Rather
than operating as a stand-alone energy storage device, supercapacitors work
well as low-maintenance memory backup to bridge short power interruptions. The
virtue of ultra-rapid charging and delivery of high current on demand makes the
supercapacitor an ideal candidate as a peak-load enhancer for hybrid vehicles,
as well as fuel cell applications.
The charge time of a
supercapacitor is about 10 seconds. The charge characteristic is similar to an
electrochemical battery and the charge current is, to a large extent, limited
by the charger. The initial charge can be made very fast, and the topping
charge will take extra time. Provision must be made to limit the initial
current inrush when charging an empty supercapacitor. The supercapacitor cannot
go into overcharge and does not require full-charge detection; the current
simply stops flowing when the capacitor is full.
The supercapacitor can be charged
and discharged virtually an unlimited number of times. Unlike the
electrochemical battery, which has a defined cycle life, there is little wear
and tear by cycling a supercapacitor. Nor does age affect the device, as it
would a battery. Under normal conditions, a supercapacitor fades from the
original 100 percent capacity to 80 percent in 10 years. Applying higher
voltages than specified shortens the life. The supercapacitor functions well at
hot and cold temperatures.
The self-discharge of a
supercapacitor is substantially higher than that of an electrostatic capacitor
and somewhat higher than the electrochemical battery. The organic electrolyte
contributes to this. The stored energy of a supercapacitor decreases from 100
to 50 percent in 30 to 40 days. A nickel-based battery self-discharges 10 to 15
percent per month. Li-ion discharges only five percent per month.
Supercapacitors are expensive
in terms of cost per watt. Some design engineers argue that the money for the
supercapacitor would better be spent on a larger battery. We need to realize
that the supercapacitor and chemical battery are not in competition; rather
they are different products serving unique applications.