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■Principle and Construction of Electric Double Layer Capacitor (EDLC)
This special type of capacitor has properties that are about halfway between regular capacitors and rechargeable (secondary) batteries. While a battery stores an electrical charge through a chemical reaction, the EDLC stores charge by means of an electric double layer formed by ions adhering to the surface of an activated carbon electrode. Whereas charging a rechargeable battery requires several hours, an electric double layer capacitor can be charged in a matter of seconds. Furthermore, the number of charge cycles for a battery is limited, but the electric double layer capacitor in principle has no such limitation.
■Types of electric double layer capacitors
Electric double layer capacitors are divided into the following categories, according to their construction and shape. Sizes range from small chip and coin type products to large modules combining many interconnected cylindrical or rectangular cells.
■Features and characteristics of electric double layer capacitors
An electric double layer capacitor is a charge storage device which offers higher capacitance and higher energy density than an electrolytic capacitor. Electric double layer capacitors are suitable for a wide range of applications, including memory backup in electronic devices, battery load leveling in mobile devices, energy harvesting, energy regeneration in automobiles, and more. A further increase in energy density, improved charge/discharge characteristics and thermal characteristics, as well as electrode material improvements are some of the technical challenges that still need to be addressed. The main characteristics of electric double layer capacitors are described below.
- ● Capacitance
- The surface structure of the activated carbon (pore diameter and volume, specific surface area) has a large influence on capacitance.
- ● Internal resistance
- An electric double layer capacitor can be considered an equivalent circuit where a large number of miniature capacitors having internal resistance are connected in parallel. The resistance component of the electrolyte and electrodes etc. creates the internal resistance which causes a drop in effective voltage. For large current discharge applications, internal resistance should therefore be kept as low as possible.
- ● Leakage current
- When an electric double layer capacitor is charged for an extended period of time, the charge current decreases but it does not become zero. Rather it settles at a certain constant value, which is called the leakage current. The magnitude of this current is determined by factors such as electrode material, cell construction, usage temperature etc.