Ultracapacitor Technology
Powers Electronic Circuits
B y Youngho KKim
im
im, Director of Product Development,
NESSCAP Co. Ltd., Korea
As engineering innovations continue to advance
ultracapacitors, their enhanced performance capabilities are expected to hasten the convergence of
batteries and ultracapacitors—strengthening the
combination of both specific energy storage and
pulse power design in future applications.
s the market strives for lighter, more compact wireless and portable devices with
more ingenious features crammed into an
ever-tighter space, a related quest ensues for
the next power supply innovation—a powerful, compact, long-lasting, economical and safe battery.
Although progressing toward this end, current battery technology often compromises the desired space and weight
specifications without properly satisfying peak power
requirements.
Ultracapacitors, also known as supercapacitors, offer an
alternative source that promises to circumvent the battery
scramble and extract greater efficiency from existing power
sources. Because of high price and manufacturability is-
vices. Ultracapacitors also are free from characteristic battery problems, such as limited cycle life, cold intolerance
and critical charging rates.
A
Why Ultracapacitors?
Ultracapacitors are being developed as an alternative to
pulse batteries. To be an attractive alternative, ultracapacitors must have at least one order of magnitude higher
power and a much longer shelf and cycle life than batteries. Ultracapacitors have much lower energy density than
batteries, and their low-energy density is, in most cases,
the factor that determines the feasibility of their use in a
particular high-power application.
Available for decades, a conventional electrolytic capacitor is an energy-storage device that can be compared to a
container that gradually fills with electrical energy and then
delivers it when needed in a sudden burst. Offered just recently, an ultracapacitor is a high-energy version of a conventional capacitor, holding hundreds of times more energy per unit volume or mass than the latter by using stateof-the-art materials and high-tech microscopic manufacturing processes. When fully charged, these robust devices
deliver instant power in an affordable, compact package.
Long considered an enigma because of price, the advent of inexpensive, compact ultracapacitors, characterized
by an exceptionally high surface area, excellent conductivity, and superior chemical and physical stability, herald a
new era of practical usage.
3
10
2
Energy density (Wh/kg)
10
10h
1h
NiCads
36s
Lithium
3.6s
1
10 Lead-acid
Ultracapacitors
0.1h
0
10
-1
10
10-2 1
10
0.36s
Double-layer capacitors
Aluminum
electrolytics
36ms
Dashed lines are isotherms of
characteristic time constant
2
3
10
10
Power density (Wh/kg)
4
10
Fig
ig.. 1. Power vs. energy characteristics of energy-storage devices.
Ultracapacitors as Circuit Elements
sues, this electric double layer capacitor (EDLC), also
known as a pseudo capacitor, isn’t popular among engineers. However, it offers boundless growth potential because it responds to key market and societal needs: It’s environmentally friendly, helps conserve energy, and enhances the performance and portability of consumer dePower Electronics Technology
October 2003
The equivalent circuit used for conventional capacitors
can also be applied to ultracapacitors. The circuit schematic
in Fig. 2 represents the first-order model for an
ultracapacitor. It’s comprised of four ideal circuit elements:
a capacitance C, a series resistor Rs, a parallel resistor Rp,
and a series inductor L. Rs is called the equivalent series
34
www.powerelectronics.com
ULTRACAPACITOR TECHNOLOGY
+
Rp
Rs
L
C
-
R1
Fig
ig.. 2. The first-order circuit model of an ultracapacitor. Each of the
four circuit elements is ideal.
R1
L
+
IC1
R9
T1
R4
Rn
R2
R3
C1
R5
C2
Rp
C1
Cn
C2
R6
R2
R7
T2
R8
To ultracapacitor
+
Fig
ig.. 3. The equivalent circuit of a practical ultracapacitor.
resistance (ESR) and contributes to energy loss during capacitor charging and discharging. Rp simulates energy loss
due to capacitor self-discharge, and is often referred to as
the leakage current resistance. Inductor L results primarily
from the physical construction of the capacitor and is usually small. However, in many applications, it can’t be neglected—particularly those operating at high frequencies
or subjected to hard switching.
Resistor Rp is always much higher than Rs in practical
capacitors. Thus, it often can be neglected, particularly in
high-power applications. In that case, the impedance of
the Fig. 2 circuit model is:
-
Fig
ig.. 4. Block diagram of an active balancing circuit.
Z = R + i (2pfL-1/2pfC), where L is the inductance in
[Henrys]. The impedance is purely resistive when 2pfL-1/
2pfC = 0, or f = 1/2 p(LC)1/2. This particular frequency is
referred to as the resonance frequency of the capacitor.
Thus, the impedance of circuit is simply the resistance at
self-resonance. However, ultracapacitors exhibit non-ideal
behavior, which result primarily from the porous material
used to form the electrodes that cause the resistance and
Booth #945
CIRCLE 227 on Reader Service Card or visit freeproductinfo.net/pet
www.powerelectronics.com
35
Power Electronics Technology
October 2003
ULTRACAPACITOR TECHNOLOGY
capacitance to be distributed such that the electrical response mimics transmission line behavior. Fig. 3 shows a
more realistic circuit representing the real ultracapacitor’s
electrical response.
Vmax
Capacitive
DC Behavior of Ultracapacitors
Ultracapacitors used in electric drivelines to load-level
the battery experience large-steady (transient) dc, much
like the battery, rather than small amplitude ac signals. The
dc charge or discharge time (tdisch) of the capacitor is related to the fundamental characteristic frequency (fAC in
Hz) of the ac voltage on the capacitor by tdisch » 1/4fAC. Hence,
for several backup time applications, the ac signals are lower
than 10Hz.
In testing ultracapacitors, it’s convenient to model them
as a simple series RC circuit when inductive effects are
unimportant. In this case, Q = CV, E = 1/2 CV2 and Vo – V
= iR + (Qo – Q / C), where Q is charge on the capacitor, V
is voltage on capacitor, E is energy stored in the capacitor,
and Vo and Qo are voltage and charge at t = 0, respectively.
Vmin
rating. Because sustained overvoltage can cause an
ultracapacitor to fail, the voltage across each cell in series
stack must not exceed the maximum continuous working
voltage rating of individual cells in the stack. The designer
must either reduce the “rate of charge” being delivered to a
cell, or completely stop charging a cell whose voltage approaches its surge voltage rating.
The easiest way to reduce the current that’s charging an
ultracapacitor cell is to divert some of it around the cell.
One such method employs a passive bypass component.
The other, more complicated procedure uses an active
bypass circuit. After the stack has been held at voltage for
a period of time, voltage distribution then becomes a
Many system applications require that capacitors be
connected together, in series and/or parallel combinations,
to form a “bank” with a specific voltage and capacitance
CIRCLE 229 on Reader Service Card or freeproductinfo.net/pet
CIRCLE 228 on Reader Service Card or freeproductinfo.net/pet
October 2003
Tdisch
Fig
ig.. 5. Discharge profile of an ultracapacitor.
Stacking Ultracapacitors
Power Electronics Technology
Resistive
36
www.powerelectronics.com
ULTRACAPACITOR TECHNOLOGY
6
200
4
2
0
0
Voltage
Current
Power
-100
-2
Power (kW)
Current (A), Voltage (V)
100
-4
-200
-6
-300
0
60
120
180
240
300
360
Time (sec)
Fig
ig.. 6. A 54-V/175-F NESSCAP ultracapacitor bank module and 6-kW cycling data for 42-V vehicle application.
function of internal parallel resistance. The cells with
higher leakage current should have lower cell voltages,
and vice versa in a series stack of ultracapacitors.
One technique to compensate for variations in parallel
resistance is to place a bypass resistor in parallel with each
cell, sized to dominate the total cell leakage current. This
effectively reduces the variation of equivalent parallel resistance between the cells. The active balancing circuit has
Booth #945
CIRCLE 230 on Reader Service Card or visit freeproductinfo.net/pet
www.powerelectronics.com
37
Power Electronics Technology
October 2003
ULTRACAPACITOR TECHNOLOGY
an active switching device, like a bipolar transistor or a MOSFET, connected in series with each bypass element ladder. The switches are controlled by voltage-detection circuits
that only turn a switch “on” when the
voltage across that particular cell approaches a value just slightly below
the continuous working voltage rating of the cell. This is called the bypass threshold voltage. Fig. 4 depicts
a typical block diagram of an active
Booth #1028
charging-current diversion circuit.
An ultracapacitor’s voltage profile
has a capacitive and resistive component. This can be represented by dV
= i (R+dt/C), where dV is allowable
change in voltage in Volt, i is the current in amperes, R is ESR in Ohms, dt
is charge or discharge time in seconds,
and C is capacitance in Farads.
The number of ultra-capacitor
cells required can be determined by
the system variables, such as allowable
Booth #
CIRCLE 231 on Reader Service Card or visit freeproductinfo.net/pet
Power Electronics Technology
October 2003
38
change in voltage (max. and min. voltage), current (or power) and required
duration time.
Ultracapacitor Applications
Ultracapacitors benefit many applications, from those involving short
power pulses, to those requiring
low-power support of critical
memor y systems. Whether used
alone or with other power sources,
ultracapacitors provide an excellent
solution in several system configurations and high-power applications,
such as cellular electronics, power
conditioning, uninterruptible power
supplies (UPS), industrial lasers,
medical equipment, and power electronics in conventional, electric and
hybrid vehicles.
In electrical vehicle applications,
ultracapacitors permit faster acceleration, increased range, and extend battery life by freeing it from stressful
high-power tasks. In addition,
ultracapacitor technology now can do
load-leveling to extend the life of EV
batteries and provide the high power
essential for EV acceleration. For example, a vehicle might use this power
burst to accelerate and climb a steep
hill. Ultracapacitors also can absorb
regenerative braking energy and thus
limit the otherwise very high charging current to the battery.
UPS: Ultracapacitors provide
short-term support for UPS. With less
energy storage capability than a battery, an ultracapacitor isn’t a viable
substitute in UPS as a long-term
power source. However, for shortterm support, its instant power and
rapid response capability allows it to
act as a bridge during power outages
until an alternative source kicks in,
such as a generator or other backup
power supply. In addition, ultracapacitors can serve as a load-leveling
function by absorbing power surges
and spikes and then releasing clean
quality power essential for precision
high-tech equipment.
Peak pulse power: Ultracapacitors
are optimal for applications requiring
a high energy burst not achievable
with a battery alone, such as with conwww.powerelectronics.com
HIGH VOLTAGE HIGH CURRENT
1 mA to 100 kA
SEMICONDUCTORS
FOR YOUR HIGH CURRENT AND/OR
HIGH VOLTAGE REQUIREMENTS
FAST RECOVERY
AVAILABLE
Booth #836
High Voltage
Silicon Rectifiers
High Voltage Diodes
Voltage Multipliers
Custom Assemblies
Full Wave Bridges
The First True
Surface Mount
High Voltage
Diode
3000V fast recovery rectifier
available on a 12mm tape
■ Silicon Rectifiers
- Air-Cooled
- Water-Cooled
■ Assemblies
- Thyristor
- SCR
- Rectifier
- Bridge Rectifier
Our experienced engineers will help
you find the perfect solution for your
high voltage, high current application.
HV Component Associates
P.O. Box 2245
Farmingdale, NJ 07727
(732) 938-4499
(732) 938-4451 FAX
info@hvca.com
www.hvca.com
CKE
To Order A Free
Catalog Or Samples,
Visit our Web Site
P.O. Box 211
Lucernemines, PA 15754
(724) 479-3533
(724) 479-3537 FAX
e-mail: info@cke.com
www.cke.com
CIRCLE 232 on Reader Service Card or visit freeproductinfo.net/pet
sumer electronics and wireless devices. For example, unlike analog
equipment that draws steady current, digital wireless communication
devices often burden the battery with
short, heavy current spikes during
transmission mode. An added
ultracapacitor can provide the intermittent pulse power while the battery
supplies the steady current. End users benefit from an extended battery
life and lengthened time between
charges. Other electronics with ultracapacitor applications include VCRs,
CD players, electronic toys, security
systems, computers, scanners, smoke
detectors, microwaves and coffee
makers.
Quick-charge applications: Ultracapacitors charge in seconds whereas
batteries often require hours, thus potentially benefiting quick-charge applications. Wireless power tools employing an ultracapacitor can be
charged just before use. Moving toys,
such as miniature racing cars, also
www.powerelectronics.com
benefit from the fast-charge properties of the ultracapacitor.
Memory backup applications:
Widely used in consumer electronics,
small ultracapacitors defend against
data loss during short-period power
outages or, in the case of portable devices, during battery replacement.
For this, an ultracapacitor is superior to a battery because it’s less expensive and requires no replacement during the device’s lifetime.
All-weather quick start: Today’s
car batteries meet peak power requirements during engine startup,
even in the coldest weather conditions. Ultracapacitors, unaffected by
weather, can supply the secondslong peak power, thus permitting
the battery to be downsized and its
useful life extended. The current
catalytic converter in vehicles sends
untreated exhaust gas into the environment for a few minutes until it
warms and begins functioning.
Ultracapacitors quickly preheat the
39
catalytic converter and enable it to
function immediately.
Applications to decrease battery size:
Often, power loss in batteries can be
traced to voltage “spikes” that occur
when a device is activated or draws a
higher-than-average current. Adding
an ultracapacitor to handle voltage
spikes allows either a prolonged life
or smaller battery—whatever is application optimal.
Implications
Although some ultracapacitor
applications, such as memory backup,
are already in widespread use, many
of the applications just discussed
are still in the initial phase of adoption. Higher-voltage ultracapacitor
technology looms on the horizon, and
the implications are enormously farreaching.
PETech
For more information on this article,
CIRCLE 342 on Reader Service Card
Power Electronics Technology
October 2003