the
electrolytic paste without a real good insulator between them. When the
oxide-deficient capacitor is used the next time, if you are lucky, the
oxide layer is recreated and the capacitor is reformed (rejuvenated).
The reformation of the oxide layer is not instantaneous and can take
hours. While it is reforming, the capacitor is "leaky." The leakage
current creates heating. If you are unlucky the heating can be
sufficient to boil the electrolytic paste and the capacitor explodes or
at least discharges its guts through the blow-out plug.
If you have already plugged the unit into AC power and it survived, you
were lucky. If a device like a photoflash has been idle for several
years, it might be a good idea to plug it in briefly (a second or two)
then let it sit for a while, and repeat this several times over an hour
or so before leaving it plugged in. That allows some reforming of the
oxide layer without allowing too much prolonged high-power leaking.
Until the capacitor is reformed, the drain on the battery-operated power
supply might be too much to allow it to work properly. If the unit is
left plugged into the AC source for a couple days, the capacitor should
completely reform and then the battery circuit might work. If not, it
could be that the capacitor does need replaced, but it also could be
that the battery-operated power supply is defective.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu