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.