Suppose I have a device that utilizes a supercapacitor.
How long will it take to wear out the supercapacitor so that it needs replacement?
In general capacitor lifetime (including supercapacitors) is dependent on three things:
If you want the capacitor to last a long time, limit the applied voltage, keep it cool, and limit the output current. All this should be in the datasheet of your capacitor.
If the capacitor is, say, ceramic or tantalum, the electrolyte is a solid and the cap will basically never go bad. If it's an electrolytic, then it contains fluid which will evaporate and eventually cause the cap to fail. In an electrolytic double-layer super-capacitor, the electrolyte is a combination of a fluid and of activated carbon, so it is mildly vulnerable to evaporation.
More important, though, is the voltage derating of the capacitor. If the voltage burns the cap up on the first use, you won't need to worry about evaporation. A capacitor is a carefully constructed device which separates two conductive films with a thin, thin layer of insulating material over a wide area (folded or rolled into a package). Decrease the separation, and you've got higher capacitance with the same area. This thin separation is vulnerable to high voltages; that's why capacitors have specific voltage maximums, often printed on the case. In a supercap, this barrier is often just nanometers thick, and the dielectric will not insulate high voltages across this short distance.
Using a cap at close to its maximum voltage will cause it to fail more quickly than using it at a lower voltage, this tradeoff is known as a derating curve. It should be available from your capacitor manufacturer.
A capacitor is negatively affected by heat. It causes the electrolyte to evaporate more quickly, causes the dielectric to be weakened, and it can damage the thin conducting elements in the capacitor. Both environmental heat and self-heating effects should be considered. If the capacitor is discharged very rapidly, the small resistance of the foil and leads will be inconsequential compared to the square of the current.
Every capacitor has different characteristics. You should check out the datasheet of you capacitor to figure out when it needs replacement.
The characteristics of your capacitor will change over time, under certain conditions. Usually, (super)capacitors have a parameter called endurance / load life (in hours) or life cycles (in cycles). An example of what you can find in a datasheet:
After 1000 hours application of 5.5V DC at +85°C, the capacitor shall meet the following limits:
Capacitance change: ±30% of initial measured value
Internal resistance: < 4 times of initial specified value
So, in the case above, you can decide if a change of ±30% of the initial capacitance is still suitable for your application. If not, you should replace the capacitor after 1000 hours of operation under those conditions.
An extremely good reference here
Which includes the comment:
Here is a diagram from the above reference which puts some figures on various effects.
The 'key' does not make full sense. The significant effect of voltage on lifetime is worth noting.
This reference is about supercap aging - not immediately apparent.
Appears to be the middle of a wider discussion.
Some related discussion here