Jan Břínek, Zdeněk Kozáče
ek
KB
B micro s.r.o.
Design off Powe
er Systems with
w Superc
S
capaciitors
N
Návrh
nap
pájecích ssystémů se superrkapacitorry
_____________________________
_____________________
___________
____________________ Vy
ysoké učen
ní technické v Brně 2012
INDEX
1. SUPERCAPACITOR VS BATTERY BY THE MIT (USA) EXPERTS ................................... 3 2. PRINCIPLE OF A SUPERCAPACITOR .................................................................................... 3 3. THE BENEFITS OF USING SUPERCAPACITORS ................................................................. 5 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 VERY HIGH EFFICIENCY ............................................................................................................ 5 HIGH CURRENT CAPABILITY ..................................................................................................... 5 WIDE VOLTAGE RANGE ............................................................................................................. 5 WIDE TEMPERATURE RANGE..................................................................................................... 6 CONDITION MONITORING (SOC & SOH) .................................................................................. 6 LONG CYCLE LIFE ...................................................................................................................... 6 LONG OPERATIONAL LIFE.......................................................................................................... 6 LIFE EXTENSION FOR OTHER ENERGY SOURCES ....................................................................... 6 EASE OF MAINTENANCE ............................................................................................................ 7 STRAIGHTFORWARD INTEGRATION ........................................................................................... 7 TEN REASONS = DESIGN FLEXIBILITY ....................................................................................... 7 4. SUPERCAPACITOR MARKET .................................................................................................. 7 5. RECENT DEVELOPMENT OF SUPERCAPACITORS ........................................................... 8 6. SUPERCAPACITOR CHARACTERISTICS.............................................................................. 8 6.1 6.2 POWER RIDE-THROUGH APPLICATIONS ..................................................................................... 9 APPLICATION EXAMPLES ........................................................................................................... 9 7. DESIGN OF A SUPERCAPACITOR-BASED POWER SYSTEM ......................................... 11 7.1 EQUIVALENT SC MODELS FOR POWER ELECTRONICS APPLICATION ...................................... 11 7.1.1 RC Equivalent Model....................................................................................................... 11 7.1.2 Three Branch Model ........................................................................................................ 12 7.1.3 Time Domain Model ........................................................................................................ 12 7.2 MANAGEMENT OF SUPERCAPACITOR POWER SYSTEMS .......................................................... 13 7.2.1 Passive Balancing............................................................................................................ 13 7.2.2 Active Balancing .............................................................................................................. 14 7.2.3 Voltage Initialization ....................................................................................................... 14 7.2.4 Topology of supercapacitor energy storage system ........................................................ 14 8. CONCLUSION.............................................................................................................................. 15 1. SUPERCAPA
ACITOR VS
V BATT
TERY BY THE MIT (USA) EX
XPERTS
Almost e
everything we
e use require
es a battery (computers, mobile cell phones,
p
flashhlights, hybrid electric
cars, personal enterrtainment device, and esspecially spa
ace electronics). As funcctionality incrreases in
the digittal age, so has
h
our relia
ance on the
e traditional battery. The
e battery hass not progre
essed far
beyond the basic de
esign develo
oped by Alesssandro Voltta in the 19th century, uuntil just now
w. Recent
works att the world‘ss top laborattories offer tthe most ec
conomically viable
v
alternaative to con
nventional
batteriess in more than 200 yearrs. The sup
percapacitor (ultracapacittor, electrochhemical dou
uble layer
capacito
or-EDLC) is both a battery and a ccapacitor. Supercapacito
ors could alllow laptops and cell
phones to be charge
ed in a minu
ute. Unlike l aptop batterries, which start
s
to lose their ability to
t hold a
after a year or
o two (severral hundred ccharge/disch
harge cycles)), supercapaacitors have hundreds
h
charge a
of thoussands of cha
arge/discharrge cycles a
and could sttill be going strong longg after the device
d
is
obsolete
e. 'Theoretic
cally, there's
s no proces
ss that woulld cause the
e supercapaacitor to ne
eed to be
replaced
d.'
2. PRINCIPLE OF A SUPERCAPA
ACITOR
This intro
oduction con
ncerns Electrrochemical D
Double Layerr Capacitors (EDLCs). Foor brevity, the second
most popular word fo
or them – su
upercapacito
ors is mainly used. The th
hird most poopular term for them ultracapa
acitors - is offten used in heavy electrrical applications. Include
ed in the disccussion and forecasts
are so-ccalled Asym
mmetric Elec
ctrochemical Double Lay
yer Capacito
ors (AEDLC
Cs) better known
k
as
supercab
batteries.
Fig
g.1 Comparisson of Capacitor Structures
Superca
apacitors store electricity
y by physica lly separatin
ng positive and negative charges — different
from battteries which do so chem
mically. The ccharge they hold is like the
t static eleectricity that can build
up on a balloon, bu
ut is much greater than
nks to the extremely
e
high surface area of their interior
materialss.
The main advantage
e of the supe
ercapacitors is their supe
er fast rate of
o charge andd discharge,, which is
determin
ned solely by
b their phys
sical propertties. A batte
ery relies on
n a slower cchemical rea
action for
energy.
age of the supercapacito
or is that currrently they store
s
a smal ler amount of
o energy
The main disadvanta
attery does.
than a ba
Superca
apacitors are
e very good at efficientlyy capturing electricity
e
from regenerattive braking, and can
deliver p
power for accceleration just as quickkly. With no
o moving pa
arts, they alsso have a very
v
long
lifespan - 500,000 plus charge/re
echarge cyccles. Supercapacitors are
e currently uused for wind
d energy,
solar ene
ergy, and hyydro energy storage.
s
A superccapacitor po
olarizes an electrolytic
e
so
olution to sto
ore energy electro
e
staticcally. Though it is an
electroch
hemical device, no chem
mical reactio
ons are invo
olved in its energy
e
storaage mechaniism. This
mechaniism is highly reversible
e, and allow
ws the supe
ercapacitor to be chargged and discharged
hundreds of thousan
nds of times.
Once the
e supercapa
acitor is charrged and ene
ergy stored, a load (the electric veh icle's motor)) can use
this enerrgy. The amount of enerrgy stored is very large compared
c
to a standard capacitor be
ecause of
the enorrmous surfacce area creatted by the po
orous carbon electrodes and the sm
mall charge se
eparation
created by the dielecctric separato
or.
A superrcapacitor ca
an be viewed as two non reactive porous plates, or
o collectors, suspended within an
electrolyyte, with a vo
oltage potenttial applied a
across the collectors. In an individuaal supercapa
acitor cell,
the applied potential on the pos
sitive electrod
de attracts the
t negative ions in the electrolyte, while the
potentiall on the neg
gative electro
ode attracts tthe positive ions. A diele
ectric separaator between
n the two
electrode
es prevents the
t charge frrom moving between the
e two electrod
des.
Fig. 2
Electrica
al energy sto
orage devices, such as ccapacitors, store
s
electrical charge onn an electrod
de. Other
devices, such as electrochemic
e
cal cells or batteries, utilize
u
the electrode to create, by chemical
reaction,, an electrica
al charge at the
t electrode
es. In both of
o these, the ability to sto re or create electrical
charge iss a function of the surfac
ce area of th e electrode. For example
e, in capacitoors, greater electrode
surface a
area increases the capac
citance or en
nergy storage
e capability of
o the device .
As a sttorage devicce, the sup
peracapacito r relies on the micros
scopic chargge separatio
on at an
electroch
hemical interrface to store
e energy. Sin
nce the capa
acitance of th
hese devicess is proportional to the
active electrode are
ea, increasin
ng the electrrode surface
e area will increase thee capacitance, hence
increasin
ng the amou
unt of energy
y that can b
be stored. Th
his achievem
ment of high surface are
ea utilizes
materialss such as acctivated carbon or sintere
ed metal pow
wders. However, in both ssituations, th
here is an
intrinsic limit to the porosity
p
of these materia ls, that is, there is an upper limit to thhe amount of
o surface
area tha
at can be atta
ained simply by making ssmaller and smaller partiicles. An alteernative method must
be develloped to incrrease the acttive electrode
e surface are
ea without increasing thee size of the device. A
much m
more highly efficient
e
elec
ctrode for ele
ectrical enerrgy storage devices couuld be realiz
zed if the
surface a
area could be significantly increased..
Superca
apacitor Adv
vantages
 V
Virtually unlim
mited life cyc
cle - cycles millions of tim
me -10 to 12 year life
 L
Low impedance
 C
Charges in seconds
s
 N
No danger of
o overcharge
e
 V
Very high rattes of charge
e and discha
arge
 H
High cycle efficiency (95% or more)
 S
Super capaccitors and ultra capacitorss are relative
ely expensive
e in terms of cost per wattt
Supercapacitor Disadvantages





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.
Requires sophisticated electronic control and switching equipment
The high performance characteristics of supercapacitors allow the system designer to develop hybrid
power system solutions that cost less and perform better than non-hybrid solutions. In the use of a
single energy device to satisfy the entire power specification of an application required designers to
either design for power (at times providing excess energy), or design for energy (at times providing
inadequate power).
3. THE BENEFITS OF USING SUPERCAPACITORS
The benefits of using supercapacitor technology in various designs of power systems can be
summarized as follows.
3.1 VERY HIGH EFFICIENCY
Supercapacitors are highly efficient components. Their coulombic efficiency (defined as the total
charge removed divided by the total charge added to replenish the charge removed) is greater than
99%, even at very high currents, meaning that little charge is lost when charging and discharging the
supercapacitor. Round-trip efficiency (RTE) is also very high, due to the low equivalent series
resistance (ESR). At a 5 second rate, discharging to ½ of voltage in 5 seconds, and recharging at the
same rate until the ultracapacitor is fully charged, round-trip efficiency is greater than 70%. At a 10
second rate, RTE is greater than 80%. This result is not only a more efficient use of energy, but less
heating, and therefore potentially less overhead for cooling energy storage.
3.2 HIGH CURRENT CAPABILITY
Supercapacitors are designed with a very low equivalent series resistance (ESR), allowing them to
deliver and absorb very high current. The low ESR of modern supercapacitors allows them to be
charged very quickly, making them well suited for regenerative braking applications and other quickcharge scenarios. The inherent characteristics of the supercapacitor allow it to be charged and
discharged at the same rates, something no battery can tolerate.
In case of the need to charge the energy storage device quickly (in applications like regenerative
braking and quick-charge toys), the supercapacitor can be charged as quickly as the system will allow,
within reasonable limits based only on simple resistive heating.
In battery-based systems, you can only charge as fast as the battery will accept the charge. This limits
the system to only low to moderate charging rates, and may also limit how frequently one can charge,
a significant issue in braking systems. Furthermore, the battery does not self-limit this charging rate,
therefore you as the systems designer must manage this charging. In some cases, the designer may
need the extra energy you get with a battery. In these cases, the designer can combine a
supercapacitor and a battery to get the best of both, optimizing the system design.
Examples include consumer electronics such as digital cameras, in which an inexpensive alkaline
battery is combined with a supercapacitor, and automotive applications such as hybrid power trains. In
both examples, the high power pulses are provided by the supercapacitor, while the large energy
requirement is provided by the battery.
3.3 WIDE VOLTAGE RANGE
Because they are capacitors, supercapacitors are not confined to a narrow voltage window. Designers
need only consider the voltage range of the system, which can be much wider than the narrow voltage
range required by a battery. The supercapacitor can operate at any voltage below its maximum
continuous operating voltage. To achieve higher voltages, multiple cells are placed in series, and are
operated at or below their total series maximum voltage. There is no risk of over-discharging the
supercapacitor, and in fact there is additional safety for service personnel, who can fully discharge a
supercapacitor system before servicing, reducing the electrical hazard. In some systems such as fuel
cells, the ability of the supercapacitor to track with the fuel cell's voltage is a significant benefit over
battery/fuel cell systems, where the fuel cell wants to operate over a voltage range that is wider than
that tolerated by batteries.
3.4 WIDE TEMPERATURE RANGE
Since supercapacitors operate without relying on chemical reactions, they can operate over a wide
range of temperatures. On the high side, they can operate up to 65°C, and withstand storage up to
85°C, without risk of thermal runaway. On the low side, they can deliver power (with slightly increased
resistive losses) as cold as -40°C, well below the cold performance threshold of batteries.
The excellent cold performance of supercapacitors is an excellent fit for engine-starting applications.
When combined with batteries, you can implement a system that meets the energy requirements with
a battery (such as powering lights and stereos while the engine is off) and the power requirements
with the supercapacitor (such as turning the engine over when it is cold, or when the battery may be
discharged from powering lights and stereos while the engine is off).
3.5 CONDITION MONITORING (SOC & SOH)
Determining battery state of charge (SOC) and state of health (SOH) is a significant factor in designing
robust battery systems, requiring sophisticated data acquisition, complex algorithms, and long-term
data integration. In comparison, it is very simple to determine the SOC and SOH of supercapacitors.
Since the energy stored in a capacitor is a function only of capacitance and voltage, and the
capacitance is constant (relatively speaking), a simple open-circuit voltage measurement defines state
of charge. Since capacitance is relatively stable, voltage alone effectively determines SOC. Because
of the relatively slow change in capacitance and equivalent series resistance over time, occasional
calculations of capacitance and ESR can be used to determine SOH. A short (2-10 sec) discharge at
any constant current can provide sufficient data to calculate capacitance and ESR. Since these values
change slowly, this SOH data point, when combined with an open-circuit voltage measurement for
SOC, yields all the information required to determine the condition of the superacapacitor.
3.6 LONG CYCLE LIFE
The energy storage mechanism of a supercapacitor is a highly reversible process. The process moves
charge and ions only. It does not make or break chemical bonds. It therefore is capable of hundreds of
thousands of complete cycles with minimal change in performance. Cycle depth is also not an issue,
so supercapacitors can be micro-cycled (cycled less than 5% of their total energy) or full cycled
(cycled greater than 80% of their total energy) with the same long life. They can be cycled
infrequently, such as in an uninterruptible power supply system where they may only be discharged a
few times a year, or they may be cycled very frequently, as in a hybrid vehicle.
3.7 LONG OPERATIONAL LIFE
Since there are no chemical reactions, the energy storage mechanism of a supercapacitor is a highly
stable process. It is therefore capable of many years of continuous duty with minimal change in
performance. Long-term storage is not an issue, since the supercapacitor can (and should) be stored
completely discharged.
The long cycle life and long operational life make the supercapacitor a lifetime component for most
applications. Battery replacement is considered normal routine maintenance, costing time and money.
In most cases, supercapacitors are installed for the life of the system.
3.8 LIFE EXTENSION FOR OTHER ENERGY SOURCES
Energy sources such as batteries, specialty engines, and fuel cells don't perform well in transient
conditions. For some components, transients can significantly shorten life. Coupling a supercapacitor
with these energy sources off-loads many of these transients from the main energy source. The
benefits are a smaller main energy source, and one that has potentially much longer life. The life cycle
cost of the battery associated with a supercapacitor-battery system may be much lower than that of a
battery-only system.
3.9 EASE OF MAINTENANCE
Supercapacitors require basically no maintenance. They have no memory effects, cannot be overdischarged, and can be held at any voltage at or below their rating. If kept within their wide operating
ranges of voltage and temperature, there is no recommended maintenance.
3.10 STRAIGHTFORWARD INTEGRATION
The inherent nature of supercapacitors makes system integration relatively easy, much easier than
with batteries. Systems integration with respect to the supercapacitor is primarily focused on keeping
the supercapacitor within its wide operating limits of voltage and temperature. Supercapacitors can be
placed in series or in parallel. When installed in parallel, no extra management is necessary. When
placed in series, a voltage management circuit is often used to keep the voltage of each cell within
operating limits. Voltage management circuitry is often used in battery systems as well, however, a
supercapacitor management system need only prevent cells from exceeding their rated voltage. This
is typically done with a simple voltage-sensitive current-bypass circuit. No control is necessary to keep
cells above a minimum voltage, since supercapacitors have no lower voltage limit. For installations
that are conservative with respect to individual cell voltage, no management system may be needed.
Recent technology improvements have significantly decreased variations in performance from cell to
cell, reducing the need for management systems, with the potential to eliminate them completely.
3.11 TEN REASONS = DESIGN FLEXIBILITY
These ten reasons give the designer an additional flexibility when designing the system.
Supercapacitors can be used as the only energy storage in a system, or can be used to augment
other energy sources in a hybrid system. They can be charged and discharged quickly, allowing them
to be used in a variety of system architectures. Though the high power density of the supercapacitor is
offset by its low energy density, appropriate systems design with supercapacitors accounts for the
high power/low energy density by using the supercapacitor as an intermittent power cache rather than
a continuous power source. Where one previously traded system performance against the size of a
single component, one can now strive to meet the optimum system performance by balancing two
components; an energy cache, and a power cache.
4. SUPERCAPACITOR MARKET
Half of the manufacturers and intending manufacturers of supercapacitors/supercabatteries (EDLC,
AEDLC) are in East Asia, 28% are in North America but Europe is fast asleep at only 7%. Yet, being
used for an increasing number of purposes in electric vehicles, mobile phones, energy harvesting,
renewable energy and other products of the future, this market is roaring up to over $11 billion in ten
years with considerable upside potential. Supercapacitors are a curiously neglected aspect of
electronics and electrical engineering with a multi-billion dollar market rapidly emerging. For example,
for land, water and airborne electric vehicles, there are about 200 serious traction motor
manufacturers and 110 serious traction battery suppliers compared to just a few supercapacitor
manufacturers. In all, there are no more than 66 significant supercapacitor manufacturers with most
concentrating on the easier small ones for consumer electronics such as power backup. However, in a
repetition of the situation with rechargeable batteries, the largest part of the market has just become
the heavy end, notably for electric and conventional vehicles.
Maxwell (USA), NESSCap (Korea), Panasonic (Japan), Epcos (Germany), ECOND (Russia), and
NEC (Japan), Ao-wei (China), Shuangdeng (China) are the main manufactures of supercapacitors.
Saft (France), Batscap (France) Superfarad (Sweden), Okamura lab (Japan) have been doing
research on supercapacitors. Manufacturers, such as Maxwell, NessCap, Epcos, Econd, offer to sell
supercapacitor module, which is a package of a group of supercapacitors in series and contains
supercapacitor management equipment, giving much more feasibility in application.
5. RECENT DEVELOPMENT OF SUPERCAPACITORS
In terms of energy density, existing commercial electric double-layer capacitors range from arend 0.5
to 10Wh/kg. Especially, the Okamura lab has developed the Nanogate capacitor which is
characterized by making poresin the carbon with the ions in the electrolyte solution which has an
energy density of 20~60Wh/kg, while EEStor claims the in examples will offer capacities on the order
of 200 to 300Wh/kg. For comparison, a conventional lead acid battery is typically 30~40Wh/kg,
modern lithium-ion batteries are about 150~200Wh/kg. The power density of supercapacitors in
commercial use is around 4~7kW/kg, which is much higher than other energy storage devices, for
example 1.8kW/kg for lithium-ion batteries. High power density combined with long life cycle makes
supercapacitors ideal devices for peak power application.
Researchers have used computer simulations to elucidate how supercapacitors are able to store
electric charge. The work could open new ways of designing future carbon materials with higher
energy densities, which could vastly reduce charging times in various applications including electric
cars and consumer electronics.
The past decade has seen several important developments, including the discovery that controlling
the porosity of microporous carbon electrodes leads to an increase in ion adsorption, and thus
capacitance. However, while many experimental studies have since confirmed this effect, no one had
been able to quantitatively explain or visualise the details of the ion electro-adsorption involved.
Now, researchers from France, the UK and the US have used realistic molecular-scale computer
simulations to examine how ions are arranged in carbide-derived carbon electrodes, which has cast
light on the origin of supercapacitance at the atomic scale.
In order to examine the behavior of ionic liquids at the interface with carbon electrodes, the team
designed a realistic simulation cell consisting of an ionic liquid electrolyte surrounded by two porous
electrodes which could be held at a constant potential. When two different voltages were applied
between the electrodes, the team could compare how ions are organised in the electrified pores of
electrodes and how they behave at a planar graphite surface.
6. SUPERCAPACITOR CHARACTERISTICS
Supercapacitors come in a variety of sizes, for example a 10 F/2.7 V supercapacitor is available in a
10 × 30mm, 2-terminal radial can with an ESR (Effective Series Resistance) of 25 mΩ, while a 350
F/2.5 V supercapacitor with an ESR of 1.6 mΩ is available in a D-cell battery form factor.
One advantage supercapacitors offer over batteries is their long life. A capacitor's cycle life is quoted
as greater than 500,000 cycles; batteries are specified for only a few hundred cycles. This makes the
supercapacitor an ideal "set and forget" device, requiring little or no maintenance.
Two parameters of the supercapacitor that are critical to an application are cell voltage and initial
leakage current. The manufacturers of supercapacitors rate their leakage current after 100 hours of
applied voltage, while the initial leakage current in those first 100 hours may be as much as 50 times
the specified leakage current.
The voltage across the capacitor has a significant effect on its operating life. When used in series, the
supercapacitors must have balanced cell voltages to prevent over-charging of one of the series
capacitors. Passive cell balancing is a popular and simple technique. The disadvantage of this
technique is that the capacitor discharges through the balancing resistor when the charging circuit is
disabled. The rule of thumb for this scheme is to set the balancing resistor to 50 times the worst case
leakage current, estimated at 2 μA/F.
An alternative is to use a non-dissipative, active cell-balancing circuit, such as the LTC3225 IC, to
maintain cell voltage. The LTC3225 presents less than 4 μA of load to the supercapacitor when in
shutdown mode and less than 1 μA when input power is removed. It also features a programmable
charging current of up to 150 mA, charging two series supercapacitors while balancing the voltage on
the capacitors.
6.1 PO
OWER RID
DE-THROU
UGH APP LICATION
NS
To proviide a consta
ant voltage to
o the load, a DC/DC con
nverter is required betweeen the load
d and the
supercap
pacitor. As th
he voltage across the su
upercapacitor decreases, the current drawn by the DC/DC
converte
er increases to maintain
n constant p
power to the
e load. The DC/DC co nverter drop
ps out of
regulatio
on when its in
nput voltage reaches the minimum op
perating volta
age (VUV).
To estim
mate the requ
uirements for the superccapacitor, the
e effective ciircuit resistannce (RT) nee
eds to be
determin
ned. RT is the
e sum of the capacitors' E
ESRs and the circuit distribution resisstances,
Assumin
ng 10% of the input powe
er is lost in tthe effective circuit resisttance when the DC/DC converter
c
is at VUVV, the worst-ccase RT is
The volta
age required
d across the Supercapaci
S
itor at VUV threshold of the DC/DC co
converter is
The requ
uired effectivve capacitan
nce can then
n be calcula
ated based on
o the requirred ride-thro
ough time
(TRT), an
nd the initial voltage
v
on th
he capacitor (VC(0) ) and VC(UV)
The ESR
R of a superccapacitor dec
creases with
h higher frequ
uency. Manu
ufacturers us ually specify
y the ESR
at 1 kHzz, while som
me manufactu
ures publish both the va
alue at DC and at 1 kHzz. The capac
citance of
supercap
pacitors also
o decreases as frequencyy increases and is usually specified at DC. When using a
supercap
pacitor in a ride-through
r
application w
where the po
ower is being
g sourced forr seconds to minutes,
use the e
effective cap
pacitance and
d ESR meassurements att a low freque
ency, such aas 0.3 Hz.
6.2 APPLICATION EXAM
MPLES
Figure 3 shows two series conne
ected 10 F/2 .7 V superca
apacitors cha
arged to 4.8 V that can hold up 20
W. The LTC3225 is used to charge the supe
ercapacitors at 150 mA and
a maintainn cell balanciing, while
the LTC
C4412 provid
des an automatic switch
hover functio
on. The LTM
M4616 dual output swittch mode
μModule
e DC/DC con
nverter generrates the 1.8
8 V and 1.2 V outputs.
Fig
g. 3: 5-V pow
wer ride-through application
Figure 4 shows a 12
2 V power system
s
that u
uses six 10 F/2.7 V supe
ercapacitors in series ch
harged by
three LT
TC3225s set to 4.8 V, and
d a charging current of 15
50 mA.
Fig.4: 5-V pow
wer ride-throu
ugh application
The thre
ee LTC3225ss are powerred by three
e floating 5-V
V outputs ge
enerated by the LT1737
7 fly back
controlle
er. The outpu
ut of the stac
ck of six sup ercapacitors
s is set up in a diode-OR
R arrangement via the
LTC4355
5 dual ideal--diode contro
oller. The LT
TM4601A μM
Module DC/DC regulator produces 1.8
8 V at 11
A from th
he OR'd outp
puts. The LTC4355's MO
ON1 in this ap
pplication is set
s for 10.8 V
V.
Conclus
sion:
Superca
apacitors are
e meeting the needs of power ride-through
h applicatio ns
requirem
ments are in the seconds
s to minutes range. Supe
ercapacitors offer long liife,
light weight and en
nvironmentallly friendly s olutions whe
en compared to batteriees.
LTC3225
5 provides a compact, low noise so
olution to ch
harging and cell balanciing
supercap
pacitors.
where the time
low main
ntenance,
To this end, the
series connected
7. DESIGN OF A SUPERCAPACITOR-BASED POWER SYSTEM
When designing a supercapacitor-based power system, building a proper model for supercapacitor,
taking the appropriate methodology of voltage management, choosing a right topology of power
system, and knowing the dynamic terminal behavior are very important factors for the performance of
power system. Power electronics device, such as DC/DC converter, is an indispensable part for the
power system.
In power electronics applications, designers concern more about the dynamic parameters of
supercapacitors for they are often used for high duty cycle applications. Regarding that parameters on
data sheet supplied by manufacturers are the static value. Hence, a supercapacitor testing method is
needed in order to test the dynamic characteristic parameters. Due to the low cell voltage of
supercapacitor (0.9~3.3V), 0.9V per cell with an aquaeous electrolyte and 2.3 to 3.3V per cell with an
organic elektrolyte, a series connection of supercapacitor cells is necessary to obtain higher voltage.
However, the unequal distribution of cell voltage will affect the performance and lifetime of the cell.
There are several voltage balancing strategies. Another way to overcome the problem is so-called
Voltage Initialization. It has yet to be decided that the consistency of supercapacitors in order to
choose a proper voltage balancing strategy.
7.1 EQUIVALENT SC MODELS FOR POWER ELECTRONICS APPLICATION
In the application of power electronics, an equivalent model which reflects the terminal behaviour of
supercapacitors is desired in simulation with the purpose of further studying the characteristics of
supercapacitor based power system.
From the circuit theory point of view the supercapacitor is a nonlinear parametric device and its
precise mathematical modelling leads to a nonlinear differential equation with variable parameters
(parametric), which generally has no exact solution. It is given by the nonlinearity of its basic coulombvolt characteristic that makes its static and dynamic capacitances voltage dependent. In many
practical applications for supply purposses, the precise nonlinear model is usually much simplified
(linearized).
7.1.1
RC EQUIVALENT MODEL
The RC equivalent model for EDLC is the RC serial model as shown in Figure 5. This method
assumes ideal linear behaviour of the supercapacitor and it neither reflects physical aspects, nor the
influence of voltage or temperature on supercapacitors. However, this model can be applied in the low
required precision.
R
C
Fig.5: RC Equivalent model
7.1.2
THREE BRANCH MODEL
Another model shown as Figure 6 which is based on the physical aspects and the desire of practical
engineering is proposed.
Ri
R1
Rd
Rlea
Vci
Ci0
Ci1*Vci
Cd
C1
Fig.6: Three branch model
The model has three well distinct RC time constants covering the desired time range. Each of the
three branches has a distinct time constant differing from the others in more than an order of
magnitude which will result in an easily measurable model. The first branch, with the elements Ri, Ci0,
and the voltage-dependent capacitor Ci1 (in F/V), dominates the immediate behavior of the DLC in the
time range of seconds in response to a charge action. The second branch, with parameters Rd and
Cd, dominates the terminal behavior in the range of minutes. Finally, the third branch, with parameters
Rl and Cl, determines the behavior for times Langer than 10minutes. The equivalent circuit model
reflects the physics of the double-layer charge distribution. First, the resistive element represents the
resistivity of carbon particles. The capacitive element represents the capacitance between carbon and
electrolyte. Second, the capacitance of the double-layer charge distribution depends on the potential
diference across the material, and according to measurements, in the practical voltage range of the
device, the DLC capacitance varies linearly with the capacitor terminal voltage. Third, double layer
charge distribution shows self-discharge. It is possible to determine the parameters of the model using
measurements at the DLC terminals which is very practical for engineering. This model shows good
agreement with experimental data and is possible for gaining the parameters. Temperature influences
can be added to this model making it an efficient way to study its application in power electronic
circuits and automotive applications.
7.1.3
TIME DOMAIN MODEL
Ci
L
Cp2
Cp1
Re
Rv
CR
RL
Rp2
Rp1
Ri
Fig.7: Time domain model
Ri
Ca
Cv
The model shown as Figure 7 takes into account frequency, voltage and temperature dependencies of
capacitance, series resistance, redistribution of electrical charges on the electrode surface and
leakage current. It is also on the base of the second model. The first rounded circuit-block takes into
account the electrolyte ionic resistance temperature dependence in the low frequency range. The
parallel capacitance Ci has been used to cancel the contribution of Ri(T) in high frequency range. For
low frequency the first circuit block behavior is close to that of resistance Ri(T). The relationship
between Ri and the temperature can be established from experimental results by using EIS. The
second rounded circuit block is introduced to increase the value of capacitance of the average
frequencies. Their behaviour is the one of a phase shifter. The third rounded circuit block describes
the leakage current and the internal charge redistribution. The self discharge behaviour of
supercapacitors is an important factor because it determines the duration time of stored energy on
open circuit. The supercapacitor self discharge is also a function of temperature. It is neceséry to use
two different time constant circuits RC by elements Rp1Cp1, Rp2Cp2 which depend on the voltage
and on the operating temperature. It also includes a parallel RL resistance, which gives the long time
leakage current contribution.
It depends on the practical application to decide a proper model which meets the requirements.
Generally speaking, the RC serial model can be used in a system with low precision, the three branch
model is fit for the application which has requirement on dynamic characteristics while the time domain
model is often used for precise theoretical study but it is not easy to find all the parameters and
dependencies. It is suggested that the three branch model is the most appropriate choice for power
electronics applications.
7.2 MANAGEMENT OF SUPERCAPACITOR POWER SYSTEMS
When connecting many capacitors in series, the issue of voltage balancing inevitably comes into play.
Basically there are two reasons for an imbalance of voltages in a serial string of supercapacitors:
 deviations from the nominal capacitance of the capacitors
 deviations in self discharge performance.
While the first topic is mainly important during dynamic performance of the capacitor string, the latter
topic dominates for static capacitor performance during constant voltage phases. A cell management
circuit maximizes the performance and life of supercapacitors installed in series. Generally speaking,
there are two ways in voltage balancing, one is passive balancing and the other is active balancing.
Another way to overcome the problem caused by unbalance of voltage is so-called voltage
initialization
7.2.1
PASSIVE BALANCING
Fig.8: Passive balancing
A passive balancing system is designed to overwhelm the inherent variations in leakage current by
installing a resistor in parallel with each other. The resistor is typically sized at 10 times the average
leakage current of the cell. The benefits to this balancing method are simplicity and low cost. The
drawback of this technique is slow response due to the linearity of leakage current with voltage and
high parasitic losses due to the 10-time additional leakage current. Passive balancing is mainly used
in low-duty cycle applications such as in backup power systems. Fig. 8 shows a simple balancing
network with resistors. Another possible passive balancing is by using Zener diodes instead of
resistors. This type of balancing with Zener diodes has a greater power consumption due to necessary
overvoltage state of the whole block for the proper balancing.
7.2.2
A
ACTIVE BA
ALANCING
G
Fig.9
9: Active bala
ancing
In contra
ast to passive
e solutions, an
a active ba
alancing circu
uit behaves nonlinearly
n
aand works to force the
cells to have an equ
ual voltage, resulting in tthe most effective use of
o the superccapacitor striing. Fig.9
shows a simplified diagram of an active balancing circuit incorrporating a comparatorr. In this
configura
ation, each circuit
c
stretches across ttwo cells, co
omparing the
eir voltage aand moving charge
c
to
equalize
e the two ce
ells. A numbe
er of schem
mes are used
d to achieve
e active balaancing and many
m
are
patented
d. Active bala
ancing circuit is required
d in high dutty-cycle applications andd where low parasiticl
osses arre necessaryy.
7.2.3
V
VOLTAGE INITIALIZA
ATION
The principle of volta
age initializattion is that a ll capacitors are balance
ed at the uppper voltage lim
mit of the
capacito
or module. As
A a consequ
uence, when
n the module
e is discharg
ged, the indiividual capac
citors will
adopt diffferent voltag
ges on a low
wer level. Wh
hen recharge
ed to the upp
per voltage, all the capacitors will
be balan
nced again. Provided
P
thatt the capacittances of individual capac
citors changee slowly with
h time, an
occasion
nal initializatiion of the mo
odule will ke
eep the capacitors balanc
ced at the uppper working
g voltage.
This is shown by Fig.10.
Fig.10:
F
Princiiple of voltag
ge initializatio
on
h more feasible and lesss expensive. However, it demands soound consistency and
This metthod is much
high qua
ality of superccapacitors.
7.2.4
T
TOPOLOG
GY OF SUPE
ERCAPACIITOR ENER
RGY STOR
RAGE SYST
TEM
The term
minal voltage
e of supercapacitor chan
nges significa
antly when being
b
chargeed or discharged. For
this reasson, a powe
er electronics
s device - D
DC/DC conv
verter is necessary to foorm a superrcapacitor
power syystem. The non-isolated
d Buck-Boosst bidirectiona
al DC/DC co
onverter shoown as Fig. 11 is the
first choiice for that. The advanta
age of this to
opology is high efficiency
y, high reliabbility, low po
ower loss,
less expensive and small
s
in size.
D2
S2
C
S1
Supercapacitor bank
D1
DC-Link
Bidirectional DC/DC converter
Fig.11: Supercapacitor energy storage system topology
This bidirectional Buck-Boost DC/DC converter allows the power transfer in both directions. This
feature enables the process of charging and discharging through one unit. The current from the
supercapacitor is fully controlled by bidirectional DC/DC converter, and the voltage of DC link is
dependent on the control result of it. When charging the supercapacitor bank, the DC/DC converter
works in Buck mode, and supplies a constant charge current. The power flows from DC-link to
supercapacitor bank. When discharging, the DC/DC converter works in Boost mode, and keeps the
voltage of DC-link constant. The power flows from supercapacitor bank to DC-link.
8. CONCLUSION
Unlike the battery systems, the supercapacitor banks require a balancing system in all cases because
of the following reasons:
(i)
Leakage current is relatively high in comparison to the energy stored in the supercapacitor
(ii)
Leakage current depends on temperature and voltage
(iii) Discharging the supercapacitor to zero volts, the natural dispersal in term of capacitance leads
necessary to a voltage dispersal once charged
When comparing the passive and active balancing by the effectiveness, simplicity, cost and reliability
viewpoints, the passive resistive balancing appears to be the best solution especially from the cost,
simplicity and even reliability viewpoints.