Supercapacitors and Ultracapacitors: Complete Guide to EDLC Technology, Selection, and Applications
Last Updated: February 2026 | Reading Time: 16 minutes
Supercapacitors — also called ultracapacitors or electric double-layer capacitors (EDLCs) — occupy a unique position in the energy storage spectrum. They store 10 to 100 times more energy than conventional capacitors, deliver and absorb energy 10 to 100 times faster than batteries, and survive hundreds of thousands of charge-discharge cycles without degradation. No battery chemistry can match that cycle life, and no conventional capacitor can match that energy density.
For engineers designing backup power systems, regenerative braking circuits, energy harvesting front-ends, or grid stabilization equipment, supercapacitors solve problems that neither batteries nor traditional capacitors can address alone. But selecting and applying them requires understanding their unique characteristics, limitations, and design considerations.
This guide covers supercapacitor construction, key specifications, sizing calculations, and practical application guidelines.
A supercapacitor stores energy electrostatically at the interface between an electrode and an electrolyte — the same fundamental mechanism as any capacitor. What makes supercapacitors different is the extraordinary surface area of their electrodes and the nanoscale thickness of the charge separation layer.
1. Electric Double-Layer Capacitors (EDLCs)
The most common type. EDLC electrodes are made from activated carbon with surface areas of 1,000 to 3,000 m²/g. When voltage is applied, ions in the electrolyte migrate to the electrode surface and form two layers of charge (the "double layer") separated by roughly 0.3 to 0.8 nm — orders of magnitude thinner than any manufactured dielectric film. Since capacitance is proportional to area and inversely proportional to distance, this combination of enormous area and nanoscale separation produces capacitances measured in Farads rather than microfarads.
EDLCs store energy through purely electrostatic means. No chemical reactions occur, which is why they achieve essentially unlimited cycle life.
Key EDLC manufacturers and series:
- Nichicon — JJD, JLC, and EVerCAP series (2.5V to 3.0V cells, 1F to 3,000F)
- Panasonic — Gold Cap and HB/HL series
- Eaton — XL60, HV, and PM series (supercapacitor modules and cells)
- Murata — DMF, DMH series (small form factor, coin cell type)
2. Pseudocapacitors
These use electrode materials — typically metal oxides like MnO2 or RuO2, or conductive polymers — that undergo fast, reversible Faradaic (redox) reactions at the surface. Energy density is 2 to 3 times higher than EDLCs, but cycle life drops to roughly 100,000 to 500,000 cycles.
3. Hybrid Supercapacitors
These combine one EDLC electrode (activated carbon) with one battery-type electrode (lithium-intercalation or metal oxide). Energy density approaches some battery chemistries while maintaining faster charge/discharge rates and better cycle life. Examples include lithium-ion capacitors (LICs) from Taiyo Yuden and JM Energy.
Understanding where supercapacitors fit relative to conventional capacitors and batteries is essential for correct application.
| Parameter | Conventional Capacitor | Supercapacitor (EDLC) | Lithium-Ion Battery |
|---|
| Energy density | 0.01–0.1 Wh/kg | 1–10 Wh/kg | 100–265 Wh/kg |
| Power density | 10,000+ W/kg | 1,000–10,000 W/kg | 250–1,000 W/kg |
| Capacitance range | pF to mF | 1F to 12,000F | N/A |
| Charge time | Nanoseconds to microseconds | Seconds to minutes | 30 minutes to hours |
| Cycle life | Essentially infinite | 500,000 to 1,000,000+ cycles | 500–2,000 cycles |
| Voltage per cell | Rated voltage (wide range) | 2.5V to 3.0V (aqueous: 1.2V) | 3.2V to 4.2V |
| Self-discharge | Very low | Moderate (5–40% per month) | Low (2–5% per month) |
| Operating temperature | -55°C to +125°C (type dependent) | -40°C to +65°C (typical) | 0°C to +45°C (typical) |
| Cost per Wh | Very high | $5–20/Wh | $0.10–0.50/Wh |
| Cost per kW | Very low | $5–20/kW | $50–200/kW |
Key insight: Supercapacitors win on power density and cycle life. Batteries win on energy density and cost per Wh. Conventional capacitors win on voltage flexibility, frequency response, and extreme temperature range. The best designs often combine two or three of these technologies.
Rated in whole Farads — a dramatic contrast to the microfarad and nanofarad values of conventional capacitors. Common cell capacitances range from 1F (coin cells for RTC backup) to 3,000F or more (large cylindrical cells for industrial and automotive use). Tolerance is typically -0/+20% or -10/+30%.
Individual EDLC cells are rated at 2.5V, 2.7V, or 3.0V depending on the electrolyte chemistry. Exceeding the rated voltage — even briefly — causes electrolyte decomposition, gas generation, and rapid degradation. Unlike electrolytic capacitors where occasional overvoltage may be tolerable, supercapacitors must never exceed their rated voltage. For higher system voltages, cells are connected in series with voltage balancing (discussed below).
ESR determines power delivery capability and internal heating. Supercapacitor ESR values range from under 1 milliohm for large industrial cells to several ohms for small coin cells. ESR is specified at 1 kHz (AC ESR) or as DC resistance (DCR). The DC resistance is typically 1.5 to 2 times the AC ESR.
Power delivery: Peak current = V_rated / ESR. A 2.7V, 3,000F cell with 0.29 milliohm ESR can theoretically deliver over 9,000 amps — though practical limits are much lower due to thermal constraints.
Supercapacitors have significantly higher leakage current than conventional capacitors. A typical 2.7V, 100F cell may have a specified leakage of 0.05 to 0.5 mA after 72 hours at rated voltage and 25°C. This leakage limits the usefulness of supercapacitors for long-term energy storage (weeks to months). For backup power holding times measured in hours to days, leakage must be factored into the sizing calculation.
EDLC cycle life is typically specified at 500,000 to 1,000,000 cycles with less than 30% capacitance loss and less than 100% ESR increase. In practice, many EDLC cells exceed these specifications. This is orders of magnitude better than any rechargeable battery and makes supercapacitors ideal for applications with frequent charge-discharge cycling.
Independent of cycling, supercapacitors age through electrolyte decomposition. Calendar life is strongly temperature-dependent, following the same Arrhenius relationship as electrolytic capacitors: life roughly doubles for each 10°C reduction in temperature. Typical rated life is 1,500 to 2,000 hours at maximum temperature and voltage, translating to 10 to 15 years at moderate conditions.
The energy stored in a supercapacitor is:
E = ½ * C * V²
Where:
E = energy in Joules (1 J = 1 Ws)
C = capacitance in Farads
V = voltage in Volts
Example: A 100F, 2.7V cell stores ½ * 100 * 2.7² = 364.5 J = 0.101 Wh
For a constant power load, the discharge time from initial voltage V_i to final voltage V_f is:
t = ½ * C * (V_i² - V_f²) / P
Where:
t = discharge time in seconds
C = capacitance in Farads
V_i = initial voltage (fully charged)
V_f = minimum acceptable voltage
P = load power in Watts
Example: How long can a 100F, 2.7V supercapacitor power a 10W load down to 1.35V (50% voltage)?
t = ½ * 100 * (2.7² - 1.35²) / 10 = ½ * 100 * (7.29 - 1.82) / 10 = 27.3 seconds
Note that 75% of the stored energy is delivered in the first 50% voltage drop. Supercapacitor systems typically use DC-DC converters to maintain a regulated output voltage as the supercapacitor discharges.
For a constant current load:
t = C * (V_i - V_f) / I
Where:
I = discharge current in Amps
Example: 100F cell, 2.7V to 1.35V, at 5A constant current:
t = 100 * (2.7 - 1.35) / 5 = 27.0 seconds
Working backward from a power requirement:
C = 2 * P * t / (V_i² - V_f²)
Example: 50W load for 10 seconds, voltage range 48V to 36V (series-connected module):
C = 2 * 50 * 10 / (48² - 36²) = 1,000 / (2304 - 1296) = 1,000 / 1,008 = 0.99F
For a 48V module, you would need at least a 1F module — or equivalently, 18 series cells of 2.7V each, each cell requiring 18F capacitance (series capacitance = C_cell / n_series).
Since individual EDLC cells are limited to 2.5-3.0V, most practical applications require series-connected cells. A 48V module requires 18 cells of 2.7V each. A 400V DC bus requires approximately 150 cells.
Manufacturing tolerances cause each cell to have slightly different capacitance and leakage current. In a series string, the cell with the lowest capacitance sees the highest voltage during charging. Without balancing, this cell will exceed its rated voltage and degrade rapidly, creating a cascade failure as the weakened cell places even more stress on its neighbors.
Passive balancing (resistive): Parallel resistors across each cell bleed off excess voltage. Simple and reliable, but wastes power continuously. The balancing resistor value is typically chosen so that its bleed current is 5 to 10 times the worst-case cell leakage current mismatch.
Active balancing: Uses transistor circuits or switched-capacitor networks to transfer energy from higher-voltage cells to lower-voltage cells. More complex but dramatically more efficient. Essential for high-power systems where passive balancing losses are unacceptable.
Zener/TVS clamping: Zener diodes or TVS devices across each cell clamp the voltage at a safe maximum. Acts as a safety backup rather than a primary balancing method.
Most commercial supercapacitor modules from Eaton, Maxwell (now part of Tesla's energy division), and Nichicon include integrated passive or active balancing circuits.
Supercapacitors provide seconds to minutes of backup power for graceful shutdown, data saving, or bridging to generator startup. Unlike batteries, they work reliably at -40°C and don't require temperature-controlled enclosures. Common applications include PLC memory backup, server holdup power, SSD power-loss protection, and industrial control system ride-through.
Electric and hybrid vehicles, elevators, cranes, and industrial vehicles use supercapacitors to capture braking energy that would otherwise be dissipated as heat. The high power density and cycle life of supercapacitors make them ideal for absorbing the high-current, short-duration energy pulses generated during braking. Maxwell/Tesla BCAP series and Skeleton Technologies SkelCap series dominate this application.
Solar, vibration, thermal, and RF energy harvesters produce intermittent, low-power energy. Supercapacitors buffer this energy for periodic high-power transmission bursts in wireless sensors and IoT devices. Small coin-cell supercapacitors (Murata DMF series, Panasonic Gold Cap) are common in these applications.
Supercapacitor banks provide short-duration power injection to stabilize voltage and frequency on electrical grids during sudden load changes or renewable energy fluctuations. These systems typically use large module arrays providing megawatts of power for seconds.
Supercapacitors bridge the gap between power failure and diesel generator startup (typically 5 to 15 seconds). This application eliminates the battery — and its associated maintenance, temperature sensitivity, and limited cycle life — from the UPS system for short-duration bridging needs.
When selecting a supercapacitor for your application, evaluate these parameters in order:
- Required voltage — Determines cell count in series (and therefore module size, cost, and balancing complexity)
- Required energy or holdup time — Determines capacitance per cell
- Peak current — Determines maximum acceptable ESR
- Operating temperature range — Narrows manufacturer and series options; derate at elevated temperatures
- Leakage current — Critical for long-holdup or energy-harvesting applications
- Physical form factor — Cylindrical cells, prismatic cells, coin cells, or pre-built modules
- Calendar life requirements — Temperature and voltage derating extend life significantly
There is no difference. "Supercapacitor" and "ultracapacitor" refer to the same technology. "Ultracapacitor" was popularized by Maxwell Technologies (now part of Tesla) as a product name, while "supercapacitor" is the more common generic term used in academic literature and by most manufacturers. The IEEE and IEC standards use "electrochemical capacitor" or "electric double-layer capacitor (EDLC)" as the formal terms.
In some applications, yes. Supercapacitors can replace batteries for short-duration backup power (seconds to minutes), high-cycle applications (hundreds of thousands of cycles), extreme temperature environments (-40°C to +65°C), and applications where maintenance-free operation over 10+ years is essential. They cannot replace batteries where high energy density is needed (hours of runtime), where low self-discharge is critical (months of storage), or where cost per Wh is the primary constraint. Many modern designs use supercapacitors and batteries together in hybrid configurations — the supercapacitor handles peak power demands while the battery provides sustained energy.
Not very long compared to a battery. Self-discharge rates vary by manufacturer, cell size, and temperature, but a typical EDLC will lose 5 to 20% of its voltage in the first 24 hours and 20 to 40% within a month. This is fundamentally different from batteries, which can hold charge for months. For backup power applications, you must factor self-discharge into your sizing calculations by treating the starting voltage as the charged voltage minus the self-discharge loss over the expected idle period.
Exceeding the rated voltage causes electrolyte decomposition, producing gas inside the sealed cell. This increases internal pressure, increases ESR, reduces capacitance, and can ultimately cause the cell to vent or rupture. Unlike electrolytic capacitors, which can tolerate brief overvoltage transients, supercapacitor degradation from overvoltage is rapid and cumulative. Even a few percent of overvoltage sustained over time will drastically shorten cell life. Voltage balancing in series stacks is essential to prevent any individual cell from exceeding its rating.
Supercapacitors are charged with a constant-current/constant-voltage (CC-CV) profile, similar to lithium-ion batteries. Apply a constant current until the voltage reaches the rated value, then hold that voltage while the current tapers. Unlike batteries, supercapacitors can accept extremely high charge currents — limited only by the ESR and the thermal capacity of the cell. For fast charging, charge at the maximum current the cell ESR and your power source can support. For maximum cell life, limit the charge voltage to 80 to 90% of the rated voltage and keep the temperature below 40°C.
