The supercapacitor resembles a regular capacitor with the exception that it offers very high capacitance in a small package. Energy storage is by means of static charge rather than of an electro-chemical process that is inherent to the battery. Applying a voltage differential on the positive and negative plates charges the supercapacitor. This concept is similar to an electrical charge that builds up when walking on a carpet. The supercapacitor concept has been around for a number of years. Newer designs allow higher capacities in a smaller size.
Whereas a regular capacitor consists of conductive foils and a dry separator, the supercapacitor crosses into battery technology by using special electrodes and some electrolyte. There are three types of electrode materials suitable for the supercapacitor. They are: high surface area activated carbons, metal oxide and conducting polymers. The high surface electrode material, also called Double Layer Capacitor (DLC), is least costly to manufacture and is the most common. It stores the energy in the double layer formed near the carbon electrode surface.
The electrolyte may be aqueous or organic. The aqueous variety offers low internal resistance but limits the voltage to one volt. In contrast, the organic electrolyte allows 2.5 volts of charge, but the internal resistance is higher.
To operate at higher voltages, supercapacitors are connected in series. On a string of more than three capacitors, voltage balancing is required to prevent any cell from reaching over-voltage.
The amount of energy a capacitor can hold is measured in microfarads or µF. (1µF = 0.000,001 farad). While small capacitors are rated in nano-farads (1000 times smaller than 1µF) and pico-farads (1 million times smaller than 1µF), supercapacitors come in farads.
The gravimetric energy density of the supercapacitor is 1 to 10Wh/kg. This energy density is high in comparison to a regular capacitor but reflects only one-tenth that of the nickel-metal-hydride battery. Whereas the electro-chemical battery delivers a fairly steady voltage in the usable energy spectrum, the voltage of the supercapacitor is linear and drops evenly from full voltage to zero volts. Because of this, the supercapacitor is unable to deliver the full charge.
If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment cuts off, the supercapacitor reaches that threshold within the first quarter of the discharge cycle. The remaining energy slips into an unusable voltage range. A DC-to-DC converter could correct this problem but such a regulator would add costs and introduce a 10 to 15 percent efficiency loss.
Rather than operate as a main battery, supercapacitors are more commonly used as memory backup to bridge short power interruptions. Another application is improving the current handling of a battery. The supercapacitor is placed in parallel to the battery terminal and provides current boost on high load demands. The supercapacitor will also find a ready market for portable fuel cells to enhance peak-load performance. Because of its ability to rapidly charge, large supercapacitors are used for regenerative braking on vehicles. Up to 400 supercapacitors are connected in series to obtain the required energy storage capacity.
The charge time of a supercapacitor is about 10 seconds. The ability to absorb energy is, to a large extent, limited by the size of the charger. The charge characteristics are similar to those of an electrochemical battery. The initial charge is very rapid; the topping charge takes extra time. Provision must be made to limit the current when charging an empty supercapacitor.
In terms of charging method, the supercapacitor resembles the lead-acid battery. Full charge occurs when a set voltage limit is reached. Unlike the electrochemical battery, the supercapacitor does not require a full-charge detection circuit. Supercapacitors take as much energy as needed. When full, they stop accepting charge. There is no danger of overcharge or 'memory'.
The supercapacitor can be recharged and discharged virtually an unlimited number of times. Unlike the electrochemical battery, there is very little wear and tear induced by cycling and age does not affect the supercapacitor much. In normal use, a supercapacitor deteriorates to about 80 percent after 10 years.
The self-discharge of the supercapacitor is substantially higher than that of the electro-chemical battery. Supercapacitors with an organic electrolyte are affected the most. In 30 to 40 days, the capacity decreases from full charge to 50 percent. In comparison, a nickel-based battery discharges about 10 percent during that time.
Supercapacitors are relatively expensive in terms of cost per watt. Some design engineers argue that the money would be better spent in providing a larger battery by adding extra cells. But the supercapacitor and chemical battery are not necessarily in competition. Rather, they enhance one another. Advantages
Virtually unlimited cycle life - can be cycled millions of time.
Low impedance - enhances load handling when put in paralleled with a battery.
Rapid charging -supercapacitors charge in seconds.
Simple charge methods - no full-charge detection is needed; no danger of overcharge.
Linear discharge voltage prevents use of the full energy spectrum.
Low energy density - typically holds one-fifth to one-tenth the energy of an electrochemical battery.
Cells have low voltages - serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series.
High self-discharge - the rate is considerably higher than that of an electrochemical battery.