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What's
the role of the Supercapacitor?
The supercapacitor resembles a regular capacitor with the exception that
it offers very high capacitance in a small package. Energy storage is
by means of static charge rather than of an electro-chemical process that
is inherent to the battery. Applying a voltage differential on the positive
and negative plates charges the supercapacitor. This concept is similar
to an electrical charge that builds up when walking on a carpet. The supercapacitor
concept has been around for a number of years. Newer designs allow higher
capacities in a smaller size.
Whereas a regular capacitor consists of conductive foils and a dry separator,
the supercapacitor crosses into battery technology by using special electrodes
and some electrolyte. There are three types of electrode materials suitable
for the supercapacitor. They are: high surface area activated carbons,
metal oxide and conducting polymers. The high surface electrode material,
also called Double Layer Capacitor (DLC), is least costly to manufacture
and is the most common. It stores the energy in the double layer formed
near the carbon electrode surface.
The electrolyte may be aqueous or organic. The aqueous variety offers
low internal resistance but limits the voltage to one volt. In contrast,
the organic electrolyte allows 2.5 volts of charge, but the internal resistance
is higher.
To operate at higher voltages, supercapacitors are connected in series.
On a string of more than three capacitors, voltage balancing is required
to prevent any cell from reaching over-voltage.
The amount of energy a capacitor can hold is measured in microfarads or
µF. (1µF = 0.000,001 farad). While small capacitors are rated
in nano-farads (1000 times smaller than 1µF) and pico-farads (1
million times smaller than 1µF), supercapacitors come in farads.
The gravimetric energy density of the supercapacitor is 1 to 10Wh/kg.
This energy density is high in comparison to a regular capacitor but reflects
only one-tenth that of the Nickel-metal-hydride battery. Whereas the electro-chemical
battery delivers a fairly steady voltage in the usable energy spectrum,
the voltage of the supercapacitor is linear and drops evenly from full
voltage to zero volts. Because of this, the supercapacitor is unable to
deliver the full charge.
If, for example, a 6V battery is allowed to discharge to 4.5V before the
equipment cuts off, the supercapacitor reaches that threshold within the
first quarter of the discharge cycle. The remaining energy slips into
an unusable voltage range. A DC-to-DC converter could correct this problem
but such a regulator would add costs and introduce a 10 to 15 percent
efficiency loss.
Rather than operate as a main battery, supercapacitors are more commonly
used as memory backup to bridge short power interruptions. Another application
is improving the current handling of a battery. The supercapacitor is
placed in parallel to the battery terminal and provides current boost
on high load demands. The supercapacitor will also find a ready market
for portable fuel cells to enhance peak-load performance. Because of its
ability to rapidly charge, large supercapacitors are used for regenerative
braking on vehicles. Up to 400 supercapacitors are connected in series
to obtain the required energy storage capacity.
The charge time of a supercapacitor is about 10 seconds. The ability to
absorb energy is, to a large extent, limited by the size of the charger.
The charge characteristics are similar to those of an electrochemical
battery. The initial charge is very rapid; the topping charge takes extra
time. Provision must be made to limit the current when charging an empty
supercapacitor.
In terms of charging method, the supercapacitor resembles the lead acid
battery. Full charge occurs when a set voltage limit is reached. Unlike
the electrochemical battery, the supercapacitor does not require a full-charge
detection circuit. Supercapacitors take as much energy as needed. When
full, they stop accepting charge. There is no danger of overcharge or
'memory'.
The supercapacitor can be recharged and discharged virtually an unlimited
number of times. Unlike the electrochemical battery, there is very little
wear and tear induced by cycling and age does not affect the supercapacitor
much. In normal use, a supercapacitor deteriorates to about 80 percent
after 10 years.
The self-discharge of the supercapacitor is substantially higher than
that of the electro-chemical battery. Supercapacitors with an organic
electrolyte are affected the most. In 30 to 40 days, the capacity decreases
from full charge to 50 percent. In comparison, a Nickel-based battery
discharges about 10 percent during that time.
Supercapacitors are relatively expensive in terms of cost per watt. Some
design engineers argue that the money would be better spent in providing
a larger battery by adding extra cells. But the supercapacitor and chemical
battery are not necessarily in competition. Rather, they enhance one another.
Advantages
· Virtually unlimited cycle life - can be cycled millions of time.
· Low impedance - enhances load handling when put in paralleled
with a battery.
· Rapid charging -supercapacitors charge in seconds.
· Simple charge methods - no full-charge detection is needed; no
danger of overcharge.
Limitations
· Linear discharge voltage prevents use of the full energy spectrum.
· Low energy density - typically holds one-fifth to one-tenth the
energy of an electrochemical battery.
· Cells have low voltages - serial connections are needed to obtain
higher voltages. Voltage balancing is required if more than three capacitors
are connected in series.
· High self-discharge - the rate is considerably higher than that
of an electrochemical battery.
Information
provided by Maxwell Technologies, Inc
About
the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver
BC.
Mr. Buchmann has a background in radio communications and has studied
the behavior of rechargeable batteries in practical, everyday applications
for two decades. Award winning author of many articles and books on batteries,
Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery
analyzers and PC software. For product information please visit www.cadex.com.
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