Hi,
for projects with very low average current consumption but "large" current peaks and battery with high internal resistance (example: coin cell powered IR remote) it is good to have large electrolyte capacitor to cover the increased current demand. But from datasheets it looks like large electrolyte capacitor has large leakage current. For example datasheet of my electrolyte capacitor says "0.01CV or 3uA (after 2 minutes) whichever is greater" where C is rated capacity and V is rated voltage. From various sources I know leakage current is (much) less at voltage lower than the rated one and it decreases with time for which voltage is applied. So I tried to measure "real" leakage current of various capacitors I have. I connected 4700uF/6.3V cap to 2 AA batteries via a 10k resistor and I measured voltage over the resistor. I was decreasing quickly as the cap charged and settled on about 3mV after a few minutes meaning about 0.3uA of "leakage current". I left it connected and after a day made the measurement again. I measured 0V with my DMM. So I tried to swap the 10k resistor for 100k and measured again - this time I got 0.1mV indicating leakage around 1nA!
Possible explanations:
My measurements are somehow wrong - maybe mains hum or resistor noise keep the cap charged and the pulses of "charging current" are so short the DMM does not notice?
The leakage current drops quickly and it is negligible after a few hours - the leakage is important only when the cap is charged only for brief period of time?
When external voltage is applied the leakage current is minimal. If disconnected from power source the leakage will rise again discharging the cap?
If you have old electrolytics that haven't been charged for a decade or more, you need to "reform"
them by gradually raising the voltage and monitoring the leakage current, allowing the electrolytic
reactions to complete again - otherwise severe leakage can happen and overheat or damage the
capacitor. This is more important for high voltage capacitors where the power dissipated in the
leakage can be considerable.
This reforming happens whenever a cap is powered up, but to a much lesser extent if its been used
recently - which is exactly what your measurements show.
The actual insulating layer in an aluminium electrolytic is the anodized surface of the aluminium
foil, a layer of alumina a few nanometers thick for low voltage capacitors. Since alumina is an
extremely good insulator, and since the layer is incredibly thin, you get very high levels of capacitance
in a small cap. The applied voltage maintains the layer from slow degradation by the electrolyte,
hence the reforming process.
Of course the spec in the data sheet is for MAX leakage, allowing for full voltage, full temp rating, and nominal aging.... hopefully. A brand new cap at room temp and etc should be better.
OK, I did a few more experiments. I think I can conclude leakage current of caps is greatly overestimated. I think the best experiment was the one with op amp. I used LM2904 powered from 9V (single rail) in this configuration:
The bias current of the op amp is about 37 nA (using 10M resistor instead of C1 led to ~370mV at output of the op amp). I discharged 1000uF/6.3V cap. At first the voltage on the cap (measured as voltage on output of op amp) increased by about 1V/8h - which means nearly all of the bias current was used to charge the cap. Then we left for weekend. When we returned voltage was close 6V but it still grows - about 0.1V/8h. So the leakage current of the cap is still lower than 37 nA bias current but close to it. It is consistent with the fast leakage current grows when close to the rated voltage. But despite the voltage is around 6.2V now it is still less than the bias current - and that is much much less than expected leakage current (but I must admit there is hardly 20°C here - it would be surely worse @ higher temperature).
Leakage is always much larger at higher temperatures, maybe increasing exponentially with temperature (not
sure what the relation is for Al electrolytics, but semiconductor leakage is exponential. The specs have
to cover manufacturing spread and working life too... A brand new component at room temperature isn't
going to be the worst case!
