I've looked at a few leakage current specifications for electrolytic capacitors, and they all seem to specify the value as something like this:
I < 0.01 CV or 3 (μA) after 2 minutes, whichever is greater
Here are a few example datasheets: Panasonic, Multicomp, Nichicon, Rubycon.
Am I right in thinking that the leakage current is a product of capacitance and voltage, i.e. for a 100µF cap on a 5V supply I'd be looking at a leakage current of \$I = 0.01\times100µF\times5V=5\times10^{-6}A = 5µA\$.
Or is that CV unit something totally different?
Additionally, why the long time delay for this rating when a capacitor typically charges in seconds or less?
Best Answer
The leakage spec- in this case 0.01CV (or 3\$\mu\$A) is the product of rated voltage and rated capacitance, not applied voltage. The 3\$\mu\$A, of course, means "whichever is higher" (aka "worse"). So if your cap is rated at 10V/100\$\mu\$F, leakage would be less than 10\$\mu\$A.
SP's rule #1 of data sheet interpretation is:
If a spec can be interpreted in two ways, and one is worse than the other, the worse one is the correct way.
The actual leakage of an electrolytic cap may be much less than the rated value or a bit less. Chances are a higher voltage rated capacitor will have lower leakage when operated at a much lower than rated voltage, but it is not guaranteed, nor will it necessarily last if the capacitor is continuously operated at lower than rated voltage.
The (relatively) long time is, of course, because the initial leakage may be quite a bit higher than spec and it may take some time to drop down to the guaranteed value. This is because the dielectric in an electrolytic cap is actually a very, very thin oxide layer on the etched aluminum plates and it can develop pinholes etc. that are anodized away when voltage is applied.
Here is what United Chemicon has to say about leakage:
A strong 'forming' effect of this type is relatively uncommon with modern parts, and seemed to happen a lot more often in olden days when parts were sitting for some time before being used. Maybe the modern electrolyte is better controlled or more pure, or has preservative additives.
Edit: Note @Dave's comment that the units of the 0.01 parameter must be 1/s.