“Supercapacitors” (or electric double layer capacitors) have been around for a long time. Early uses were as an alternative to rechargeable batteries for retaining memory contents in domestic equipment. They were even made to look a bit like batteries in some cases, rather than electrolytic capacitors, such as these Panasonic RG series ones:
This particular range has a maximum operating voltage of 3.6V and capacitances of 0.22F up to 1.5F depending on the physical size. The benefits of using a supercapacitor (or electric double layer capacitor or gold capacitor depending on what the manufacturer chooses to call them) are fast charging, virtually unlimited charge/discharge cycles, and low self-discharge. The disadvantages are cost, considerably smaller capacity than a battery and a voltage that drops steadily during discharge rather than maintaining the voltage until almost discharged. This last point may be important for your application. A 3V backup battery would stay close to the nominal 3V for most of its discharge time before rapidly dropping when fully discharged, as shown in this discharge curve for the Panasonic VL2020 lithium rechargeable battery.
By contrast, a capacitor (super or otherwise) will reduce its voltage linearly with a constant discharge current (dV/dt = i/C where i is the discharge current and C is the capacitance). So, if you charge a capacitor to 3.3V and have a memory cut off at 2V, you will be able to squeeze almost all the useful energy out of the lithium rechargeable battery. But you’ll only reach 60% of the total stored energy in a capacitor before the memory possibly fails, or your real-time clock/calendar (RTCC) stops keeping time. So, you need to bear that in mind when comparing batteries with capacitors and selecting a capacitor size. However, the considerably lower storage capacity is more likely to be the limiting factor rather than the end discharge voltage.
Taking the above VL2020 as an example, at 100µA it would last around 200 hours or 8 days. A 0.22F capacitor charged to 3.3V and then discharged to 2V at 100µA would last 2860 seconds or 0.8 hours. So, there is clearly a significant difference in the energy storage capacity of a battery compared to a supercapacitor. Even thought the shape and size of the devices is not identical in this example, the figures are representative. You would need to be discharging at considerably less than 100µA to have a useful storage capacitor in something as small as 0.22F, assuming you wanted a working time measured in days, not minutes.
Normally small supercapacitors would be used as backup batteries, so the current drain is likely to be sub-microamp even with an RTCC. Also, larger devices are available if required. The real benefit comes from the huge number of charge/discharge cycles that a supercapacitor can endure compared to a conventional battery.
Moving to larger capacitors, they have other uses, such as providing a high burst of current. They are also viable as power energy sources for short term use. While they won’t replace batteries completely, they are useful where you require rapid charging. There is a small cordless screwdriver, for example, which uses supercapacitors. The main benefit is that it can be charged in 60 seconds. However, the energy to charge it has to come from somewhere so to charge it in 60 seconds implies the energy capacity is small or the charger is very powerful.
The ability to rapidly charge and discharge supercapacitors is one of the reasons companies such as Maxwell Technologies are suggesting they can be used to harvest energy from regenerative braking systems or provide a high power burst to help with surge applications. I have used supercapacitors in just such an application where a high current was required from batteries which couldn’t actually deliver the high current. While I could have used larger batteries, I didn’t need the extra storage capacity and it would have increased the size of the product too much.
The largest single capacitor from Maxwell is the 3400F and can deliver 2000A. It has a projected life of 1,000,000 cycles and 10 years at full voltage. Like most supercapacitors, the voltage is low – only 2.85V. Any higher voltage supercapacitors are likely to be multiple capacitors in series. Ready built high voltage modules are available which can be the size of a truck battery. For example, a 125V 63F module which is larger than 2′ x 1′ x 1′ and weighs over 130lb.
When using multiple capacitors in series to increase the voltage, you need to consider charge balancing, unless you buy a module where that is taken care of for you. This is a similar problem to that of series secondary lithium cells when charging – appropriate circuitry is needed to prevent one cell in the set being overcharged. In the case of supercapacitors you need to prevent one capacitor exceeding its rated voltage bearing in mind, the capacitors will have different values due to manufacturing tolerances. Active cell balancing can be used or simply resistors. Resistors have the disadvantage that they waste power.
I don’t think supercapacitors are going to replace batteries in many applications, but they do seem to have a niche market of providing surge energy and accepting surge energy to charge them.