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Power cycling tests for accelerated ageing of ultracapacitors
Microelectronics Reliability 46 (2006) 1445–1450 www.elsevier.com/locate/microrel
Power cycling tests for accelerated ageing of ultracapacitors O. Briat*, W. Lajnef, J-M. Vinassa, E. Woirgard Laboratoire IXL CNRS UMR 5818 – ENSEIRB Université Bordeaux 1 351 Cours de la Libération 33405 Talence Cedex - FRANCE
Abstract In this paper, a test methodology is presented and evaluated in order to quantify the lifetime of ultracapacitors in case of active power cycling. The test profiles are specified for taking into account both the high charge-discharge levels and the periodicity of typical HEV applications. Before cycling, preliminary tests have been done with Maxwell BCap 2600F devices and have proved that pulsed current profiles can be predetermined in order to induce a given self-heating. Then, during power cycling, periodic characterization tests have been done in order to follow the evolution of cell parameters. The results at 25,000 cycles illustrate that impedance real part and capacitance are significant parameters for ageing quantification. Furthermore, the trend of these results shows that the self-heating acts as an accelerating factor for ageing.
1.
Introduction
Available storage devices like ultracapacitors offer excellent performances for a potential use as peak power source in hybrid and electric vehicles (HEV) applications. Their main features are a high capacitance up to 5000F for a 2.7V cell, a very low internal resistance <1 mΩ, and a wide range of operating temperature [-40°C; 70°C]. Therefore, they are able to supply and accept high power pulses very efficiently during few seconds [1]. Ultracapacitors can be combined with batteries, or another energy source, leading to a hybrid source where the battery acts as an energy tank and is discharged with constant current for long time duration. The ultracapacitors meet the transient power requirements which are growing needs in Hybrid and Electric Vehicles applications. For a best use of ultracapacitors, it is important to know their electro-thermal behaviour and to determine the parameters that have an effect on their reliability. * Corresponding author : [email protected] Tel: +33(0)5 40 00 26 08; Fax: +33(0)5 56 37 15 45
0026-2714/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.microrel.2006.07.008
Regarding manufacturer data and academic studies, test procedures and electric models are not really standardized. Also, few works are devoted to the thermal behaviour. Concerning reliability, data are very poor and anyway not well adapted for pulsed current cycles. Therefore, the aim of this study is to elaborate a methodology to assess the ultracapacitors reliability. First, the pulsed current profiles that will be used in power cycling tests are specified in terms of peak and RMS values and according to the periodicity of typical power requirements of micro- and mild-hybrids vehicles. After a brief description of the test platform, preliminary results concerning the validation of the pulsed current profiles and the characterization procedures are presented. Finally, the characterization results obtained during power cycling tests are presented with a focus on electrical parameters changes with the number of cycles.
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O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
2.
Current profiles specification
The current profiles specification is based on both ultracapacitors features (allowable RMS value of current profiles due to thermal limits, periodicity due to low specific energy) and on HEV requirements (current pulses up to 800A for few seconds) [2]. According to acceleration/braking or start/stop operations for normalised HEV profiles, a typical period of 1min has been chosen. The pulse width depends on the vehicle type and is about 0.5 to 2s for micro-hybrids and about 2 to 10s for mild-hybrids. According to theses specifications, a general current profile has been proposed for power cycling and is illustrated in Fig. 1. The pulse width τ which defines the duty cycle α is determined from the desired RMS value and peak value of the current.
cycling, thermal characterization and ageing study. The impedance analyser Zahner IM6 is associated with a power booster in order to ensure measurement accuracy of very low impedances in the [10mHz-1kHz] investigated frequency range. This tool is associated to a post-processing algorithm which allows the parameters identification of equivalent electrical models from obtained frequency responses. Finally, a connecting and mounting plate has been specially designed to ensure the electrical connection and to avoid the DUT cooling by heat conduction in the large-section cables. In this aim, high thermal resistances are inserted between the ultracapacitor terminals and cables by reducing the section for very short lengths.
Fig. 1. Current profiles definition
In practice, such a profile can not be chargeneutral due to losses induced by non-reversible electrochemical reactions. In spite of very good charge efficiency, these losses have to be compensated in order to perform a significant number of cycles during power cycling. Thus, instead of constant charge duration, the charge will be stopped when the maximum voltage value chosen for power cycling tests will be reached. In these conditions, the current profile is not exactly symmetric. However, thanks to this solution, a voltage drift during power cycling is avoided. 3.
Dedicated test platform for ultracapacitors
The test platform for ultracapacitors is illustrated in Fig. 2 and is made of a dynamic power tester, a specific impedance analyser and a climatic chamber. The power tester Digatron MBT is a fast dynamic charge/discharge system composed of two independent power circuits (20 V/±400 A). This tool is useful for constant current tests for model validation, power
Fig. 2. Overview of the test platform and details on the DUT mounting plate
4.
Validation of the power cycling profiles
The main objective of power cycling is to induce a self-heating of the ultracapacitor under safe operating conditions. In this aim, we have chosen to predetermine the RMS value of the current profiles which leads to a maximum working temperature of 60°C at the beginning of power cycling tests. First, we have focused on the definition of the rated current value. As it is not clearly expressed in manufacturer’s datasheets, we defined this rated value as the magnitude of a rectangular current (consecutive charge/discharge without rest) which induces a maximum self-heating. Thus, for a given ambient temperature and thanks to an electro-thermal model of the ultracapacitor, iterative simulations have allowed us
O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
Then, on the basis of the simulation results, power cycling tests have been made. Fig. 3 shows the thermal response of a Maxwell BCap 2600F element with a 400A profile. In our case, as the internal temperature is not accessible, we have monitored the external hot-spot temperature which is located on the terminals. 62
voltage between the beginning and the end of the cycling. This difference reveals a diminution of the device capacitance. 2.6
2.4 2.2 Vscap (V)
to determine the rated current value that were used for power cycling tests [3].
1447
Initial After power cycling
2 1.8
1.6
60 1.4
58 62
0
54
61
52
Température (°C)
Temperature (°C)
56
50 48 46 44 40
0
20
59
40
119 Time (min)
60 80 Time(min)
120
100
120
Fig. 3. Experimental validation of the thermal response with a 400A current profile
The results of Fig. 3 confirm that the average value of the temperature is 60°C in steady-state. At the timescale of one cycle, we can observe variations around the steady-state temperature. For a given RMS current value, these variations will be sharp as the charge and discharge duration is reduced. This behaviour may have an impact on the thermal stress of the device and consequently on the ageing acceleration. However, additional tests are required to confirm this point. 5.
40
60
80
120 100 Time (s)
140
160
180
200
Fig. 4. Voltage response change during power cycling
60
58 118
42
20
Characterization method for ageing study
In order to quantify the ultracapacitors ageing, electrical characterization procedures are done periodically during the power cycling. Initially, they were based on both constant current and impedance spectroscopy tests. Then, a first campaign of power cycling tests has been investigated in order to refine this characterization method. 5.1. Constant current tests The time-domain analysis of the voltage response to constant current profiles (charge/discharge with rest period) can be used to determine the influence of power cycling on the ultracapacitor's main features. From the results presented in Fig. 4, we can observe a small increase of the rising and falling slopes of the
Furthermore, we can see that the final value of the voltage at the end of rest, after a charge or a discharge, differs between these two curves. This is typically due to an increase of the equivalent serial resistance of the device since the upper and the lower voltage limits have been fixed during the test. Although these results allow a first qualitative analysis of the parameters evolution with the number of cycles, another characterization method is proposed. 5.2. Impedance spectroscopy Impedance spectroscopy is a useful experimental method for the parameters identification of dynamic electric models [4]. Also, it is interesting to integrate this method in a periodical characterization procedure in order to follow the evolution of these parameters during power cycling tests. In this aim, Fig. 5 shows the change of the impedance of a Maxwell BCap 2600F element after power cycling. A noticeable increase of the real part is observed which has an effect on power losses and so on the roundtrip efficiency. At low frequency, a change of the imaginary part is noticed which illustrates a capacitance decrease and so less stored energy.
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O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450 Ultracapacitor tested with 400 A current profile 0.8
0
120
0.7
-1
Re(Z) (mΩ )
0.6
-2
Initial impedance Impedance after cycling
0.5
-3
0.4
-4
0.3
-5
-1
10
1
0
2
10 10 Frequency (Hz)
10
R (DC) Re (100 mHz)
40 20 0
0
90 Relative capacitance (%)
70 60
50
30 20
0.3
0.2 0
1
10 10 Frequency (Hz)
2
10
Fig. 6. Effect of the rest time on the impedance real part after power cycling
Power cycling results and exploitation
Power cycling tests have been done in a climatic chamber at 40°C and characterization tests have been done periodically according to the previously presented methodology. From constant current tests, we have extracted R(DC) and C(DC) and from impedance spectroscopy, real and imaginary part of the impedance Re(100mHz) and C(10mHz) have been selected as representative parameters for the ageing quantification.
0
5000
15000 10000 Number of cycles
20000
25000
Fig. 8. Capacitance losses during power cycling
0.4
-1
C (DC) C (10 mHz)
40
10
0.5
10
25000
80
0
-2
20000
Ultracapapcitor tested with 400 A current profile
Number of cycles =0 Number of cycles=10 000, Rest time=24h Number of cycles=10 000, Rest time=48h Number of cycles=10 000, Rest time=72h
10
15000 10000 Number of cycles
100
0.7
0.6
5000
Fig. 7. Resistance increase during power cycling
In order to obtain reliable results, the experimental conditions are key issues. Therefore, before each characterization test, the device is kept in open circuit state at 20°C for a sufficient time. The influence of the rest time is illustrated in Fig. 6. Also, these results show a regeneration phenomenon of the device performances as function of the rest time since the impedance real parts measured 48h and 72h after the end of the power cycling are lower than the one measured 24h after. Following results presented in this paper were obtained with 24h rest time.
Re(Z) (mΩ )
80
60
-6
Fig. 5. Impedance changes after power cycling
6.
100
-7 3 10
0.2 0.1 -2 10
Relative resistance (%)
140
Im(Z) (mΩ)
1
0.9
3
10
After 25,000 cycles, Fig. 7 shows that the increase of the impedance real part measured at 100mHz part is about 35%. A roughly 20% increase of the DC resistance is also observed. Also, Fig. 8 illustrates that the capacitance fading reaches 12% at DC and 10mHz. Although these results validate the power cycling method, as we have to distinguish very low variations of resistance and capacitance, in our case, DC results are not sufficiently accurate for ageing quantification. In order to complete these first results, we have focused on the influence of the profile discontinuity. Thus, two new ultracapacitors have been cycled respectively with 200A and 400A current profiles. They have the same RMS value (200A) which leads to terminals temperatures of 60°C at the beginning of the power cycling. Therefore, Fig. 9 shows the increase of the impedance real part at 100mHz for the tested ultracapacitors. Roughly, 20% and 30% increase is recorded respectively for the 200A and the 400A current profiles. Like for the impedance real part, Fig. 10 shows that 10% and 12% of capacitance losses are recorded for the ultracapacitors tested respectively for the 200A and 400A current profiles.
0,5
140
120
100
80
200A profile 400A profile
60
40
0,4 300A profile 400A profile
0,3
0,2
0,1
20 0 0
5000
15000 10000 Number of cycles
0 0
25000
20000
5000
15000 10000 Number of cycles
20000
25000
Fig. 11. Resistance increase during power cycling
Fig. 9. Resistance increase during power cycling
2 800
100
2 400
90 80
300A profle 400A profile
2 000
200A profile 400A profile
70
Capacitance (F)
Capacitance / initial capacitance (n=0 cycles) (%)
1449
160
Re (100mHz) (mΩ )
Resistaance/ initiale resistance (n=0 cycles) (%)
O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
60
50 40
1 600 1 200 800
30
500
20 10 0 0
0 0 5000
15000 10000 Number of cycles
20000
5000
25000
10000 15000 Number of cycles
20000
25000
Fig. 12. Capacitance losses during power cycling
Fig. 10. Capacitance losses during power cycling
The difference observed for these results may reveal an influence of the profile discontinuity on ageing. In fact, even if a common RMS current value is supposed to lead to the same heating, the temperature shapes during a cycle are different and can induce differences in ageing mechanisms. However, this effect has to be verified by additional power cycling tests. After 25,000 cycles, the temperature of the device tested with the 400A current profile has reached 70°C. So, in these conditions, the power cycling tests can not be continued although manufacturer end-of-life specifications, which are 100% increase of the impedance real part and 20% losses of the capacitance, are not reached. So, we have reduced the RMS current value in order to obtain 55°C terminal temperature and we have performed additional power cycling tests with two new components with 300A and 400A profiles. Fig. 11 shows that increases of 22% and 26% are recorded for the impedance real part after 25,000 cycles for 300A and 400A respectively. Then, as illustrated in Fig. 12, the capacitance loss reaches 16% and 17% for these two profiles.
7.
Conclusion
In this paper, an experimental method has been investigated and evaluated in order to accelerate the ageing tests of ultracapacitors. It is based on the selfheating induced by repetitive pulsed current profiles. First, these pulsed current profiles are specified and tests have proved that a thermal steady-state can be obtained from a given RMS value of the current profiles which can be determined thanks to electrothermal models. Then, ultracapacitors have been cycled. A specific characterization method has been applied periodically on the tested components. The results obtained from the first 25,000 cycles have demonstrated that impedance real part and capacitance values are significant parameters for ageing quantification. In these experimental conditions and according to the tendency of the obtained results, ultracapacitors expected lifetime will be lower than the 500,000 cycles announced by the manufacturers for soft operating conditions. Anyway, current shape has a clear impact on the ultracapacitor ageing.
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O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
References [1] A. Burke: "Ultracapacitors: why, how, and where is the technology". Journal of power sources, Vol. 91, Issue 1, pp 37-50, November 2000. [2] "Freedom CAR Ultracapacitor Test Manual", DOE/NE ID. Revision 0. September 21, 2004. [3] W. lajnef, J.-M. Vinassa, S. Azzopardi, O. Briat, A. Guedon, C. Zardini, "First step in the reliability of ultracapacitors used as power source in hybrid electric vehicles", ESREF 04, Zurich, Switzerland, 4-8 October 2004 [4] Eckhard Karden, Stephan Buller, Rick W. De Doncker, “A frequency-domain approach to dynamical modelling of electrochemical power sources”, Electrochemica Acta, Volume 47, Issues 13-14, Pages 2347-2356, 25 May 2002.
Power cycling tests for accelerated ageing of ultracapacitors O. Briat*, W. Lajnef, J-M. Vinassa, E. Woirgard Laboratoire IXL CNRS UMR 5818 – ENSEIRB Université Bordeaux 1 351 Cours de la Libération 33405 Talence Cedex - FRANCE
Abstract In this paper, a test methodology is presented and evaluated in order to quantify the lifetime of ultracapacitors in case of active power cycling. The test profiles are specified for taking into account both the high charge-discharge levels and the periodicity of typical HEV applications. Before cycling, preliminary tests have been done with Maxwell BCap 2600F devices and have proved that pulsed current profiles can be predetermined in order to induce a given self-heating. Then, during power cycling, periodic characterization tests have been done in order to follow the evolution of cell parameters. The results at 25,000 cycles illustrate that impedance real part and capacitance are significant parameters for ageing quantification. Furthermore, the trend of these results shows that the self-heating acts as an accelerating factor for ageing.
1.
Introduction
Available storage devices like ultracapacitors offer excellent performances for a potential use as peak power source in hybrid and electric vehicles (HEV) applications. Their main features are a high capacitance up to 5000F for a 2.7V cell, a very low internal resistance <1 mΩ, and a wide range of operating temperature [-40°C; 70°C]. Therefore, they are able to supply and accept high power pulses very efficiently during few seconds [1]. Ultracapacitors can be combined with batteries, or another energy source, leading to a hybrid source where the battery acts as an energy tank and is discharged with constant current for long time duration. The ultracapacitors meet the transient power requirements which are growing needs in Hybrid and Electric Vehicles applications. For a best use of ultracapacitors, it is important to know their electro-thermal behaviour and to determine the parameters that have an effect on their reliability. * Corresponding author : [email protected] Tel: +33(0)5 40 00 26 08; Fax: +33(0)5 56 37 15 45
0026-2714/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.microrel.2006.07.008
Regarding manufacturer data and academic studies, test procedures and electric models are not really standardized. Also, few works are devoted to the thermal behaviour. Concerning reliability, data are very poor and anyway not well adapted for pulsed current cycles. Therefore, the aim of this study is to elaborate a methodology to assess the ultracapacitors reliability. First, the pulsed current profiles that will be used in power cycling tests are specified in terms of peak and RMS values and according to the periodicity of typical power requirements of micro- and mild-hybrids vehicles. After a brief description of the test platform, preliminary results concerning the validation of the pulsed current profiles and the characterization procedures are presented. Finally, the characterization results obtained during power cycling tests are presented with a focus on electrical parameters changes with the number of cycles.
1446
O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
2.
Current profiles specification
The current profiles specification is based on both ultracapacitors features (allowable RMS value of current profiles due to thermal limits, periodicity due to low specific energy) and on HEV requirements (current pulses up to 800A for few seconds) [2]. According to acceleration/braking or start/stop operations for normalised HEV profiles, a typical period of 1min has been chosen. The pulse width depends on the vehicle type and is about 0.5 to 2s for micro-hybrids and about 2 to 10s for mild-hybrids. According to theses specifications, a general current profile has been proposed for power cycling and is illustrated in Fig. 1. The pulse width τ which defines the duty cycle α is determined from the desired RMS value and peak value of the current.
cycling, thermal characterization and ageing study. The impedance analyser Zahner IM6 is associated with a power booster in order to ensure measurement accuracy of very low impedances in the [10mHz-1kHz] investigated frequency range. This tool is associated to a post-processing algorithm which allows the parameters identification of equivalent electrical models from obtained frequency responses. Finally, a connecting and mounting plate has been specially designed to ensure the electrical connection and to avoid the DUT cooling by heat conduction in the large-section cables. In this aim, high thermal resistances are inserted between the ultracapacitor terminals and cables by reducing the section for very short lengths.
Fig. 1. Current profiles definition
In practice, such a profile can not be chargeneutral due to losses induced by non-reversible electrochemical reactions. In spite of very good charge efficiency, these losses have to be compensated in order to perform a significant number of cycles during power cycling. Thus, instead of constant charge duration, the charge will be stopped when the maximum voltage value chosen for power cycling tests will be reached. In these conditions, the current profile is not exactly symmetric. However, thanks to this solution, a voltage drift during power cycling is avoided. 3.
Dedicated test platform for ultracapacitors
The test platform for ultracapacitors is illustrated in Fig. 2 and is made of a dynamic power tester, a specific impedance analyser and a climatic chamber. The power tester Digatron MBT is a fast dynamic charge/discharge system composed of two independent power circuits (20 V/±400 A). This tool is useful for constant current tests for model validation, power
Fig. 2. Overview of the test platform and details on the DUT mounting plate
4.
Validation of the power cycling profiles
The main objective of power cycling is to induce a self-heating of the ultracapacitor under safe operating conditions. In this aim, we have chosen to predetermine the RMS value of the current profiles which leads to a maximum working temperature of 60°C at the beginning of power cycling tests. First, we have focused on the definition of the rated current value. As it is not clearly expressed in manufacturer’s datasheets, we defined this rated value as the magnitude of a rectangular current (consecutive charge/discharge without rest) which induces a maximum self-heating. Thus, for a given ambient temperature and thanks to an electro-thermal model of the ultracapacitor, iterative simulations have allowed us
O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
Then, on the basis of the simulation results, power cycling tests have been made. Fig. 3 shows the thermal response of a Maxwell BCap 2600F element with a 400A profile. In our case, as the internal temperature is not accessible, we have monitored the external hot-spot temperature which is located on the terminals. 62
voltage between the beginning and the end of the cycling. This difference reveals a diminution of the device capacitance. 2.6
2.4 2.2 Vscap (V)
to determine the rated current value that were used for power cycling tests [3].
1447
Initial After power cycling
2 1.8
1.6
60 1.4
58 62
0
54
61
52
Température (°C)
Temperature (°C)
56
50 48 46 44 40
0
20
59
40
119 Time (min)
60 80 Time(min)
120
100
120
Fig. 3. Experimental validation of the thermal response with a 400A current profile
The results of Fig. 3 confirm that the average value of the temperature is 60°C in steady-state. At the timescale of one cycle, we can observe variations around the steady-state temperature. For a given RMS current value, these variations will be sharp as the charge and discharge duration is reduced. This behaviour may have an impact on the thermal stress of the device and consequently on the ageing acceleration. However, additional tests are required to confirm this point. 5.
40
60
80
120 100 Time (s)
140
160
180
200
Fig. 4. Voltage response change during power cycling
60
58 118
42
20
Characterization method for ageing study
In order to quantify the ultracapacitors ageing, electrical characterization procedures are done periodically during the power cycling. Initially, they were based on both constant current and impedance spectroscopy tests. Then, a first campaign of power cycling tests has been investigated in order to refine this characterization method. 5.1. Constant current tests The time-domain analysis of the voltage response to constant current profiles (charge/discharge with rest period) can be used to determine the influence of power cycling on the ultracapacitor's main features. From the results presented in Fig. 4, we can observe a small increase of the rising and falling slopes of the
Furthermore, we can see that the final value of the voltage at the end of rest, after a charge or a discharge, differs between these two curves. This is typically due to an increase of the equivalent serial resistance of the device since the upper and the lower voltage limits have been fixed during the test. Although these results allow a first qualitative analysis of the parameters evolution with the number of cycles, another characterization method is proposed. 5.2. Impedance spectroscopy Impedance spectroscopy is a useful experimental method for the parameters identification of dynamic electric models [4]. Also, it is interesting to integrate this method in a periodical characterization procedure in order to follow the evolution of these parameters during power cycling tests. In this aim, Fig. 5 shows the change of the impedance of a Maxwell BCap 2600F element after power cycling. A noticeable increase of the real part is observed which has an effect on power losses and so on the roundtrip efficiency. At low frequency, a change of the imaginary part is noticed which illustrates a capacitance decrease and so less stored energy.
1448
O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450 Ultracapacitor tested with 400 A current profile 0.8
0
120
0.7
-1
Re(Z) (mΩ )
0.6
-2
Initial impedance Impedance after cycling
0.5
-3
0.4
-4
0.3
-5
-1
10
1
0
2
10 10 Frequency (Hz)
10
R (DC) Re (100 mHz)
40 20 0
0
90 Relative capacitance (%)
70 60
50
30 20
0.3
0.2 0
1
10 10 Frequency (Hz)
2
10
Fig. 6. Effect of the rest time on the impedance real part after power cycling
Power cycling results and exploitation
Power cycling tests have been done in a climatic chamber at 40°C and characterization tests have been done periodically according to the previously presented methodology. From constant current tests, we have extracted R(DC) and C(DC) and from impedance spectroscopy, real and imaginary part of the impedance Re(100mHz) and C(10mHz) have been selected as representative parameters for the ageing quantification.
0
5000
15000 10000 Number of cycles
20000
25000
Fig. 8. Capacitance losses during power cycling
0.4
-1
C (DC) C (10 mHz)
40
10
0.5
10
25000
80
0
-2
20000
Ultracapapcitor tested with 400 A current profile
Number of cycles =0 Number of cycles=10 000, Rest time=24h Number of cycles=10 000, Rest time=48h Number of cycles=10 000, Rest time=72h
10
15000 10000 Number of cycles
100
0.7
0.6
5000
Fig. 7. Resistance increase during power cycling
In order to obtain reliable results, the experimental conditions are key issues. Therefore, before each characterization test, the device is kept in open circuit state at 20°C for a sufficient time. The influence of the rest time is illustrated in Fig. 6. Also, these results show a regeneration phenomenon of the device performances as function of the rest time since the impedance real parts measured 48h and 72h after the end of the power cycling are lower than the one measured 24h after. Following results presented in this paper were obtained with 24h rest time.
Re(Z) (mΩ )
80
60
-6
Fig. 5. Impedance changes after power cycling
6.
100
-7 3 10
0.2 0.1 -2 10
Relative resistance (%)
140
Im(Z) (mΩ)
1
0.9
3
10
After 25,000 cycles, Fig. 7 shows that the increase of the impedance real part measured at 100mHz part is about 35%. A roughly 20% increase of the DC resistance is also observed. Also, Fig. 8 illustrates that the capacitance fading reaches 12% at DC and 10mHz. Although these results validate the power cycling method, as we have to distinguish very low variations of resistance and capacitance, in our case, DC results are not sufficiently accurate for ageing quantification. In order to complete these first results, we have focused on the influence of the profile discontinuity. Thus, two new ultracapacitors have been cycled respectively with 200A and 400A current profiles. They have the same RMS value (200A) which leads to terminals temperatures of 60°C at the beginning of the power cycling. Therefore, Fig. 9 shows the increase of the impedance real part at 100mHz for the tested ultracapacitors. Roughly, 20% and 30% increase is recorded respectively for the 200A and the 400A current profiles. Like for the impedance real part, Fig. 10 shows that 10% and 12% of capacitance losses are recorded for the ultracapacitors tested respectively for the 200A and 400A current profiles.
0,5
140
120
100
80
200A profile 400A profile
60
40
0,4 300A profile 400A profile
0,3
0,2
0,1
20 0 0
5000
15000 10000 Number of cycles
0 0
25000
20000
5000
15000 10000 Number of cycles
20000
25000
Fig. 11. Resistance increase during power cycling
Fig. 9. Resistance increase during power cycling
2 800
100
2 400
90 80
300A profle 400A profile
2 000
200A profile 400A profile
70
Capacitance (F)
Capacitance / initial capacitance (n=0 cycles) (%)
1449
160
Re (100mHz) (mΩ )
Resistaance/ initiale resistance (n=0 cycles) (%)
O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
60
50 40
1 600 1 200 800
30
500
20 10 0 0
0 0 5000
15000 10000 Number of cycles
20000
5000
25000
10000 15000 Number of cycles
20000
25000
Fig. 12. Capacitance losses during power cycling
Fig. 10. Capacitance losses during power cycling
The difference observed for these results may reveal an influence of the profile discontinuity on ageing. In fact, even if a common RMS current value is supposed to lead to the same heating, the temperature shapes during a cycle are different and can induce differences in ageing mechanisms. However, this effect has to be verified by additional power cycling tests. After 25,000 cycles, the temperature of the device tested with the 400A current profile has reached 70°C. So, in these conditions, the power cycling tests can not be continued although manufacturer end-of-life specifications, which are 100% increase of the impedance real part and 20% losses of the capacitance, are not reached. So, we have reduced the RMS current value in order to obtain 55°C terminal temperature and we have performed additional power cycling tests with two new components with 300A and 400A profiles. Fig. 11 shows that increases of 22% and 26% are recorded for the impedance real part after 25,000 cycles for 300A and 400A respectively. Then, as illustrated in Fig. 12, the capacitance loss reaches 16% and 17% for these two profiles.
7.
Conclusion
In this paper, an experimental method has been investigated and evaluated in order to accelerate the ageing tests of ultracapacitors. It is based on the selfheating induced by repetitive pulsed current profiles. First, these pulsed current profiles are specified and tests have proved that a thermal steady-state can be obtained from a given RMS value of the current profiles which can be determined thanks to electrothermal models. Then, ultracapacitors have been cycled. A specific characterization method has been applied periodically on the tested components. The results obtained from the first 25,000 cycles have demonstrated that impedance real part and capacitance values are significant parameters for ageing quantification. In these experimental conditions and according to the tendency of the obtained results, ultracapacitors expected lifetime will be lower than the 500,000 cycles announced by the manufacturers for soft operating conditions. Anyway, current shape has a clear impact on the ultracapacitor ageing.
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O. Briat et al. / Microelectronics Reliability 46 (2006) 1445–1450
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