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Analysis of ultracapacitors ageing in automotive application
Microelectronics Reliability 53 (2013) 1676–1680
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Analysis of ultracapacitors ageing in automotive application M. Catelani a, L. Ciani a,⇑, M. Marracci b, B. Tellini b a b
Department of Information Engineering, University of Florence, Via S. Marta 3, 50139 Florence, Italy Department DESTEC, University of Pisa, L.go L. Lazzarino, 56122 Pisa (PI), Italy
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 28 June 2013 Accepted 12 July 2013
a b s t r a c t Ultracapacitors are characterized by a high specific power and long life cycle, thus they are a very interesting design solution for on-board energy applications such as hybrid-electric vehicles. In this paper the analysis of ultracapacitors ageing in automotive application is discussed to investigate the optimal performances during the whole vehicle lifetime. To study the ageing effects on the ultracapacitors, calendar life tests and power cycling tests have been carried out. The degradation of the performance of the ultracapacitors was analyzed by means of impedance spectroscopy technique. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In many fields of application, it is nowadays fundamental to guarantee high reliability of components and systems. The knowledge of the reliability aspects allows to establish the time interval in which the item (a component, an equipment or a system) is capable of performing its function, under specified operating conditions. Thus, it is evident how the reliability of ultracapacitors represents a key issue for their use into automotive applications [1–3]. According to the nature of power demand in hybrid (HEV) and electric (EV) vehicles in a broad sense, ultracapacitors represent a very useful design solution. For specific automotive applications, they are generally used to provide for example peak power demands or to store vehicle braking energy. They can be used in combination with another electrical source, like a battery or fuel cell. When appropriately designed with a system approach, they offer excellent performance, wide operating temperature range, long life, flexible management, reduced system size, and are cost effective as well as highly reliable. The approach to the design has also to face the operating conditions typical of vehicular applications (electrical, mechanical and thermal stresses). Ultracapacitors are fundamentally viewed as maintenance-free devices that do not require costly test runs and expensive management systems vs. batteries, which require ongoing evaluation of their state of health (SOH) and state of charge (SOC). For all these aspects, the analysis of ultracapacitors ageing represents a fundamental aspect to take into account in order to assure an optimal design for reliability. The method that has been chosen to study the ageing of ultracapacitors is based on both calendar life tests and power cycling tests. The calendar life tests are performed at constant temperature with the ultracapacitors polarized at the maximum voltage and
⇑ Corresponding author. Tel./fax: +39 055 4796393. E-mail address: lorenzo.ciani@unifi.it (L. Ciani). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.07.051
maintained at zero voltage level to evaluate possible differences due to polarization voltage. Tests allow therefore to analyze high-voltage and high-temperature effects on ultracapacitor ageing [4–9]. Instead, in the power cycling tests ultracapacitors are charged and discharged at constant current allowing to assess the degradations during the actual use of SCs as peak power sources in hybrid electric applications [9–12]. Power cycling tests are carried out at constant temperature as calendar life tests. Thus, ultracapacitors are aged in a short time with the same degradation mechanisms observed in normal use. The paper is organized as follows: In Section 2 a brief overview of the failure modes and effects analysis is given with application to ultracapacitors. In Section 3, the experimental setup and measurements are discussed. Results and discussion are reported in Section 4.
2. Ultracapacitors failure modes Considering the reliability and safety aspect in automotive applications, it is rather important to know the ageing behavior of the capacitor and the corresponding failure modes. For the operation of a module, it is fundamental to know whether the capacitor end-of-life is characterized by a short circuit or an open circuit state, for instance [6,9]. The main failure modes and effects related to the ultracapacitors are summarized in Table 1. Ultracapacitors are characterized by a longer lifetime than secondary batteries and, usually, they do not have a hard end of life failure similar to batteries. The main end-of-life failure mode for a ultracapacitor is an increase in Equivalent Series Resistance (ESR) and a decrease in capacitance. The first one is due to the fact that the electrode adhesion on the collector is weakening with time and temperature and, consequently, the ion availability is reduced. On the other hand, the accessible carbon surface and the ions availability are reduced during the electrochemical cycling thus leading to a capacitance decrease [9].
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M. Catelani et al. / Microelectronics Reliability 53 (2013) 1676–1680 Table 1 The main failure modes and effects of ultracapacitors. Item
Failure modes
Failure causes
Local effect
Final effect
Ultracapacitor
Capacitance decrease
Overvoltage Overtemp. Ageing Overvoltage Overtemp. Mechanical stress Ageing Mechanical stress Overvoltage Overtemp. Overpressure
Decrease of component efficiency
Loss of system performance
Decrease of component efficiency
Loss of system performance
Fire Decrease of component efficiency
System damage Loss of system performance
Component Explosion Component Explosion Component Explosion
System damage
ESR increase
Temperature increase Capacitance/ESR out of specific Cell leakage (solvent vapor release) Cell opening Gas pressure
Electrochemical decomposition overpressure Overvoltage Overtemperature
The cell container opening due to an internal overpressure is caused by the voltage and the temperature that generate a gas pressure inside the cell which increases with the working time. When the pressure reaches a determined limit, a mechanical fuse, generally a groove on the can wall or a pressure relief, may open softly, avoiding an explosion of the device [6,9]. The failure modes criteria are dependent on the application requirements. An exposure to high level of temperatures for a long time, high applied voltage and excessive current will lead to an increase of ESR and a decrease of capacitance. In fact, reducing these parameters will lengthen the lifetime of a ultracapacitor. It is important to note that a failure mode can have more than one cause; the most likely potential independent causes for each failure mode need to be identified and described. The identification and description of failure causes as well as suggestions for their mitigation should be done on the basis of the failure effects and their severity. The more severe the effects of failure modes, the more accurately failure causes should be identified and described. Failure causes may be determined from analysis of field failures or failures in test units as proposed in Section 3 with particular attention to the ageing processes of ultracapacitors due to temperature and cell voltage strictly connected to the automotive applications.
3. Experimental activity A dedicated measurement setup was carried out to investigate the ageing of ultracapacitors as function of voltage and temperature. As reported in Section 2, the observed main parameters of a ultracapacitor are usually the Equivalent Series Resistance (ESR) and the nominal Capacitance (C). The ESR and C values are generally obtained following a six-step measurement procedure suggested by the manufacturer of the ultracapacitor. Although commonly adopted in literature [7,9], these two simple parameters may not represent the whole behaviour of the ultracapacitor and it is opinion of the authors that a more complete analysis is required to characterize the device ageing. In a recent paper [13], two of the authors presented a basic model of ultracapacitor based on a theoretical analysis discussing the frequency dependence of the electric permittivity. They deduced an equivalent parallel capacitance and conductance circuit having frequency dependence components via the experimental data obtained through an impedance spectroscopy method. For such reasons an analysis of ageing processes of ultracapacitor due to temperature and cell voltage was conducted in the frequency range 1 mHz to 10 Hz, which is typical for hybrid vehicle applications. The real and imaginary part variation of the impedance was analysed in the adopted frequency
System damage System damage
range. The devices under test are 350 F MaxwellÓ double layer capacitors (D cells) represented in Fig. 1. Their main parameters are reported in Table 2. Devices under test were inserted in a precision refrigerating/ warming test chamber at constant temperature T = +70 °C. The climatic chamber (BinderÓ MK 53) has a program control ranging from T = 40 °C up to T = +180 °C and guarantees a temperature uniformity that ranges between ±0.8 °C (@ 40 °C) and ±2.0 °C (@ +150 °C) while the declared temperature fluctuation is limited to ±0.3 °C for each selected temperature. A photograph of the climatic chamber is shown in Fig. 2. Two devices were charged at the nominal voltage V = +2.7 V, two were short-circuited (V = 0 V) while two were used to perform power cycling tests. During the power cycling tests, ultracapacitors are charged and discharged at constant current of 25 A allowing to assess the degradations during the actual use of SCs as peak power sources in hybrid electric applications [9–12]. The aim of these tests is to show the trend of impedance changes during power cycling operations and compare these changes with variations obtained ageing the ultracapacitors at constant voltage. The used current profiles and the corresponding voltage responses are shown in Fig. 3. 4. Results and discussion After verifying the linear behavior of ultracapacitors, we supplied sinusoidal currents having an amplitude of 1 A in the frequency range 1 mHz to 10 Hz. From the measured current i and
Fig. 1. Ultracapacitors under test.
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Table 2 Main parameters of tested ultracapacitors.
Table 3 Equivalent parallel admittance Y: absolute value and phase vs. frequency for virgin devices.
Nominal capacitance
Rated voltage
Resistance (ESR)
Power
Energy
350 F
2.7 V
3.2 m X
4.3 kW/kg
5.62 Wh/kg
Frequency (Hz)
|Y| (S)
Phase (Y) (°)
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10
1.86 3.73 9.22 18.27 33.99 74.39 109.73 131.82 142.79 146.63 151.21 155.79 158.46
87.66 87.10 85.73 83.04 74.55 58.80 39.36 23.14 11.24 7.34 5.28 3.27 2.29
Table 4 Equivalent parallel admittance Y after 700 h for devices charged at the nominal voltage (@ V = 2.7 V) and for the devices short-circuited (@ V = 0 V).
Fig. 2. Precision refrigerating/warming test chamber.
3
30 20 10
2
0 -10
current (A)
voltage (V)
2.5
Freq. (Hz)
|Y| (S) @ V = 2.7 V
Phase (Y) (°) @ V = 2.7 V
|Y| (S) @ V=0V
Phase (Y) (°) @ V=0V
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10
1.82 3.61 9.12 17.43 31.85 56.84 70.15 74.16 80.78 81.79 83.63 85.51 86.42
88.19 86.91 81.44 74.62 63.08 40.96 24.69 12.32 7.32 4.60 3.42 2.20 1.51
1.80 3.58 8.87 17.24 31.35 60.79 79.39 87.10 89.78 90.80 91.95 93.54 94.37
87.89 87.04 85.94 79.96 67.53 45.93 28.30 15.37 6.99 4.26 3.02 1.83 1.27
1.5 -20 1
time
-30
Fig. 3. Current profiles (green lines – right axis) and corresponding voltage responses (blue lines – left axis). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
voltage v adopting symbolic notation the equivalent admittance can be estimated as Y = G + jxC, at each frequency. As known from the literature [9], the measurements performed to characterize the ageing are sensitive to the polarization history. For these reasons we observed an increase of capacitance (compared with ‘‘virgin’’ values) for the first 24 h and then the expected monotone evolution with ageing. As a consequence, we used as the reference initial values the data obtained after 100 full charge and discharge cycles and 24 h rest time. Obtained data in terms of absolute value and phase of admittance Y are reported in Table 3. 4.1. Calendar life tests In Table 4, measured data of absolute value and phase of Y after 700 h in climatic chamber at T = +70 °C for the devices charged at the nominal voltage (@ V = 2.7 V) and for the devices short-circuited (@ V = 0 V) are reported. In Figs. 4 and 5, the plots of C(f) and G(f) vs. log(f) are shown. From the experimental results, it is possible to see that for increasing temperature the ageing processes are accelerated by the higher reactivity of the chemical components [2,6].
At the same time it is also possible to note that the ageing is function of the frequency, as shown by the trend of Figs. 4 and 5. As a consequence, the measurement of the capacitance and the ESR is clearly not exhaustive and the frequency dynamic behavior should be better taken into account to understand the ultracapacitor ageing as proposed in this paper. In particular, from Fig. 4 it is possible to see that the decrease of the capacitance is present in all the frequency range. Instead, as shown in Fig. 5, the conductance values are similar in the range 1–10 mHz while considerable variations are observed at higher frequencies. 4.2. Power cycling tests To better evaluate the ageing process of ultracapacitors for automotive applications, it is necessary to reproduce in a schematic way the stress to which the considered storage device should be subjected in hybrid vehicle drive trains. For such reasons, an experimental phase with power cycling was developed quantifying the ageing as a function of the number of charge/discharge cycles. Such ageing tests are performed in high temperature and voltage conditions but without exceeding the manufacturer limits. Thus, the failure modes obtained are supposed to be as close as possible to the real use. Power cycling tests consisted in the following steps: initial rest phase of 15 s; charge phase of the device to its rated voltage at 25 A constant current; rest phase of 15 s; discharge phase of the device to one-half its rated voltage at 25 A constant current. Devices under test were inserted in the climatic chamber at constant temperature T = +70 °C.
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300
300
250
250
capacitance (F)
capacitance (F)
M. Catelani et al. / Microelectronics Reliability 53 (2013) 1676–1680
200 150 100
200 150 100 50
50 0 -3 10
10
-2
10
-1
0
0 -3 10
1
10
10
10
-2
Fig. 4. Variation of C vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
-1
10
0
1
10
Fig. 6. Variation of C vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C; magenta line (triangles) – after 10000 power cycling tests; green line (squares) – after 20000 power cycling tests. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
160
160
140
140
120
120
conductance (S)
conductance (S)
10
frequency (Hz)
frequency (Hz)
100 80 60 40 20
100 80 60 40 20
0 -3 10
-2
-1
10
10
10
0
1
10
frequency (Hz)
0 -3 10
-2
10
-1
10
0
10
1
10
frequency (Hz)
Fig. 5. Variation of G vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 5 Equivalent parallel admittance Y after 10,000 and 20,000 cycles. Freq. (Hz)
|Y| (S) after 10,000 cycles
Phase (Y) (°) after 10,000 cycles
|Y| (S) after 20,000 cycles
Phase (Y) (°) after 20,000 cycles
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 100
1.82 3.66 9.09 17.12 33.34 68.49 95.79 110.56 117.41 119.71 122.11 125.28 126.70
87.82 84.64 86.40 78.40 72.82 53.04 34.59 19.71 9.29 5.85 4.08 2.52 1.66
1.80 3.66 9.03 17.28 28.46 62.65 88.60 95.22 103.10 104.19 106.16 107.22 108.01
82.92 84.25 86.69 78.68 61.66 47.45 31.92 16.01 7.96 5.22 3.62 2.07 1.32
In Table 5, measured data of absolute value and phase of Y after 10,000 cycles (about 350 h) and 20,000 cycles (about 700 h) are reported. The results of cycling can be influenced by the experimental conditions, in particular by the rest time during the characterization procedure [10]. To obtain a good reproducibility, due to the
Fig. 7. Variation of G vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C; magenta line (triangles) – after 10000 power cycling tests; green line (squares) – after 20000 power cycling tests. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
performance recovery phenomenon, it was demonstrated [11] that the measurements of the impedance have to be carried out after the same rest time. In Figs. 6 and 7, the plots of C(f) and G(f) vs. log(f) are shown. As shown in Figs. 6 and 7, the variation of the impedance during power cycling tests is quite different from those obtained through the calendar life tests. In particular, the observed decrease of the conductance is smaller in the power cycling tests. 5. Conclusions This paper aims at assessing the ultracapacitors ageing in automotive application by means of both calendar life tests and power cycling tests. The first one allows to evaluate performance degradation when cells are used at relatively high voltages and temperatures. The power cycling of ultracapacitors was done with a specific current profile based on the typical power requirements of hybrid electric vehicle. The behavior of the ultracapacitors is monitored by a specific measurement setup with a periodic spectroscopic analysis. The experimental results show that the ultracapacitors ageing is not only affected by temperature and applied voltage but it is also a function of the frequency. Moreover, the evolution of the
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impedance measurements in power cycling tests is quite different from those of the calendar life tests. Considering that the test conditions used in calendar life tests can be far from the normal behavior encountered in operating hybrid electric vehicles, the use of power cycling is in the opinion of the authors more appropriate to estimate the ageing process during actual vehicle operations. The characterization of the ageing process via thermally stressed power cycling can contribute to the development of a new strategy for the determination of the SOH of ultracapacitor during real operating conditions in hybrid and electric vehicles. References [1] Ciappa M, Carbognani F, Fichtner W. Lifetime prediction and design of reliability tests for high-power devices in automotive applications. IEEE Trans Device Mater Rel 2003;3(4):191–6. [2] Linzen D, Buller S, Karden E, De Doncker RW. Analysis and evaluation of charge-balancing circuits on performance, reliability, and lifetime of supercapacitor systems. Ind Appl IEEE Trans 2005;41(5):1135–41. [3] Hirschmann D, Tissen D, Schroder S, De Doncker RW. Reliability prediction for inverters in hybrid electrical vehicles. Ind Appl IEEE Trans 2007;22(6):2511–7. [4] Mizutani Y, Okamoto T, Taguchi T, Nakajima K, Tanaka K. Life expectancy and degradation behavior of Electric double layer capacitor. In: Proceedings of 7th international conference on properties and applications of dielectric materials, Nagoya, June 2003.
[5] Kötz R, Hahn M, Gallay R. Temperature behaviour and impedance fundamentals of supercapacitors. J Power Sour 2006;154:550–5. [6] Kötz R, Ruch PW, Cericola D. Ageing and failure mode of electrochemical double layer capacitors during accelerated constant load tests. J Power Sour 2010;195(3):923–8. [7] El Brouji H, Briat O, Vinassa JM, Bertrand N, Woirgard E. Comparison between changes of ultracapacitors model parameters during calendar life and power cycling ageing tests. Microelectron Reliab 2008;48(8–9):1473–8. [8] Miller JR, Klementov A, Butler S. Electrochemical capacitor reliability in heavy hybrid vehicles. In: Proceedings of 16th International Seminar On Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, USA; 2006. p. 218. [9] Gualous H, Gallay R, Al Sakka M, Oukaour A, Tala-Ighil B, Boudart B. Calendar and cycling ageing of activated carbon supercapacitor for automotive application. Microelectron Reliab 2012;52(9–10):2477–81. [10] Lajnef W, Vinassa J-M, Briat O, El Brouji H, Azzopardi S, Woirgard E. Quantification of ageing of ultracapacitors during cycling tests with current profile characteristics of hybrid and electric vehicles applications. Electric Power Appl, IET 2007;1(5):683–9. [11] El Brouji E-H, Briat O, Vinassa J-M, Bertrand N, Woirgard E. Impact of calendar life and cycling ageing on supercapacitor performance. Vehicular Technol, IEEE Trans 2009;58(8):3917–29. [12] Briat O, Vinassa J-M, Bertrand N, El Brouji H, Delétage J-Y, Woirgard E. Contribution of calendar ageing modes in the performances degradation of supercapacitors during power cycling. Microelectron Reliab 2010;50(9– 11):1796–803. [13] Barsali S, Ceraolo M, Marracci M, Tellini B. Frequency dependent parameter model of supercapacitor. Measurement 2010;43(10):1683–9.
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Analysis of ultracapacitors ageing in automotive application M. Catelani a, L. Ciani a,⇑, M. Marracci b, B. Tellini b a b
Department of Information Engineering, University of Florence, Via S. Marta 3, 50139 Florence, Italy Department DESTEC, University of Pisa, L.go L. Lazzarino, 56122 Pisa (PI), Italy
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 28 June 2013 Accepted 12 July 2013
a b s t r a c t Ultracapacitors are characterized by a high specific power and long life cycle, thus they are a very interesting design solution for on-board energy applications such as hybrid-electric vehicles. In this paper the analysis of ultracapacitors ageing in automotive application is discussed to investigate the optimal performances during the whole vehicle lifetime. To study the ageing effects on the ultracapacitors, calendar life tests and power cycling tests have been carried out. The degradation of the performance of the ultracapacitors was analyzed by means of impedance spectroscopy technique. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In many fields of application, it is nowadays fundamental to guarantee high reliability of components and systems. The knowledge of the reliability aspects allows to establish the time interval in which the item (a component, an equipment or a system) is capable of performing its function, under specified operating conditions. Thus, it is evident how the reliability of ultracapacitors represents a key issue for their use into automotive applications [1–3]. According to the nature of power demand in hybrid (HEV) and electric (EV) vehicles in a broad sense, ultracapacitors represent a very useful design solution. For specific automotive applications, they are generally used to provide for example peak power demands or to store vehicle braking energy. They can be used in combination with another electrical source, like a battery or fuel cell. When appropriately designed with a system approach, they offer excellent performance, wide operating temperature range, long life, flexible management, reduced system size, and are cost effective as well as highly reliable. The approach to the design has also to face the operating conditions typical of vehicular applications (electrical, mechanical and thermal stresses). Ultracapacitors are fundamentally viewed as maintenance-free devices that do not require costly test runs and expensive management systems vs. batteries, which require ongoing evaluation of their state of health (SOH) and state of charge (SOC). For all these aspects, the analysis of ultracapacitors ageing represents a fundamental aspect to take into account in order to assure an optimal design for reliability. The method that has been chosen to study the ageing of ultracapacitors is based on both calendar life tests and power cycling tests. The calendar life tests are performed at constant temperature with the ultracapacitors polarized at the maximum voltage and
⇑ Corresponding author. Tel./fax: +39 055 4796393. E-mail address: lorenzo.ciani@unifi.it (L. Ciani). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.07.051
maintained at zero voltage level to evaluate possible differences due to polarization voltage. Tests allow therefore to analyze high-voltage and high-temperature effects on ultracapacitor ageing [4–9]. Instead, in the power cycling tests ultracapacitors are charged and discharged at constant current allowing to assess the degradations during the actual use of SCs as peak power sources in hybrid electric applications [9–12]. Power cycling tests are carried out at constant temperature as calendar life tests. Thus, ultracapacitors are aged in a short time with the same degradation mechanisms observed in normal use. The paper is organized as follows: In Section 2 a brief overview of the failure modes and effects analysis is given with application to ultracapacitors. In Section 3, the experimental setup and measurements are discussed. Results and discussion are reported in Section 4.
2. Ultracapacitors failure modes Considering the reliability and safety aspect in automotive applications, it is rather important to know the ageing behavior of the capacitor and the corresponding failure modes. For the operation of a module, it is fundamental to know whether the capacitor end-of-life is characterized by a short circuit or an open circuit state, for instance [6,9]. The main failure modes and effects related to the ultracapacitors are summarized in Table 1. Ultracapacitors are characterized by a longer lifetime than secondary batteries and, usually, they do not have a hard end of life failure similar to batteries. The main end-of-life failure mode for a ultracapacitor is an increase in Equivalent Series Resistance (ESR) and a decrease in capacitance. The first one is due to the fact that the electrode adhesion on the collector is weakening with time and temperature and, consequently, the ion availability is reduced. On the other hand, the accessible carbon surface and the ions availability are reduced during the electrochemical cycling thus leading to a capacitance decrease [9].
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M. Catelani et al. / Microelectronics Reliability 53 (2013) 1676–1680 Table 1 The main failure modes and effects of ultracapacitors. Item
Failure modes
Failure causes
Local effect
Final effect
Ultracapacitor
Capacitance decrease
Overvoltage Overtemp. Ageing Overvoltage Overtemp. Mechanical stress Ageing Mechanical stress Overvoltage Overtemp. Overpressure
Decrease of component efficiency
Loss of system performance
Decrease of component efficiency
Loss of system performance
Fire Decrease of component efficiency
System damage Loss of system performance
Component Explosion Component Explosion Component Explosion
System damage
ESR increase
Temperature increase Capacitance/ESR out of specific Cell leakage (solvent vapor release) Cell opening Gas pressure
Electrochemical decomposition overpressure Overvoltage Overtemperature
The cell container opening due to an internal overpressure is caused by the voltage and the temperature that generate a gas pressure inside the cell which increases with the working time. When the pressure reaches a determined limit, a mechanical fuse, generally a groove on the can wall or a pressure relief, may open softly, avoiding an explosion of the device [6,9]. The failure modes criteria are dependent on the application requirements. An exposure to high level of temperatures for a long time, high applied voltage and excessive current will lead to an increase of ESR and a decrease of capacitance. In fact, reducing these parameters will lengthen the lifetime of a ultracapacitor. It is important to note that a failure mode can have more than one cause; the most likely potential independent causes for each failure mode need to be identified and described. The identification and description of failure causes as well as suggestions for their mitigation should be done on the basis of the failure effects and their severity. The more severe the effects of failure modes, the more accurately failure causes should be identified and described. Failure causes may be determined from analysis of field failures or failures in test units as proposed in Section 3 with particular attention to the ageing processes of ultracapacitors due to temperature and cell voltage strictly connected to the automotive applications.
3. Experimental activity A dedicated measurement setup was carried out to investigate the ageing of ultracapacitors as function of voltage and temperature. As reported in Section 2, the observed main parameters of a ultracapacitor are usually the Equivalent Series Resistance (ESR) and the nominal Capacitance (C). The ESR and C values are generally obtained following a six-step measurement procedure suggested by the manufacturer of the ultracapacitor. Although commonly adopted in literature [7,9], these two simple parameters may not represent the whole behaviour of the ultracapacitor and it is opinion of the authors that a more complete analysis is required to characterize the device ageing. In a recent paper [13], two of the authors presented a basic model of ultracapacitor based on a theoretical analysis discussing the frequency dependence of the electric permittivity. They deduced an equivalent parallel capacitance and conductance circuit having frequency dependence components via the experimental data obtained through an impedance spectroscopy method. For such reasons an analysis of ageing processes of ultracapacitor due to temperature and cell voltage was conducted in the frequency range 1 mHz to 10 Hz, which is typical for hybrid vehicle applications. The real and imaginary part variation of the impedance was analysed in the adopted frequency
System damage System damage
range. The devices under test are 350 F MaxwellÓ double layer capacitors (D cells) represented in Fig. 1. Their main parameters are reported in Table 2. Devices under test were inserted in a precision refrigerating/ warming test chamber at constant temperature T = +70 °C. The climatic chamber (BinderÓ MK 53) has a program control ranging from T = 40 °C up to T = +180 °C and guarantees a temperature uniformity that ranges between ±0.8 °C (@ 40 °C) and ±2.0 °C (@ +150 °C) while the declared temperature fluctuation is limited to ±0.3 °C for each selected temperature. A photograph of the climatic chamber is shown in Fig. 2. Two devices were charged at the nominal voltage V = +2.7 V, two were short-circuited (V = 0 V) while two were used to perform power cycling tests. During the power cycling tests, ultracapacitors are charged and discharged at constant current of 25 A allowing to assess the degradations during the actual use of SCs as peak power sources in hybrid electric applications [9–12]. The aim of these tests is to show the trend of impedance changes during power cycling operations and compare these changes with variations obtained ageing the ultracapacitors at constant voltage. The used current profiles and the corresponding voltage responses are shown in Fig. 3. 4. Results and discussion After verifying the linear behavior of ultracapacitors, we supplied sinusoidal currents having an amplitude of 1 A in the frequency range 1 mHz to 10 Hz. From the measured current i and
Fig. 1. Ultracapacitors under test.
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M. Catelani et al. / Microelectronics Reliability 53 (2013) 1676–1680
Table 2 Main parameters of tested ultracapacitors.
Table 3 Equivalent parallel admittance Y: absolute value and phase vs. frequency for virgin devices.
Nominal capacitance
Rated voltage
Resistance (ESR)
Power
Energy
350 F
2.7 V
3.2 m X
4.3 kW/kg
5.62 Wh/kg
Frequency (Hz)
|Y| (S)
Phase (Y) (°)
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10
1.86 3.73 9.22 18.27 33.99 74.39 109.73 131.82 142.79 146.63 151.21 155.79 158.46
87.66 87.10 85.73 83.04 74.55 58.80 39.36 23.14 11.24 7.34 5.28 3.27 2.29
Table 4 Equivalent parallel admittance Y after 700 h for devices charged at the nominal voltage (@ V = 2.7 V) and for the devices short-circuited (@ V = 0 V).
Fig. 2. Precision refrigerating/warming test chamber.
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Phase (Y) (°) @ V=0V
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10
1.82 3.61 9.12 17.43 31.85 56.84 70.15 74.16 80.78 81.79 83.63 85.51 86.42
88.19 86.91 81.44 74.62 63.08 40.96 24.69 12.32 7.32 4.60 3.42 2.20 1.51
1.80 3.58 8.87 17.24 31.35 60.79 79.39 87.10 89.78 90.80 91.95 93.54 94.37
87.89 87.04 85.94 79.96 67.53 45.93 28.30 15.37 6.99 4.26 3.02 1.83 1.27
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time
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Fig. 3. Current profiles (green lines – right axis) and corresponding voltage responses (blue lines – left axis). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
voltage v adopting symbolic notation the equivalent admittance can be estimated as Y = G + jxC, at each frequency. As known from the literature [9], the measurements performed to characterize the ageing are sensitive to the polarization history. For these reasons we observed an increase of capacitance (compared with ‘‘virgin’’ values) for the first 24 h and then the expected monotone evolution with ageing. As a consequence, we used as the reference initial values the data obtained after 100 full charge and discharge cycles and 24 h rest time. Obtained data in terms of absolute value and phase of admittance Y are reported in Table 3. 4.1. Calendar life tests In Table 4, measured data of absolute value and phase of Y after 700 h in climatic chamber at T = +70 °C for the devices charged at the nominal voltage (@ V = 2.7 V) and for the devices short-circuited (@ V = 0 V) are reported. In Figs. 4 and 5, the plots of C(f) and G(f) vs. log(f) are shown. From the experimental results, it is possible to see that for increasing temperature the ageing processes are accelerated by the higher reactivity of the chemical components [2,6].
At the same time it is also possible to note that the ageing is function of the frequency, as shown by the trend of Figs. 4 and 5. As a consequence, the measurement of the capacitance and the ESR is clearly not exhaustive and the frequency dynamic behavior should be better taken into account to understand the ultracapacitor ageing as proposed in this paper. In particular, from Fig. 4 it is possible to see that the decrease of the capacitance is present in all the frequency range. Instead, as shown in Fig. 5, the conductance values are similar in the range 1–10 mHz while considerable variations are observed at higher frequencies. 4.2. Power cycling tests To better evaluate the ageing process of ultracapacitors for automotive applications, it is necessary to reproduce in a schematic way the stress to which the considered storage device should be subjected in hybrid vehicle drive trains. For such reasons, an experimental phase with power cycling was developed quantifying the ageing as a function of the number of charge/discharge cycles. Such ageing tests are performed in high temperature and voltage conditions but without exceeding the manufacturer limits. Thus, the failure modes obtained are supposed to be as close as possible to the real use. Power cycling tests consisted in the following steps: initial rest phase of 15 s; charge phase of the device to its rated voltage at 25 A constant current; rest phase of 15 s; discharge phase of the device to one-half its rated voltage at 25 A constant current. Devices under test were inserted in the climatic chamber at constant temperature T = +70 °C.
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Fig. 4. Variation of C vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
-1
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0
1
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Fig. 6. Variation of C vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C; magenta line (triangles) – after 10000 power cycling tests; green line (squares) – after 20000 power cycling tests. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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conductance (S)
conductance (S)
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frequency (Hz)
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100 80 60 40 20
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Fig. 5. Variation of G vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 5 Equivalent parallel admittance Y after 10,000 and 20,000 cycles. Freq. (Hz)
|Y| (S) after 10,000 cycles
Phase (Y) (°) after 10,000 cycles
|Y| (S) after 20,000 cycles
Phase (Y) (°) after 20,000 cycles
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 100
1.82 3.66 9.09 17.12 33.34 68.49 95.79 110.56 117.41 119.71 122.11 125.28 126.70
87.82 84.64 86.40 78.40 72.82 53.04 34.59 19.71 9.29 5.85 4.08 2.52 1.66
1.80 3.66 9.03 17.28 28.46 62.65 88.60 95.22 103.10 104.19 106.16 107.22 108.01
82.92 84.25 86.69 78.68 61.66 47.45 31.92 16.01 7.96 5.22 3.62 2.07 1.32
In Table 5, measured data of absolute value and phase of Y after 10,000 cycles (about 350 h) and 20,000 cycles (about 700 h) are reported. The results of cycling can be influenced by the experimental conditions, in particular by the rest time during the characterization procedure [10]. To obtain a good reproducibility, due to the
Fig. 7. Variation of G vs. frequency. Black line (crosses) – virgin capacitors; red line (full filled circles) – capacitors @ V = 2, 7 V after 700 h @ T = 70 °C; blue line (empty circles) – capacitors @ V = 0 V after 700 h @ T = 70 °C; magenta line (triangles) – after 10000 power cycling tests; green line (squares) – after 20000 power cycling tests. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
performance recovery phenomenon, it was demonstrated [11] that the measurements of the impedance have to be carried out after the same rest time. In Figs. 6 and 7, the plots of C(f) and G(f) vs. log(f) are shown. As shown in Figs. 6 and 7, the variation of the impedance during power cycling tests is quite different from those obtained through the calendar life tests. In particular, the observed decrease of the conductance is smaller in the power cycling tests. 5. Conclusions This paper aims at assessing the ultracapacitors ageing in automotive application by means of both calendar life tests and power cycling tests. The first one allows to evaluate performance degradation when cells are used at relatively high voltages and temperatures. The power cycling of ultracapacitors was done with a specific current profile based on the typical power requirements of hybrid electric vehicle. The behavior of the ultracapacitors is monitored by a specific measurement setup with a periodic spectroscopic analysis. The experimental results show that the ultracapacitors ageing is not only affected by temperature and applied voltage but it is also a function of the frequency. Moreover, the evolution of the
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impedance measurements in power cycling tests is quite different from those of the calendar life tests. Considering that the test conditions used in calendar life tests can be far from the normal behavior encountered in operating hybrid electric vehicles, the use of power cycling is in the opinion of the authors more appropriate to estimate the ageing process during actual vehicle operations. The characterization of the ageing process via thermally stressed power cycling can contribute to the development of a new strategy for the determination of the SOH of ultracapacitor during real operating conditions in hybrid and electric vehicles. References [1] Ciappa M, Carbognani F, Fichtner W. Lifetime prediction and design of reliability tests for high-power devices in automotive applications. IEEE Trans Device Mater Rel 2003;3(4):191–6. [2] Linzen D, Buller S, Karden E, De Doncker RW. Analysis and evaluation of charge-balancing circuits on performance, reliability, and lifetime of supercapacitor systems. Ind Appl IEEE Trans 2005;41(5):1135–41. [3] Hirschmann D, Tissen D, Schroder S, De Doncker RW. Reliability prediction for inverters in hybrid electrical vehicles. Ind Appl IEEE Trans 2007;22(6):2511–7. [4] Mizutani Y, Okamoto T, Taguchi T, Nakajima K, Tanaka K. Life expectancy and degradation behavior of Electric double layer capacitor. In: Proceedings of 7th international conference on properties and applications of dielectric materials, Nagoya, June 2003.
[5] Kötz R, Hahn M, Gallay R. Temperature behaviour and impedance fundamentals of supercapacitors. J Power Sour 2006;154:550–5. [6] Kötz R, Ruch PW, Cericola D. Ageing and failure mode of electrochemical double layer capacitors during accelerated constant load tests. J Power Sour 2010;195(3):923–8. [7] El Brouji H, Briat O, Vinassa JM, Bertrand N, Woirgard E. Comparison between changes of ultracapacitors model parameters during calendar life and power cycling ageing tests. Microelectron Reliab 2008;48(8–9):1473–8. [8] Miller JR, Klementov A, Butler S. Electrochemical capacitor reliability in heavy hybrid vehicles. In: Proceedings of 16th International Seminar On Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, USA; 2006. p. 218. [9] Gualous H, Gallay R, Al Sakka M, Oukaour A, Tala-Ighil B, Boudart B. Calendar and cycling ageing of activated carbon supercapacitor for automotive application. Microelectron Reliab 2012;52(9–10):2477–81. [10] Lajnef W, Vinassa J-M, Briat O, El Brouji H, Azzopardi S, Woirgard E. Quantification of ageing of ultracapacitors during cycling tests with current profile characteristics of hybrid and electric vehicles applications. Electric Power Appl, IET 2007;1(5):683–9. [11] El Brouji E-H, Briat O, Vinassa J-M, Bertrand N, Woirgard E. Impact of calendar life and cycling ageing on supercapacitor performance. Vehicular Technol, IEEE Trans 2009;58(8):3917–29. [12] Briat O, Vinassa J-M, Bertrand N, El Brouji H, Delétage J-Y, Woirgard E. Contribution of calendar ageing modes in the performances degradation of supercapacitors during power cycling. Microelectron Reliab 2010;50(9– 11):1796–803. [13] Barsali S, Ceraolo M, Marracci M, Tellini B. Frequency dependent parameter model of supercapacitor. Measurement 2010;43(10):1683–9.