A supercapacitor stores charge in an electric double layer only a molecule thick, giving thousands of farads in a small can. It charges in seconds, survives huge numbers of cycles and bridges short power gaps that would stress a battery.
Between the fast electrolytic capacitor and the energy dense battery sits a third device that stores far more charge than one and delivers far more power than the other. Knowing how it works helps you size backup circuits correctly.
What Is a Supercapacitor?
A supercapacitor is an electrochemical capacitor that stores energy in an electric double layer at the surface of porous carbon electrodes. It follows the same basic law as the ordinary device described in capacitor working principle, but with capacitance measured in farads instead of microfarads.
Explain that Stuff notes that the largest commercial units reach several thousand farads. It also reports that datasheets suggest you can cycle a supercapacitor perhaps a million times, far beyond any rechargeable battery.
Other common names are ultracapacitor, EDLC and gold capacitor. All of them describe the same idea of a very large electrode area separated from its ions by an extremely thin layer.
How a Supercapacitor Stores Energy
Each supercapacitor cell has two activated carbon electrodes, a porous separator and a liquid electrolyte full of ions. When voltage is applied, positive ions gather at the negative electrode and negative ions at the positive electrode.
The charge separation distance is about one molecule, and activated carbon offers a surface area around 1000 square metres per gram. Tiny distance and huge area together explain why a supercapacitor reaches such high capacitance.
No chemical reaction changes the electrode material in a pure double layer device. That is why a supercapacitor charges quickly and ages slowly compared with a battery.
The electrolyte decomposes if the cell voltage rises too far, which is why a single organic electrolyte supercapacitor cell is limited to about 2.7 to 3.0 V. Higher voltages need several cells in series.
Types: EDLC, Pseudocapacitor and Hybrid
Pure double layer storage on activated carbon.
Fast surface redox reactions on metal oxides or conducting polymers.
One carbon electrode and one lithium doped electrode.
A lithium ion capacitor typically works between about 2.2 and 3.8 V, and it must not be discharged fully like an EDLC. It offers more energy per cell but gives up some cycle life and low temperature performance.
For comparison with ordinary parts, read our overview of capacitor types. The supercapacitor sits at the far high capacitance end of that family.
Supercapacitor Energy and Usable Energy Formula
The energy stored in any capacitor is half the capacitance times voltage squared. A supercapacitor rarely discharges to zero, because the load stops working below some minimum voltage, so only part of the energy is usable.
Usable energy E = ½ × C × (Vmax² minus Vmin²)
Backup time t = E ÷ P
Example: 10 F module, 5.0 V to 3.0 V, 0.5 W load
Stored = ½ × 10 × 5.0² = 125 J
Usable = ½ × 10 × (25 minus 9) = 80 J
t = 80 ÷ 0.5 = 160 s, 2.67 min
Only 64 percent of the stored energy is usable in this example, because the last 45 J stay below 3.0 V. A boost converter can extract more energy by running the supercapacitor down to a lower voltage.
Supercapacitor Backup Time Calculator
The calculator assumes constant power and a lossless converter. In practice, reduce the result by the converter efficiency and allow for capacitance falling as the part ages.
Size a supercapacitor bank for its end of life capacitance, which is often specified at 70 to 80 percent of the initial value. A design that only just works when new will fail after a few years.
Second Example: RTC and Memory Backup
A real time clock chip drawing 2 µA at 3.3 V needs only about 6.6 µW. A 0.1 F supercapacitor charged to 3.3 V and allowed to drop to 1.8 V stores about 0.38 J usable, enough for roughly 16 hours at that load.
This is why a small coin style supercapacitor often replaces a lithium coin cell in meters and controllers. Compare it with the method in how to calculate battery backup time, which uses ampere hours instead of joules.
ESR, Peak Power and Leakage
Every supercapacitor has an equivalent series resistance that causes an instant voltage step when load current starts. The Cornell Dubilier guide explains that ESR is measured at 1 kHz with a 4 probe method, and our article on capacitor ESR covers the basics.
Maximum matched power Pmax = V² ÷ (4 × ESR)
Example: 2.7 V cell, ESR 20 mΩ
Pmax = 2.7² ÷ (4 × 0.020)
Pmax = 91.1 W
Matched power is a theoretical peak, because half of it heats the ESR. Real designs run far below it to limit heating and voltage sag.
Leakage current and self discharge are higher than in a battery, so a supercapacitor slowly loses voltage over days or weeks when left unused. Check the leakage figure after 72 hours in the datasheet for long standby designs.
Supercapacitor vs Battery vs Electrolytic Capacitor
| Property | Electrolytic Capacitor | Supercapacitor | Lithium Ion Battery |
|---|---|---|---|
| Capacitance or capacity | µF to mF | 0.1 F to thousands of F | Ampere hours |
| Energy density | Very low | About 5 to 10 Wh/kg | Over 100 Wh/kg |
| Power density | Very high | High | Moderate |
| Charge time | Milliseconds | Seconds | Hours |
| Cycle life | Very high | About a million | Hundreds to thousands |
| Cell voltage | Up to hundreds of V | About 2.7 V | About 3.6 V |
A supercapacitor fills the gap between the two, and it avoids many ageing mechanisms described in causes of battery failure. Its weakness is energy density, so it bridges seconds to minutes rather than hours.
Electrolytic parts still dominate filtering and ripple duty, as shown in electrolytic, ceramic and tantalum failure modes. The supercapacitor is a storage part, not a replacement for a fast decoupling capacitor.
Cornell Dubilier states that supercapacitor life roughly doubles for every 10 °C drop in temperature or 0.1 V drop in applied voltage. A small derating therefore buys years of extra service.
Series Strings and Cell Balancing
Because one cell only handles about 2.7 V, a 16 V module uses six cells in series for 16.2 V total. Tolerances in capacitance and leakage make the cells share voltage unevenly, so one cell can be overcharged.
Cornell Dubilier describes passive balancing with a resistor across each cell and active balancing circuits that regulate cell voltages more efficiently. Passive resistors are cheap but drain the string continuously, while active ICs draw only microamps.
5 Smart Supercapacitor Applications
In a UPS a supercapacitor bank can ride through short dips that would otherwise start battery cycling. Controllers use a similar bank to hold up the logic supply described in redundant PLC power supplies.
Power supply designers use them to extend hold up time, which slows the wear discussed in power supply ageing in automation. A buck converter or linear regulator then turns the falling bank voltage into a steady rail.
Always limit inrush current when a discharged supercapacitor bank is connected to a live bus. A current limited charger or a series resistor with bypass prevents fuse blowing and contact welding.
Selecting and Installing a Supercapacitor
- Required usable energy calculated between Vmax and Vmin.
- End of life capacitance and ESR used in the sizing.
- Cell voltage derated for the maximum ambient temperature.
- Balancing method chosen for series strings.
- Charger current limited and voltage clamped.
- Leakage current acceptable for the standby period.
- Polarity marked clearly on the board and case.
Mount the parts away from hot resistors and heatsinks, because heat is the main enemy of electrolyte life. For small decoupling duties, read how decoupling capacitors work instead, since a supercapacitor has too much ESR for high frequency noise.
- Very high power density.
- Charges in seconds.
- About a million cycles.
- Wide temperature range.
- No thermal runaway like lithium cells.
- Low energy density.
- Low cell voltage needs series strings.
- Higher self discharge.
- Voltage falls linearly during discharge.
- Needs balancing and inrush control.
Cornell Dubilier Technical Guide PDF
Double Layer Animation Video
Supercapacitor FAQ
A supercapacitor is a capacitor that stores charge in an electric double layer on porous carbon electrodes. It offers capacitance from a fraction of a farad to thousands of farads in a compact package.
It sits between electrolytic capacitors and batteries in energy and power. Typical uses are backup power, pulse loads and energy recovery in machines and vehicles.
A battery stores energy through chemical reactions, while an EDLC supercapacitor stores it electrostatically in the double layer. That difference allows much faster charging and far longer cycle life.
The trade off is energy density, which is roughly ten times lower or more. So the supercapacitor bridges seconds to minutes, while the battery covers hours.
The organic electrolyte breaks down if the voltage across the double layer rises too high. Makers therefore rate most EDLC cells around 2.7 to 3.0 V at normal room temperature, with lower limits when hot.
Higher voltages therefore need several cells in series with balancing. A 16 V supercapacitor module, for example, uses six cells of 2.7 V each.
Use E equals half C times the difference of Vmax squared and Vmin squared. The result is in joules when C is in farads and voltages are in volts.
Divide that energy by the load power in watts to get the backup time in seconds. Allow extra margin for converter losses, ESR drop and capacitance loss with ageing.
ESR is the internal series resistance of the electrodes, electrolyte and terminals. It causes an immediate voltage drop equal to current times ESR when a load is applied.
Low ESR allows higher peak power and less heating during pulse loads. It rises slowly over the life of the part, so always check the end of life values in the datasheet.
Cells have slightly different capacitance and leakage, so they do not share the total voltage equally. The weakest cell can then exceed its rating and age rapidly.
Passive resistors or active balancing ICs keep each cell voltage within safe limits. Cornell Dubilier notes that active balancing regulates cell voltage more efficiently during repeated cycling.
Yes, many controllers use a supercapacitor to keep the clock and retentive memory alive during short outages. It avoids replacing a backup battery every few years.
The hold time is usually hours to days, depending on supercapacitor size, load current and leakage. For longer outages or full process control, a battery backed supply is still required.
Related Articles
- Capacitor Working Principle
- Capacitor Types Explained
- Capacitor ESR Explained
- How to Calculate Battery Backup Time
- UPS Working Principle
External References
- Supercapacitor Technical Guide, Cornell Dubilier
- How Do Supercapacitors Work, Explain that Stuff
- Supercapacitor, Wikipedia
What We Learn Today
- A supercapacitor stores charge in a double layer on porous carbon, giving farads of capacitance, fast charging and about a million cycles.
- Usable energy equals half C times the difference of Vmax squared and Vmin squared, and backup time equals that energy divided by load power.
- Low cell voltage, ESR, self discharge and balancing in series strings must all be considered when sizing a backup bank.