2013_Voltage and Temperature Impacts on Leakage Current in Calendar Ageing of Supercapacitors

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Voltage and Temperature Impacts on LeakageCurrent in Calendar Ageing of Supercapacitors

Mohamed Ayadi*, Akram Eddahech, Olivier Briat, Jean-Michel VinassaUniv. Bordeaux, IMS, UMR 5218 CNRS, F-33400Talence, FranceLaboratoire IMS, 351 cours de la libration, Bat A31, 33400 Talence, Bordeaux, France

Tel: (+33) 5 40 00 26 13, Fax: (+33) 5 56 37 15 45*e-mail: [email protected]

AbstractIn this paper, we investigate the monitoring of the

leakage current of supercapacitors during calendar ageing. First,

results of periodic characterization tests during calendar ageing

up to 5000 hours are presented. Then, a model of leakage current

is proposed and validated. Finally, the impact of both the

temperature and voltage is highlighted and quantified.

Keywords-Supercapacitor; calendar ageing; leakage current;

self-discharge.

I. INTRODUCTION

Electric Double-Layer Capacitors (EDLC), orsupercapacitors (SC), are based on the electrostatic storage ofthe energy at the interface between an electrolyte and a porouscarbon electrode [1], [2].

SCs are interesting storage components thanks to their highspecific power and efficiency, low-cost and relatively goodlifetime. Thus, they are well suitable for use as peak power

sources, for instance in Hybrid Electric Vehicles (HEV) or forthe pitch control of wind turbines.

However, one of the main challenges still remains in theunderstanding of their ageing mechanisms and thedevelopment of new characterization methods [3]. Moreoverthan the power cycling ageing mode, the SC performances interms of energy and power capabilities decreases also duringrest time (calendar mode). The decay of the performancesdepends on both the temperature and the voltage as highlightedin this paper.

In this work, we propose a method for investigatingcalendar ageing of supercapacitor based on the continuousmeasurement of the leakage current. In the first part, calendarageing experiments and characterization methods arepresented. Then, a model simulating the leakage current duringcalendar ageing is developed and presented. Furthermore, weinvestigate temperature and voltage impacts on the leakagecurrent during ageing.

II.

CALENDAR AGEING AND CHARACTERIZATION RESULTS

A. Experimental Setup

Supercapacitors are aged at constant voltage andtemperature. During calendar ageing, SCs are placed inside a

climatic chamber in order to control the ambient temperature.In this study, SCs have been tested with three differenttemperatures (40, 50 and 60C) and three voltage values (2.5V,2.7V and 2.8 V). The temperature and voltage values havebeen selected in order to accelerate the ageing [4]. Thedynamic behavior of a SC is strongly related to the mobility ofions the electrolyte used and the porosity effects of the porouselectrodes [5]. The operating voltage of the SC is limited by thestability range of the electrolyte. In our case, the tested cellshave a rated operating voltage of 2.7V.

In this test, the stored energy of the SC is sustained bymaintaining the voltage at a constant value. During the wholeexperiment, a dedicated floating test bench is able to accurately

measure the resulting leakage current, which is supposed to bestrongly linked to the SC ageing [6].

B. Characterization Method

In order to quantify the evolution of the SC performances interms of internal resistance and capacitance, characterizationtests are made periodically both in frequency (AC) and time-domains (DC). In the case of AC characterization, anelectrochemical workstation is used for ElectrochemicalImpedance Spectroscopy (EIS) tests in a wide frequency range,from 10 mHz to 10kHz. For DC characterization, a methodbased on the SC charging and discharging at constant currentaccording to the IEC62576 standard is used to determine the

Equivalent Series Resistance (ESR) and capacitance [7].

C. First Results of Calendar Ageing Tests

In this part, we focused only on the evolution of thecapacitance with ageing calculated from the imaginary part ofthe cell impedance at 10 mHz. The impact of the voltage and ofthe temperature on the capacitance during calendar ageing upto about 5000h is presented in Fig. 1a and Fig. 1b respectively.Fig.1. (a) shows that at fixed temperature, respectively 50C,the voltage does not affect the ageing beginning (before 1500h).

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However, after 1800 hours the voltage influence becomesmore visible. Fig.1. (b) illustrates the high impact of thetemperature on ageing at a constant voltage of 2.7V. Thus, a10C temperature rise affects significantly the capacitance loss.

0 1000 2000 3000 400082

84

86

88

90

92

94

96

98

100

Ageing time (h)

Normalizedcapacitance

(%)

2.5V

2.7V

2.8V

(a)

0 1000 2000 3000 400082

84

86

88

90

92

94

96

98

100

Ageing time (h)

Normalizedcapacitance(%)

40C

50C

60C

(b)

Figure 1. Temperature and voltage impact on calendar ageing ofsupercapacitor.

III. INVESTIGATION OF LEAKAGE CURRENT DURINGAGEING

In order to study SCs calendar ageing, rather thancapacitance decline and resistance increase investigation fromcharacterization [8], the leakage current revealed to be animportant parameter for the study of ageing electrochemicalcells. In fact, this current is measured during all ageing test andwill indicate devices deficiency once its performancesdegrades.

In our work, we focused on the floating current whichcorresponds to the current absorbed by the SC in order tomaintain a constant voltage during ageing. This current is veryrepresentative of the leakage current. In effect, thequantification of measured current is very sensitive at thebeginning due to the coincidence of a leakage current evolving

rapidly with ageing and a compensation current of therelaxation phenomenon. Therefore, experiments on self-discharge and impacts of temperature and voltage on thisparameter are given below.

A. Ageing Voltage Effect on Leakage Current

The quantification of the leakage current is very sensitive atthe beginning. In effects, it is evolving rapidly to compensate

the relaxation phenomenon [9]. Nevertheless, at a moreadvanced stage of ageing, leakage current becomes morestable. This behavior is dependent on the device voltageageing. For example, we see in Fig. 2 that the leakage current isless than 1mA for 2.5V and 2mA for 2.7V after 1800 hours ofageing.

500 1000 1500 2000 2500 3000 3500 4000 4500 50000

10

20

30

40

50

Time (h)

Leackage

current(m

A)

2.5V

2.7V

2.8V

Figure 2. Ageing voltage effect on leakage current at 50C.

B. Leakage Current Proposed Model

In this section, we developed a model for Leakage currentsimulation during ageing. The model is represented by

equation (1),(1)

Physically, the use of square root of the time is related todiffusion phenomenon [10]. However, the exponential functionis employed to follow the rapid drop of the current. Fig. 3.Shows a comparison between the model simulation andexperimental results from SC ageing at 2.7 V and 50C.

500 1000 1500 2000 2500 3000 3500 4000 4500 50000

5

10

15

20

25

Time (h)

leackage

current

(m

A)

simulated

measured

Figure 3. Comparison between simulated and measured leakage current

during ageing at 2.7V and 50C.

1000 2000 3000 4000 50000

1

2

4

6

8

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The model has been validated on the three test voltage.Table. 1 represents model identified parameters from thesedifferent ageing voltages at 50C.

TABLE I. MODEL ESTIMATED-PARAMETERS FOR DIFFERENTVOLTAGES.

Parameters/Voltage A B C D

2.5V 7.922 -0.011 -0.028 2.2792.7V 18.391 -0.009 -0.036 3.277

2.8V 59.950 -0.018 -0.062 5.072

Parameters A and D confirmed the logic of the dependencyof the ageing on the device voltage. Actually, these parametersincreased with voltage increase.

C. Self-Discharge and Temperature Effects on Leakage

Current

In order to study thermal behavior of the leakage currentduring ageing, we begin this part with a short focus on

temperature effect on self-discharge. In fact, self-discharge ofSCs is a very slow process which cannot be well determined byimpedance measurements [11].

The progression of the open circuit voltage (OCV) after afull charge of the SC is analyzed for 48 hours, results show aquasi-exponential decrease with time and time constantdepends on several parameters, essentially temperature andcharging current [12]. Actually, to neglect current effect and inorder to process similarly as in floating conditions duringageing, we chose a low charging current namely, 2A. The SCOCV is presented in Fig. 4 at two temperatures. OCV plotsrevealed that device voltage changes significantly during thefirst hours and self-discharge is accelerated with increase in

temperature. Basically, we obtained a lower OCV with highertemperature after 48 hours of self-discharge test.

We observed a similar behavior when plotting the leakagecurrent from calendar ageing of a SC aged at 2.7V (see Fig.5).In fact, SC aged at 50C required a leakage current more thanthe SC aged at 40C.

0 10 20 30 40 501.8

2

2.2

2.4

2.6

2.8

Time (h)

V

oltage

(V

)

40C

50C

Figure 4. Self-discharge for SC at 2.7V and different temperatures.

0 1000 2000 3000 40000

5

10

15

Time (h)

Leakage

current(mA)

50C

40C

Figure 5. Temperature effect on leakage current for SC aged at 2.7V.

In order to study this similar behavior, we used the samemodel form described with (1) to simulate the self-discharge

phenomenon. Results are presented in Fig. 6 and showed thatthis model matched very well self-discharge behavior. Fromthe identified parameters, we see that the time constant of theexponential term is twice higher (-0.0185) than this of theleakage current.

0 10 20 30 40 502

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

Time (h)

Vo

ltage

(V)

simulated

measured

Figure 6. Comparison between simulated and measured voltage during self-discharge at 2.7V and 50C.

From this study, we will simulates an equivalent circuitmodel represented in Fig. 7 and developed here to model self-

discharge taking into account diffusion phenomenon [13]. Nextequations illustrate model relationships,

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Figure 7. Equivalent circuit model for SC at self-discharge.

The first branch which includes R1 and C1 is the mainbranch. The differential capacitance C1is the sum of a constantcapacitance C0and a voltage dependent capacitance k u,

(2)

The relation of three branch current can be described by:

(3)

(4)

The relation between current and voltage for the delayedbranch is:

(5)

(6)

This model has been validated on a SC charged until 2.7V.Results are presented in Fig. 8 and parameters have beenidentified for the test running under 50C.

0 10 20 30 40 502.2

2.3

2.4

2.5

2.6

2.7

Time(h)

Voltage(V)

measured

simulated

Figure 8. Model validation.

Temperature impact on SC leakage current

Fig.9 illustrates that a temperature increase on SC underageing test, conducted to a rise on leakage current. Thisjustifies the acceleration of self-discharge at high temperature.From this we confirm the significance of leakage current in SCbehavior monitoring. Similarly, if the temperature decreasesthe leakage current decreased also.

0 5 10 1535

40

45

50

55

Time (h)

Temperature(C)

0 5 10 1535

40

45

50

55

Time (h)

Temperature(C)

0 5 10 155

10

15

20

Time (h)

Current(mA)

0 5 10 150

5

10

Time (h)

Current(mA)

Figure 9. Variation of leakage current after a slow variation of temperature.

When supercapacitor reaches end of life, ESR resistancewill increase rapidly and so there is probably a drop on thedevice voltage. Hence, leakage current will behaves differently.Therefore, leakage current could represent a real-timeindication on device health.

IV.

CONCLUSIONS

In this work, we studied SCs calendar ageing. Experimentalresults have been highlighted. We were interested in the studyof leakage current as a source for calendar ageing monitoring.Basically, a focus on the factors influencing the leakage currentduring ageing have been represented, mainly voltage andtemperature impact. Moreover, a model simulating its behaviorthrough ageing has been developed.

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2013_Voltage and Temperature Impacts on Leakage Current in Calendar Ageing of Supercapacitors

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