Function of an EDLC capacitor

How does an EDLC work?

Ultracap-Knowledge | Rainer Hake | reading time: 2 minutes

Electro-physical principle of an electric double layer capacitor

Double layer capacitors, or ultracaps, consist of two electrodes which are immersed in an electrolyte system. When a voltage is applied that is smaller than the decom­position potential of the electrolyte, both electrodes attract ions of reversed polari­zation. They create a zone of immovable current collectors with a thick­ness of only a few molecular layers. This effect was described by Hermann von Helmholtz in 1879.

The electrodes, with the load-collector layer as a dielectric separator, behave like two capa­citors that are con­nected in series via the two electrically con­ductive electrolytes. The energy is stored electro-statically, in contrast to batteries which store and supply electrical energy chemically.

The capacitance of a capacitor depends not only on the thick­ness of the insu­lation layer (the spacing), but also on the size of the electrode surfaces. The activated carbon commonly used has a surface area of up to 3,000 m2/g. In combi­nation with separator layers with thick­nesses of only a few nano­metres, a very large capa­citance is achieved on a very small area.

The most important parameters of the ultracaps are the capa­citance C and the equivalent serial resistance ESR. The idealised equivalent circuit of a capacitor is shown below:

Idealised equivalent circuit of a capacitor

The capacitance of a capacitor is specified in Farad (F) - named after Michael Faraday.

A capacitor has the capacistance of 1F when it is charged to 1V in 1s with a current of 1A.

1 F 1 As/V or 1 Farad = 1 ampere second per volt

The stored energy (work W) of a capacitor results from:
W = 1/2 C U2

The usable energy is derived from the voltage difference at the capacitor
in accordance with the following formula:
Wusable = 1/2 C (U2max - U2min)


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Why are components like ultracaps called different names?

The variety of terms is explained by the English-language terms commonly used in the electronics industry, which are often mixed with German terms or used synonymously. In some cases, manufacturers have also introduced artificial terms to better distinguish themselves from competitors. Here are the most important examples:

Double layer capacitors (DSK) are referred to synonymously as:

  • EDLC (Electric Double Layer Capacitor)
  • Supercapacitors = Supercaps
  • Ultracapacitors = Ultracaps
  • Goldcap™ [Panasonic]
  • Boostcap™  [Maxwell]
  • Greencap™  [Samwha]
  • PURIXEL™  [Pureechem] for supercap cells
  • PURETRON™  [Pureechem] for supercap modules

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