[lifesaviors] FYI -- Supercapacitors 1 of 3
- From: "Lion Kuntz" <lionkuntz@xxxxxxxxx>
- To: lifesaviors@xxxxxxxxxxxxx
- Date: Sat, 28 Jun 2003 12:55:25 -0800
http://www.batteryuniversity.com/partone-8.htm
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.
--
__________________________________________________________
Sign-up for your own FREE Personalized E-mail at Mail.com
http://www.mail.com/?sr=signup
Other related posts:
