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The double layer capacitor(memory retention capacitor, Capattery) is a special capacitor type that uses the double layer charge storage phenomena first investigated by Helmholtz in his theory publicated in 1879. It describes how charges have a tendency to attract each other and build up an electric field over two materials' contact surfaces.

Buildup of a double layer capacitor cell

      _____________
  +  |_____________|conductive rubber plate
     |  |   c   |  |
     |  +-------+i | - rubber frame
     |  |   c   |  |
  -  |-------------|
     `-------------'

The areas marked with a 'c' contain a mix of active carbon granula and an electrolyte. The layer marked 'i' is an ion conductive membrane. When the capacitor is connected to a power supply the carbon in the anode(+) gets a positive charge and the cathode(-) gets a negative charge, this causes a migration of ions through the membrane where the negative ions will remain around the surface of the positively charged carbon fragments and the positive ions will be stored in the negative carbon. Due to the granular nature the carbon has a large surface area, in one of these cells the surface area can be over 400m2.
One of these cells have a maximum voltage of 1.2v, since the water in the electrolyte starts to separate in to hydrogen and oxygen. To use higher voltages, the cells can be stacked.

These capacitors have a high internal resistance(ESR) from several ohms up to several hundred ohms. This limits the maximum output current. They do however have a low self-discharge, a high temperature stability and a long lifetime and are therefore used for memory backup and similar applications where a high current isn't important.

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