A Comparative Study of Different Fast Charging methodologies for Lithium-Ion Batteries Based on Aging Process

EVS28 KINTEX, Korea, May 3-6, 2015 A Comparative Study of Different Fast Charging methodologies for Lithium-Ion Batteries Based on Aging Process Mohamed Abdel Monem 1,2, Khiem Trad 2, Noshin Omar 1, Omar Hegazy 1, Bart Mantels 2, Peter Van den Bossche 1 and Joeri Van Mierlo 1 1 Vrije Universiteit Brussel, Pleinlaan 2, Brussels, 1050, Belgium 2 VITO, Unit of Energy Technology, Boeretang 200, Mol 2400, Belgium E-mail corresponding author: mohamed.abdel.monem@vub.ac.be Abstract The charging profile and the quality of the charging system play a key role in the lifetime and the reliability of the battery. In this research, high power 7 Ah LiFePO 4-based cells (LFP) have been used to investigate the impact of the fast charging technique on the battery s lifetime and the charging time. The charging time is an important factor to save time and cost during the battery operation. Four charging techniques have been used: Constant Current - Constant Voltage (CC-CV), Constant Current-Constant Voltage with Negative Pulse (CC-CVNP), Multistage Constant Current-Constant Voltage (MCC-CV) and Multistage Constant Current-Constant Voltage with Negative Pulse (MCC-CVNP). A comparative study between these techniques based on aging process is presented in this study. As expected the obtained results showed that the battery s aging rate and charging time depend on the charging profile. It has been shown that a CC- CVNP has a positive effect on battery s capacity fading compared to the other fast charging techniques. This research has provided an extended analysis to select the proper charging method that can be used to design an enhanced charging system for lithium-ion batteries. Keywords: Lithium-ion Batteries, Negative Pulse fast charging technique, Multistage fast charging technique, Capacity Fade 1 Introduction Recently, the use of the lithium ion batteries has been significantly increased, especially for portable electronic, electric vehicle and grid applications. As known, the lithium iron phosphate battery (LiFePO 4), also called LFP battery, consists of LiFePO 4 as a positive electrode material and typically graphite as a negative electrode [1]. Several features make LiFePO 4 material attractive for use in lithium ion batteries such as intrinsic safety, low toxicity, high cycle-lifetime, high power capability, reliability, large availability of materials, low cost and flat voltage profile [1] [3]. Many factors affect the lifetime of a rechargeable battery and the charging methodology plays an effective role in maintaining battery performance and lifespan [4]. The constant current constant voltage (CC-CV) charging method is widely used in commercial chargers for lithium-ion batteries [5], [6]. In case of CC-CV, the battery is charged by using a constant current (CC) until the battery terminal voltage reaches a EVS28 International Electric Vehicle Symposium and Exhibition 1

pre-defined value (e.g. 3.65 V), and then a constant voltage (CV) is used until the charging current is gradually decreasing to the pre-defined value (e.g. 0.01I t). I t (A) is the reference test current, which is expressed as [C n (Ah)/1(h)], where C n is the rated capacity of the cell and n is the time base (h). Many attempts have been proposed to investigate the impact of the charging profile on the battery performance and lifespan. According to [7], the impact of the charging profile on the aging mechanism of the commercial 18650 Lithium-ion battery cells has been addressed by using three charging profiles: constant current (CC) charging, constant power (CP) charging and multistage constant current (MCC) charging. The results show that fast charging accelerates capacity fading and decreases the cycle life of the battery, they then suggest a charging profile which uses low current to charge the initial 10% capacity and near the end of the charge. In [8], a multistage fast charging, method has been used for a high power LFP batteries to reduce the charging time and increase the cycle life. Another charging profile is proposed in [9], where the authors present the effect of charging and discharging pulses on the cycling behavior of commercial lithium-ion batteries. The results reveal that the charging profile, which involves pulse charging with short relaxation periods and short discharge, is helpful in reducing the charging time, increasing cycle life and eliminating the concentration polarization. However, still more investigations should be done to select the optimal charging technique for lithium-ion batteries. This paper presents the results obtained by aging 8 commercial 7Ah LFP batteries with different charging profiles (CC-CV, CC-CVNP, MCC-CV and MCC-CVNP). The capacity fading, the charging time and the thermal behavior will be discussed. The paper is organized as follows. Fast charging profiles are described in section 2, while section 3 presents the results and discussion. Section 4 shows the conclusion. 2 Fast charging profiles Eight commercial lithium ion EIG 7Ah high power cells have been used in this study. The cells have a pouch format with LiFePO 4-based cathode and Carbon-based anode. Table 1 shows the electrical parameters of the EIG cells. According to the manufacturer, these cells are suitable for use in hybrid electric vehicles (HEV) and can be charged and discharged by using the high current rate (20I t) [10]. Before cycling, all the cells have been subjected to the preconditioning test which consists of 4 standard cycles within the manufacturer recommendations [11]. The life cycle test consists of a constant current discharge at 5 current rate (35A) until the cut-off voltage (2 V) followed by 15min rest time. Then the battery is charged according to the charging profiles that will be described below. The tests have been carried out at ~ 25 C by using a PEC battery tester. The charging profiles can be summarized in 8 cases as follows: Case 1: CC-CV fast charging profile applied at 4I t; Case 2: CC-CVNP fast charging profile applied by using 40 charging pulses at 4I t followed by a negative pulse at 2I t; Case 3: CC-CVNP fast charging profile applied by using 20 charging pulses at 4I t followed by a negative pulse at 2I t; Case 4: MCC-CV fast charging profile applied by using two CC charging stages at 1I t and 0.5I t; Case 5: MCC-CV fast charging profile applied by using three CC charging stages at 2I t, 1I t and 0.5I t; Case 6: MCC-CV fast charging profile applied by using four CC charging stages at 3I t, 2I t, 1I t and 0.5I t; Case 7: MCC-CV fast charging profile applied by using four CC charging stages at 4I t, 3I t, 2I t and 1I t; Case 8: MCC-CVNP fast charging profile applied by using four CC charging stages at 4I t, 3I t, 2I t, 1I t and one negative pulse at 2I t after each charging stage. In all cases, the constant voltage charge step ends when the current decreases to 0.01I t. The life cycle test is interrupted at regular intervals (each 200 cycles) to conduct a reference performance test (RPT) to measure changes in the electrical performance of the cells (see Figure 1). The RPT consists of a capacity test based on CC- CV technique for charging and CC technique for discharging (1I t charge and 0.5I t, 1I t, 2I t, 4I t and 5I t discharge). In this paper only the 1I t discharge capacity will be discussed. EVS28 International Electric Vehicle Symposium and Exhibition 2

Figure 1: Flow chart of the battery testing procedure Table 1: Electrical parameters of EIG cell [10] Nominal Voltage Nominal Capacity Energy Density Power Density (DOD50%, 10sec) 3.2 V 7 Ah 180 Wh/L 4700 W/L 3 Results and discussion In this study, the discharge capacity, the charging time and the thermal behavior, have been used to follow-up the batteries ageing and to compare between the fast charging techniques. The initial discharge capacity of all used batteries has been measured to facilitate the comparison between the cells. Figure 2 shows the discharge capacity retention of the lithium ion batteries charged by the eight charging profiles as a function of the number of cycles. After 1500 cycles, case 1 (CC- CV) shows the most important capacity decrease (24%) compared to the other fast charging cases (between 11% and 16%), which uses either multistage CC or negative pulse during the charging process. The 7 other cases show a similar capacity decrease (~16%) with a difference less than 5%. The obtained results of CC-CVNP fast charging technique (case 2 and case 3) revealed a slightly higher remaining capacity (89%) compared to the other techniques. Furthermore, the impact of the MCC-CV fast charging techniques such as case 4, case 5, case 6 and case 7 - on the capacity fading of the lithium-ion batteries are almost the same (~13%) except case 8 (~16%). As a result, the fast charging technique by using variable constant current levels such as CC charging with negative pulse or multistage CC without negative pulse is helpful to reduce the capacity fading of a lithium ion battery. The charging time was also investigated as it is an important factor to save time and cost during the battery operation. The evolution of the charging capacity and the charging time of the different fast charging cases is shown in Figure 3 and Figure 4. Compared to the standard 1I t charging method recommended by the manufacturer (CC-CV technique), the fast charging techniques studied here show of a decrease of the charging time (~ 40%) except case 4 and case 5 (see Table 2). In order to reduce charging time with MCC-CV fast charging profile, the first constant current stage should be selected 3I t. In Figure 3, the results reveal that the CC-CV fast charging technique (case 1) achieves the shortest charging time (~27min), compared to the other fast charging techniques. However, after 1500 cycles, one can observe that the CC-CV fast charging technique leads to higher charging capacity fading (~29% of initial charging capacity) compared to the other fast charging techniques (~12% -15% of initial charging capacity) as shown in Figure 4. From case 2 to case 8, although the charging capacity fading of these cases is almost the same, the charging time of the CC-CVNP fast charging technique (case 2 and case 3) is lower than the other fast charging techniques as shown in Figure 3 and Figure 4. EVS28 International Electric Vehicle Symposium and Exhibition 3

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Table 2: Percentage of reduction in charging time as a comparison between fast charging techniques and manufacturer s recommended standard charge at 1I t. Cases Charging time reduced/increased by Case 1 +65% Case 2 +47% Case 3 +48% Case 4-35% Case 5-8% Case 6 +21 Case 7 +32 Case 8 +25 + ve: Decreased - ve: Increased On the other hand, the temperature at the surface of each cell has been measured to investigate the impact of the fast charging technique on the battery s temperature. Figure 5 shows the cell temperature evolution during the first cycle. Case 1, case 2 and case 3 recorded a higher temperature rise of around 4.5 C compared to the other cases. However, in all fast charging techniques, the temperature measurements revealed that the temperature of all cells was almost stable during charging and therefore no significant impact of the temperature rise on the aging rate of the cells. 4 Conclusion Different fast charging techniques for a high power LFP battery cells have been proposed in this study. Three factors, charging time, discharge capacity retention and temperature rise, have been used to evaluate the impact of the fast charging technique on the performance of lithium-ion battery cells. An extended cycle life test was carried out to study degradation effects. After a total of 1500 cycles, the results reveal that: 1. Using variable CC stages during charging - such as CC-CVNP or MCC-CV fast charging techniques - is helpful to decrease the charging time and to increase the cycle life of the lithium ion battery; 2. The CC-CVNP fast charging technique might be a good solution to get a higher discharge capacity retention and shorter charging time compared to the other cases; 3. For the MCC-CV fast charging technique without negative pulse, there is no significant effect of the number of CC stages on the capacity fading of the lithium-ion battery cells, but the charging time decreases with increasing both of the number of CC stages and the current rate; 4. For the MCC-CVNP fast charging technique, the negative pulse with MCC showed a negative impact on the charging time and the capacity fading. EVS28 International Electric Vehicle Symposium and Exhibition 5

Acknowledgments We acknowledge Flanders Make for the support to our team. We also acknowledge Vito for the support to this project. References [1] D. Anseán, M. González, J. C. Viera, J. C. ÁlvarC. Blanco, and V. M. García, Evaluation of LiFePO4 batteries for Electric Vehicle applications, New Concepts Smart Cities Foster. Public Priv. Alliances (SmartMILE), 2013 Int. Conf., 2013. [2] W. J. Zhang, Structure and performance of LiFePO4 cathode materials: A review, J. Power Sources, vol. 196, no. 6, pp. 2962 2970, 2011. [3] D. Jugović and D. Uskoković, A review of recent developments in the synthesis procedures of lithium iron phosphate powders, J. Power Sources, vol. 190, pp. 538 544, 2009. [4] A. Al-Haj Hussein and I. Batarseh, A review of charging algorithms for nickel and lithium battery chargers, IEEE Trans. Veh. Technol., vol. 60, no. 3, pp. 830 838, 2011. [5] R. C. Cope and Y. Podrazhansky, The art of battery charging, Fourteenth Annu. Batter. Conf. Appl. Adv. Proc. Conf. (Cat. No.99TH8371), pp. 233 235, 1999. [6] N. Omar, M. Daowd, P. van den Bossche, O. Hegazy, J. Smekens, T. Coosemans, and J. van Mierlo, Rechargeable energy storage systems for plug-in hybrid electric vehicles-assessment of electrical characteristics, Energies, vol. 5, pp. 2952 2988, 2012. [7] S. S. Zhang, The effect of the charging protocol on the cycle life of a Li-ion battery, J. Power Sources, vol. 161, no. April, pp. 1385 1391, 2006. [8] D. Anseán, M. González, J. C. Viera, V. M. García, C. Blanco, and M. Valledor, Fast charging technique for high power lithium iron phosphate batteries: A cycle life analysis, J. Power Sources, vol. 239, pp. 9 15, 2013. [9] J. Li, E. Murphy, J. Winnick, and P. a. Kohl, The effects of pulse charging on cycling characteristics of commercial lithium-ion batteries, J. Power Sources, vol. 102, pp. 302 309, 2001. [10] EIG Batteries. Available online: http://www.eigbattery.com/_eng/designer/skin/03/01.asp (accessed on 09 October 2014).pdf.. [11] N. Omar, M. Daowd, O. Hegaz, G. Mulder, J. M. Timmermans, T. Coosemans, P. Van den Bossche, and J. Van Mierlo, Standardization work for BEV and HEV applications: Critical appraisal of recent traction battery documents, Energies, vol. 5, pp. 138 156, 2012. Authors Ir. Mohamed Abdel Monem received the B.Sc. (excellent with honors) and M.Sc. degrees in Electrical Engineering from Helwan University, Cairo, Egypt. He is currently working toward the Ph.D. degree in the Department of Electrical Engineering and Energy Technology (ETEC), Vrije Universiteit Brussel (VUB), Belgium. His current research interests include Second-Life Batteries, Battery Characterization, Systems Modelling, Parameter Estimation, Power Electronics, Renewable Energy, Control Systems and Battery Management System. Dr. Khiem Trad has received her PhD in materials science from the university of Bordeaux1 (France) and the ICMCB (France) in 2010, where she worked on new phosphate materials as positive electrode in lithium and sodium ion batteries. She joined Vito in 2014 and works as a researcher within the unit Energy Technology. She worked previously as a researcher and project leader in the automotive domain where she was responsible of cells/battery packs testing (including ageing and safety tests) and designing. Prof. Dr. Eng. Noshin Omar was born in Kurdistan, in 1982. He obtained the M.S. degree in Electronics and Mechanics from Erasmus University College Brussels. He is currently the head of Battery Innovation Center of MOBI research group at Vrije Universiteit Brussel, Belgium. His research interests include applications of electrical double-later capacitors and batteries in BEV s, HEV s and PHEV s. He is also active in several international standardization committees such as IEC TC21/22.He is the author of more than 70 scientific publications. EVS28 International Electric Vehicle Symposium and Exhibition 6

Dr. Ir. Omar Hegazy received the B.Sc. (with honors) and M.Sc. degrees in Electrical Engineering from Helwan University, Cairo, Egypt. Recently, he obtained his PhD degree in July 2012 (with the greatest distinction) at the Dept. of Electrical Machines and Power Engineering (ETEC) in Vrije Universiteit Brussel (VUB), Belgium. He is an assistant professor (on leave) at Helwan University, Faculty of Engineering. Currently, Dr. Hegazy is a Postdoctoral Fellow at ETEC and MOBI team at VUB. He is the author of more than 55 scientific publications. Furthermore, he is member of IEEE and IEC standards. His current research interests include power electronics, drive systems, electric vehicles, (plug-in) hybrid electric vehicles, battery management systems (BMS), power management strategies, control and optimization techniques. committees such as IEC TC69, of which he is Secretary, and ISO TC22 SC21 Prof. Dr. ir. Joeri Van Mierlo is a full-time professor at the Vrije Universiteit Brussel, where he leads the MOBI Mobility, Logistics and automotive technology research centre (http://mobi.vub.ac.be). A multidisciplinary and growing team of 60 staff members. He is expert in the field of Electric and Hybrid vehicles (batteries, power converters, energy management simulations) as well as to the environmental and economical comparison of vehicles with different drive trains and fuels (LCA, TCO). Bart Mantels is a project responsible at the Flemish technological institute VITO in the unit Energy Technology. He is in charge of the domain of electric active components for grid governance, closely related to electric energy storage and power electronics. He graduated in Computer Science (Katholieke Universiteit Leuven, 1996). He worked in several companies as team manager before joining the smart grid activities of VITO in 2012. He is qualified for the Six sigma design and also a certified marketing manager. Prof. Dr. ir. Peter Van den Bossche graduated as civil mechanical - electrotechnical engineer from the Vrije Universiteit Brussel and defended his PhD at the same institution with the thesis "The Electric Vehicle: raising the standards". He is currently lecturer at the engineering faculties of the Vrije Universiteit Brussel, and in charge of coordinating research and demonstration projects for electric vehicles in collaboration with the international associations CITELEC and AVERE. His main research interest is electric vehicle standardization, in which quality he is involved in international standards EVS28 International Electric Vehicle Symposium and Exhibition 7