A capacitor charges in seconds, and that is not the advantage it sounds like

Everything below about supercapacitors comes from what actually happens rather than from what is supposed to.

Storing charge on a surface, not in a reaction

A battery stores energy by changing the chemical state of its electrode materials, which takes time and physically stresses them. A capacitor stores energy by holding separated charge, with ions collecting in an extremely thin layer against an electrode surface.

Nothing is transformed, so charging and discharging involve moving ions a very short distance rather than driving a reaction through a solid. That difference is why a capacitor can absorb and release energy in seconds without any of the damage a battery would suffer. It is also why the amount stored depends on surface area rather than on the mass of any reacting material.

Enormous surface area is the entire trick

Ordinary capacitors store far too little to be useful for power, because their plate area is limited by physical size. Supercapacitors use electrodes made of highly porous carbon whose internal surface area is vast compared with their outward dimensions.

Because the charge separation layer is only molecules thick, the effective capacitance becomes thousands of times larger than a conventional part. The porous structure means the ions must travel through a maze, which is what limits how fast the largest devices can respond. None of this changes the fundamental storage mechanism; it only multiplies the area over which that mechanism operates.

Fast in, fast out, but not very much

Energy density describes how much can be stored per unit mass, and here supercapacitors fall far short of lithium cells. Power density describes how quickly it can be moved, and here they comfortably exceed almost any battery chemistry available. A device sized to run something for hours would be impractically large, while one sized to deliver a huge burst is small.

That is why they appear wherever a short, hard demand must be met and disappear wherever endurance is what matters. Pairing one with a battery lets the capacitor absorb the spikes while the battery supplies the steady average draw.

The voltage never stays still

A battery holds a roughly constant voltage across most of its discharge, which makes powering electronics from it straightforward. A capacitor's voltage falls in direct proportion to the charge remaining, starting the moment current begins to flow. Extracting most of the stored energy therefore requires a converter that can maintain an output while its input voltage collapses.

That converter adds cost, size and losses, which erodes some of the advantage the capacitor provided in the first place.

It does make the state of charge trivially easy to measure, since the voltage is a direct reading of what remains.

Where they outlast everything else

Because no material is being converted, cycle life is measured in hundreds of thousands of charges rather than hundreds. They also tolerate a much wider temperature range, since the limiting factor is the electrolyte rather than a delicate reaction. For an application cycling constantly for years in an inaccessible place, that durability is worth more than energy density.

In practice, they do self-discharge relatively quickly, so leaving one charged for weeks is not a realistic expectation. Ageing shows up as falling capacitance and rising internal resistance, which is a gradual decline rather than a sudden failure.

Where they quietly appear

Memory backup is a classic use, holding settings and a clock alive for a while when the main supply disappears. Regenerative braking systems use them to absorb a large burst of energy in seconds and release it during the next acceleration. Camera flash circuits have always used capacitors, because a flash needs a very large power delivery over a very short interval.

In the datasheet, grid equipment uses banks of them to ride through momentary sags that would otherwise trip sensitive machinery. In each case the requirement is the same shape: an enormous rate for a very short time, repeated indefinitely.

The takeaway

It is a power device that people keep mistaking for an energy device.

Once you know what it is trading away, the design stops looking arbitrary.