Electrical Performances of High Power Electric Double Layer
Capacitor under Thermal and Mechanical Stress
F.V. Conte, F.Pirker
Abstract
Electric double layer capacitors are recently proposed for various automotive applications. For speeding
up the penetration in the market of this technology, reliable data about the devices’ performances in every
condition are needed. In particular severe mechanical and thermal stresses, which are normal on vehicles,
should be deeply investigated. But being relative new devices there are not yet consolidate procedures. In
literature for the mechanical stress only abuse test procedures are present. The paper tries to cover this
lack giving a procedure to analyze the electric double layer performance while a mechanical stress is
taking place. Besides, part of the preliminary laboratory results are shown.
Keywords: Component, energy storage, HEV (hybrid electric vehicle), modeling, ultra capacitor.
1. Introduction
A shift from conventional powertrain technology to the hybrid powertrain is needed to meet recent
demands for more environmentally friendly and comfortable vehicles.
The key issue for the hybridization is the energy storage device, which should have a good energy and
power density. This leads to a real mileage improvement throughout the use of the electromotor.
In the last few years, many manufacturers have presented high power electric double layer capacitors with
very high power densities and a fair energy densities [1], [2].
This allows these devices to be coupled with other energy storage devices, such as Lead Acid battery [3].
The Lead Acid battery’s higher density combined with these capacitors produces a source with fair energy
and good power performances.
A precise knowledge of the energy storage system performance under typical conditions is needed for a
correct design of the powertrain.
The stressing condition considered is a high temperature environment, combined with mechanical stresses
due to low and high frequency vibrations.
Low frequency vibrations are usually generated by the internal combustion engine whereas high frequency
vibrations usually result from the roughness of the route.
Literature has shown many test procedures [4], [5], [6] for the characterization of the supercaps and many
results are published [7], [8].
Conversely, literature also showed a lack of investigation into the electrical performances under long run,
mechanically and in case of simultaneous thermal stress.
Until now, it is not clear if the mechanical stress alone, or together with a thermal stress, could affect the
performance of the supercapacitor. So the present paper investigates the relationship between electrical
performances and the ordinary stress to which these devices are subjected.
2. Components under test
The selection of the components is based on the effective availability of the devices on the market, and on
the energy and power density performance that the manufacturers declare.
For this reason all of them have carbon based electrodes and organic electrolyte.
In particular, the study has focused on two different sizes of electric double layer capacitors, which would
be reasonable to be found in HEV powertrain applications.
These are a 5000F model, which is the biggest available and allows the absolute highest energy density,
and a middle size, of about 2000-2600F, which is also compatible with the voltage and current constrains
at the inverter interface.
The nominal performances are shown in the following list:
Manufacturer
A
B
C
D
Ah
2600
2000
5000
5000
Nominal Capacity
V
2.5
2.3
2.5
2.7
Nominal Voltage
V
2.8
2.5
2.8
2.85
Maximal Voltage
A
600
750
500
950
Maximal Current
Wh/kg
4.2989
4.45
5.1
5.69
Energy Density
0.0007
0.003
0.0004
0.0004
Internal Resistance Ohm
Kg
0.525
0.33
0.85
0.89
Weight
m
0.172 x 0.06 0.126 x 0.51 0.109 x 0.09 0.165 x 0.06 X 0.072
Dimensions
Cylindrical Cylindrical Cylindrical
Prismatic
Shape
Figure 1 Nominal data of the supercapacitors under test
3. Experimental procedure
A mechanical stress with stochastic frequency spectra and low average amplitude is normal during the life
of each components of a vehicle.
Automotive manufacturers already know that this vibration, according to the specific components could
improve or worsen the performance or the life of the component itself.
A good example of that is the first energy storage system applied on cars, the Lead acid battery.
Through the shaking the Lead acid battery improves the electrical performance slightly, because the
electrolyte stratification is avoided; but at the same time the vibration stresses the electrode plates shorting
the battery life.
Therefore automotive manufacturers developed specific procedures to check the compatibility of each
component with this stressing vehicle environment.
It would be reasonable to think that vibrations have an influence on the capacitor performances, especially
for those under test, being the liquid electrolyte absorbed in porous structure.
In literature there are Tests for Mechanical Abuse specifically defined for supercapacitor, whereas no
specifically designed procedures are defined for lasting vibrations [10].
3.1. Selfdischarge Test
Many works show the influence of temperature on the selfdischarge of the electrochemical double layer
capacitor, the present paper investigates the influence of temperature combined with mechanical stress.
It is clear that in a typical vehicle use the contemporarity of a relaxing phase, where a selfdischarge takes
place, and a vibration stress is improbable.
The hypothesis behind the selfdischarge investigation under mechanical stress condition, is that if a
mechanical stress influences the device’s electrochemical behavior, then also the selfdischarge rate would
be modified.
For this reason the following test has been done:
Each supercapacitor, after a precondition of few full charging-discharging cycles, has been fully charged
with a constant current-constant voltage method (CC-CV) and left in a temperature regulated chamber.
The temperatures used are 20°C and 43°C. Inside the climatic chamber, the supercaps have been fixed on
a vibrational test bench, thus the selfdischarge rate with and without mechanical stress has been measured.
Test
Temperature Mechanical Stress
Preconditioning
+20
No
+43
No
Hot
Characterization A
+20
No
Preconditioning
+20
No
+43
Yes
Hot + Shake
Characterization B
+43
No
Preconditioning
+20
No
+20
No
Standard
Preconditioning
+20
No
+20
Yes
Standard + Shake
Figure 2 Selfdischarge Test Schedule
The list shows how the supercap undergoes different load profiles between each selfdischarge test, used
for the device characterization.
Due to the lack of a specific on-run mechanical stress procedure, starting from the outline proposed for
batteries [10], and our laboratories’ experience in mechanical stress for automotive, we defined a
mechanical stress with a variable frequency between about 10Hz and about 60Hz, and an amplitude
between 0-20g with an average of about 4-5g.
3.2. On-run Test
The second part of investigation regards the check of supercapacitors performances during their use. In the
laboratory a possible application has been simulated where mechanical and electrical stress are
contemporarily applied to the device.
The test was thought for detecting the device endurance, and, in case, the electrical parameters variations.
Practically, at ambient standard temperature, the supercaps have been mechanically stressed with the
previous vibrational test bench, meanwhile the capacitors have been continuously charged and discharged
with the electrical test bench.
The vibration amplitude and frequency are the same as before in the selfdischarge test.
On the electrical side, the charge and discharge process are with constant current, which value is defined
in base of the capacity size. The current rate is a key parameter of this test, which should be high enough
for being representative of a real application, but on the other side, being continuously applied to the
device, should be low enough to achieve a thermal balance. We have seen that a current rate of 40mA/F
could be reasonable for this test.
Of course the voltage range is also a key issue for recreating conditions of typical use in labs; so,
concerning the hybrid electrical application (HEV), the adopted voltage thresholds, where the current
changes sign, are:
VMax = VRated - 0,1
and
VMin = VMax 2
Practically the capacitor receives a square current input, and its response is in voltage terms; an example
of how a typical voltage current profile looks, is shown for a supercapacitor model B in Figure 3.
[V]
2.5
2
1.5
1
1400
1450
time [s] 1500
1400
1450
Figure 3 Voltage and Current measurement of supercap B On-run Test
time [s] 1500
[A]
100
0
-100
As anticipated, the adopted absolute current value is 40mA/F, so the capacitors get warm during the test.
Therefore, to achieve a thermal equilibrium, the procedure has an initial phase without mechanical stress.
As shown by the infrared pictures, 30 minutes of repetitive charges and discharges are enough to achieve a
new thermal equilibrium as soon as the thermal equilibrium is achieved, the vibration test bench can be
started.
The vibration lasts for about an hour, after that the vibrational test bench is switched off again, the
electrical, instead, kept on cycling for another 30 minutes.
Figure 4 Infrared pictures of supercap D
Therefore, with this procedure it is possible to control the supercap parameter variations during each
phase, the first “warming up” one, the “shaked” one, and the “control after shaking”.
On the first test phase, the thermal transient highlights the supercap R-C parameter dependence versus the
temperature. The second phase instead shows the variation versus the mechanical stress, and the third
section of the test investigates, if the parameter modifications of the shaked phase are reversible.
4. Results
4.1. Selfdischarge test results
The main result of the selfdischarge test regards the procedure itself; it has been seen that the
preconditioning phase is very critical for the tests’ outcome.
The application of few full charge-discharge cycles, followed by a CC-CV charge, which is stopped when
the trickle current drops under a specific value, seems not to be enough.
The selfdischarge test has been done on a few devices for each manufacturer; between devices of the same
manufacturer conflicting results have been seen, and even more, the same item has given different
responses during the same test.
It has been noticed that each supercap, after a long inactivity time, shows a higher selfdischarge rate, and
after relative long usage, as long as a characterization process (about 1h), the selfdischarge is much lower,
and comparable with other measurements.
For this reason we suggest to perform a “hard” preconditioning before every selfdischarge test, even
stronger than the one suggested by the [5] for the pre-testing , or series like the on-run test for at least 1
hour.
Even if part of the data is not usable, we conclude some remarkable facts.
An important result is about the layout influence of the capacitor performances. The supercap D, being
prismatic, could be installed in a horizontal or vertical position. We notice that the selfdischarge charge is
double in the vertical position, for instance the selfdischarge rate at +43°C with mechanical stress for the
horizontal positioned one is 0,024 [%/h] respect to the 0,0425 of the vertical one.
The influence of the mechanical stress on the selfdischarge rate at ambient temperature is about 20%
higher for type A-C-D, while the model B is more sensible to the vibration and shows a selfdischarge rate
60% higher.
At last we notice that between the selfdischarge at ambient temperature and at 43°C, there is about a factor
ten of difference.
4.2. On-run test results
For the estimation of the capacitance C and the resistance R from the measurements of the on-run test we
used the simplest model, a R-C branch model, which is enough for the purpose.
The use of a more complex model [11] is only justified in case of high frequency accurate response needs,
whereas our test is characterized by a square wave with a period of about 30 s., so a simple R-C model
could, in first approximation, properly describe the supercaps behavior.
The differential equation solution of the SISO system, where the current is the input and the voltage the
output, or vice versa, together with a parameter optimization algorithm gives back the parameter R and C
required, with a relative error usually between 5 and 10%.
In detail, for the parameters determination along the test, the test is divided in parts, which are 250 s. long,
equivalent to about 4 charge-discharge cycles.
With this computational analysis we notice that there are different behaviors between the supercaps, and a
general rule is not yet possible to be defined. More tests are needed for better clarification of the
relationship between mechanical stress and supercap performances, in particular for the lifecycle. In few
cases a small capacity decrease has been seen, which might cause an accelerated ageing.
At least it seems that the mechanical stress influence on the capacitance is low.
4.3. Supercap Fault
The fragility of such devices is confirmed by the fault that happened to one supercap during the self
discharge under mechanical stress test.
In particular, at the end of the test has been noticed that one supercap type B has lost some liquid,
presumable the electrolyte, from one side, even if it appeared intact.
Figure 5 Damaged Supercap after the +43°C selfdischarge test combined with mechanical stress
After the fault, the performance of that supercap has been verified and has shown a capacity loss of about
5%, and a higher resistance with respect to the other supercap of the same type.
This capacitor type is more sensible to the mechanical stress, this is shown by the selfdischarge rate that
all the supercaps of this type have.
5. Conclusions
The mechanical stress is typical for automotive components, so the component endurance investigation
against mechanical stress is important. The paper described a method for investigating the performance of
electric double layer capacitors under mechanical stress.
The test results have shown that on a small time base the vibrations influence on the capacitor
performances could be negligible, instead of that it seems that vibration could have more consequence on
the device’s life, so further investigations for this specific issue are needed.
References
[1] L. Hofmann, W. Steiger, P. Adamis, R. Petersen, "Potential and Technical Limits of Electric Motors in
Powertrain-Applications", Volkswagen AG, Aggregatforschung (2002)
[2] Andrew Chu, Paul Braatz, “Comparison of commercial supercapacitors and high-power lithium-ion batteries for
power-assist applications in hybrid electric vehicles I. Initial characterization”, Journal of Power Sources 112
(2002) 236–246
[3] Randy B.,Wright, D.K. Jamison, “FreedomCAR TESTING
ULTRACAPACITORS”, 204th Meeting 2003 The Electrochemical Society
OF
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AABC 2002, Las Vegas February 2002
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[7] “Specification of Test Procedures for Supercapacitor in Electric Vehicle Application” , EUCAR, Traction Battery
Working Group, ECE Contract ENK6-CT2000-00088, Draft 2003.
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Applications: Introduction of New Equalisation Coefficients” , proceeding EVS 16, Beijing, China, October, 1999
[9] A.Burke, M.Miller “New Developments in Electrochemical Capacitor for Vehicle Application”, proceeding
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[10] “USABC Battery Test Procedure Manual : electric vehicle battery test procedure”, Rev.2 , 1009
[11] P.Barrade,“Simulation tools for Power Electronics :Teaching and Research”, Symplorer Workshop, 2001
Authors
Fiorentino Valerio Conte Arsenal Research, Faradaygasse 3, A-1030 Vienna, AUSTRIA,
Tel.: +43 50550-6217, Fax.: +43 505506595, e-mail: valerio.conte@arsenal.ac.at
Born 1972, graduated in Electrical Engineering from University of Pisa. In 2004 he
received the Ph.D. degree from the University of Pisa. Researcher of Arsenal Research's
Monitoring Energy and Drives business unit since 2003. His research topics are focused on
simulation and modeling of energy storage systems.
.
Franz Pirker, Arsenal Research, Faradaygasse 3, A-1030 Vienna, AUSTRIA,
Tel.: +43 50550-6233, Fax.: +43 505506595, e-mail: franz.pirker@arsenal.ac.at
Born 1968, graduated in Electrical Engineering from Vienna Technical University. In 2003
he received the MSc degree in communications from the Donau University Krems. Head of
Arsenal Research's Monitoring Energy and Drives business unit since 2000. His research
topics are focused on simulation and modeling of electrical machines.