The supercapacitor, also known as ultracapacitor or double-layer capacitor, differs from a regular capacitor in that it has very high capacitance. A capacitor stores energy by means of a static charge as opposed to an electrochemical reaction. Applying a voltage differential on the positive and negative plates charges the capacitor. This is similar to the buildup of electrical charge when walking on a carpet. Touching an object releases the energy through the finger.
There are three types of capacitors and the most basic is the electrostatic capacitor with a dry separator. This classic capacitor has very low capacitance and is mainly used to tune radio frequencies and filtering. The size ranges from a few pico-farads (pf) to low microfarad (μF).
The electrolytic capacitor provides higher capacitance than the electrostatic capacitor and is rated in microfarads (μF), which is a million times larger than a pico-farad. These capacitors deploy a moist separator and are used for filtering, buffering and signal coupling. Similar to a battery, the electrostatic capacity has a positive and negative that must be observed.
The third type is the supercapacitor, rated in farads, which is thousands of times higher than the electrolytic capacitor. The supercapacitor is used for energy storage undergoing frequent charge and discharge cycles at high current and short duration.
The supercapacitor has evolved and crosses into battery technology by using special electrodes and electrolyte. While the basic Electrochemical Double Layer Capacitor (EDLC) depends on electrostatic action, the Asymmetric Electrochemical Double Layer Capacitor (AEDLC) uses battery-like electrodes to gain higher energy density, but this has a shorter cycle life and other burdens that are shared with the battery.
Several types of electrodes have been tried and the most common systems today are built on the electrochemical double-layer capacitor that is carbon-based, has an organic electrolyte and is easy to manufacture.
All capacitors have voltage limits. While the electrostatic capacitor can be made to withstand high volts, the supercapacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible, but at a reduce service life. To get higher voltages, several supercapacitors are connected in series. Serial connection reduces the total capacitance and increases the internal resistance. Strings of more than three capacitors require voltage balancing to prevent any cell from going into over-voltage. Lithium-ion batteries share a similar protection circuit.
The specific energy of the supercapacitor ranges from 1Wh/kg to 30Wh/kg, 10–50 times less than Li-ion. The discharge curve is another disadvantage. Whereas the electrochemical battery delivers a steady voltage in the usable power band, the voltage of the supercapacitor decreases on a linear scale, reducing the usable power spectrum.
Take a 6V power source that is allowed to discharge to 4.5V before the equipment cuts off. By the time the supercapacitor reaches this voltage threshold, a linear discharge only delivers 44% of the energy; the remaining 56% is reserved. An optional DC-DC converter helps to recover the energy dwelling in the low voltage band, but this adds costs and introduces loss. A battery with a flat discharge curve, in comparison, delivers 90 to 95 percent of its energy reserve before reaching the voltage threshold.
Figures 1 and 2 demonstrate voltage and current characteristics on charge and discharge of a supercapacitor. On charge, the voltage increases linearly and the current drops by default when the capacitor is full without the need of a full-charge detection circuit. This is true with constant current supply and voltage limit that is suitable for the capacitor rated voltage; exceeding the voltage could damage the capacitor.
Figure 1: Charge profile of a supercapacitor.
The voltage increases linearly during a constant current charge. When the capacitor is full, the current drops by default.
Source: PPM Power
Figure 2: Discharge profile of a supercapacitor.
The voltage drops linearly on discharge. The optional DC-DC convertor maintains the wattage level by drawing higher current with dropping voltage.
Source: PPM Power
The charge time of a supercapacitor is 1–10 seconds. The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited by the charger’s current handling capability. The initial charge can be made very fast, and the topping charge will take extra time. Provision must be made to limit the inrush current when charging an empty supercapacitor as it will suck up all it can. The supercapacitor is not subject to overcharge and does not require full-charge detection; the current simply stops flowing when full.
Table 3 compares the supercapacitor with a typical Li-ion.
Specific energy (Wh/kg)
Specific power (W/kg)
Cost per kWh
Service life (industrial)
1 million or 30,000h
2.3 to 2.75V
Up to 10,000
–40 to 65°C (–40 to 149°F)
–40 to 65°C (–40 to 149°F)
500 and higher
$250–$1,000 (large system)
5 to 10 years
0 to 45°C (32°to 113°F)
–20 to 60°C (–4 to 140°F)
Table 3: Performance comparison between supercapacitor and Li-ion.
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