Efficient fast-charging strategies for Li-ion batteries

EVS28 KINTEX, Korea, May 3-6, 215 Efficient fast-charging strategies for Li-ion batteries D. Anseán a, V.M. García b, M. González a, J.C. Viera a, J.C. Antón a, C. Blanco a a Department of Electrical Engineering, University of Oviedo, Gijón, Spain. b Department of Physical and Analytical Chemistr, University of Oviedo, Gijón, Spain. e-mail: anseandavid.uo@uniovi.es Abstract Reducing the charging time in electric vehicles (EVs) is considered a crucial factor to promote consumer interest and to increase its marketability. However, fast charging is not tolerated by every lithium ion battery (LIB) technology, because it may produce premature aging. As a result, fast charging LIB is an increasingly important area of study. In this work, we propose a set of general strategies to attain efficient fast charge in LIB systems. This is achieved by analyzing LIB architecture designs that best accommodate for fast charge, coupled with the analysis of the most relevant charging techniques found in the literature. To evaluate the proposed charging strategies, a fast charging technique is designed. The results show that the charging technique, when applied to the selected high power lithium iron phosphate (LFP) battery permits a full recharging in approximately 2 min. Aging studies are carried out, comparing fast charging with standard testing. We found that both cells degraded similarly in terms of capacity fade, power capabilities and structural changes. These results illustrate the applicability of the proposed general charging strategies. Keywords: fast charging, lithium-ion batteries, high power batteries, charging protocols, battery aging. 1 Introduction Fast charging lithium ion batteries (LIB) has considerable advantages to the EV automotive industry, as nowadays virtually every modern EV is powered with LIB technology. Unfortunately, fast charging may affect the LIB s performance by accelerating its aging. This is because fast charging typically involves high current rates and high temperatures, namely the main factors of LIB degradation [1]. A prematurely aged LIB has negative effects on an EV, as its range and acceleration are reduced. Hence, to allow EVs to be charged quickly without reducing their performance, it becomes necessary to analyze the technical factors to attain fast charging without adding further aging to the LIB system. To achieve fast charging, the main technical factors include, (1) the utilization of an adequate LIB technology with an architecture design optimized for fast charging, and (2), the use of an appropriate fast charging protocol. In fact, the two technical factors are interrelated, as different LIB technologies have different voltage and current limitations [2], which play a key role in the design of a proper fast charging technique. Hence, both LIB technology and protocol design shall be simultaneously analyzed to achieve effective fast charge. Similarly, since EV automakers select their own LIB technology of choice, a unique protocol to achieve fast charging in every LIB system is unattainable. However, a universal strategy to develop an efficient, reliable and simple fast EVS28 International Electric Vehicle Symposium and Exhibition 1

charging technique for its applicability in any specific LIB systems is highly desired. In this work we address the important subject of fast charging. We start by analyzing the LIB architecture designs that best accommodate for fast charge. Secondly, we analyze some of the most relevant charging techniques proposed in the literature, to understand the key parameters needed to develop reliable charging processes. Thereafter, based on the previous analyses a general charging strategy is presented. The aim of the strategy is to provide a simple, general step by step guideline to design fast charging techniques that can be adapted to any specific LIB system. To validate the applicability of the proposed analyses, a practical example is provided in the last section of this work. A fast charging technique is applied to a high power lithium iron phosphate (LFP) battery optimized for fast charging. To evaluate if the fast charging protocol accelerates the aging of the cell, we compared it with a standard, 1C constant current constant voltage (i.e. CC CV) charging procedure. The main EV performance parameters, such as capacity evolution and power capabilities with cycling are compared. Additional comparisons to decipher structural degradation are also carried out, using the incremental capacity (IC) analysis. 2 Li-ion Battery Fast Charging Fundamentals Nowadays, the United States Advanced Battery Consortium (USABC) is the only organization that proposes a fast charging test oriented towards EV applications. The USABC goal of fast charging (for batteries capable of such usage) is defined as when a battery is able to return 4% of the SOC within 15 min [3]. This is approximately a charging rate of ~2C. However, the USABC does not include specifications on fast charge aging, safety, or protocol development, to name a few. Still, a good fast charging process shall be quick and simple, adding the minimal detrimental effects to the battery. Here we present the fundamentals of fast charging. For a comprehensive analysis, readers may refer to [4]. 2.1 Li-ion battery architecture design for fast-charging The LIB architecture design plays a significant role to achieve fast charging. The key factor to achieve high power performance in LIBs is to decrease the polarization resistances, so energy can rapidly be extracted. From the different electrode materials and battery configurations of choice, high power (HP) batteries accommodate best for fast charging. The choice of the HP battery considers the design of the cell from an engineering and basic science level. Fundamental engineering considerations include the design of cylindrical cells with large electrode area and reduced thickness of the electrodes [5], an even tab distribution [6], and sufficient amount and area of active material on the negative electrode to avoid Li plating. From a basic science design perspective, high ionic conductivity and reduced charge transfer potentials can optimize the performance of HP cells [7]. Additional factors must be considered, namely the use of materials optimized for high rate operation, microstructural cell design, the use of nanomaterials in the electrodes, and/or the use of conductive additives [7] [9]. Active material chemistries with high power properties such as fast kinetics and low resistivity are ideal for its use in HP cell designs. In some cases, even active materials (such as LFP) with low electronic conductivity can take advantage of the use of nanomaterial and microstructure techniques to remarkably enhance its conductive properties [9]. Nowadays, high power LIB technologies that permit fast charging are available on the market. For example, nanophosphates with coating layers show good overall performance in terms of intrinsic safety, high cycle-life time, high-power capability, flat voltage profile, reliability and low toxicity [1], [11]. The main drawback is the relatively low specific energy. Similarly, high power anode based titanium batteries (LTO) are commercially available, and show excellent charging capabilities (up to 6C) [12] and long cycle life time (over 5, cycles). Still, the main drawbacks of LTO are low specific energy and high cost, which are two major impediments for its applicability on EVs. In the research field, efforts in nanostructured materials are showing promising results on high power, laboratory-scaled LIBs [8]. 2.2 Li-ion battery fast charging techniques To achieve effective fast charging, the selected battery architecture has to be complemented with the use of an optimum charging protocol. The charging protocol should aim to shorten the charging time, improve charging efficiency, minimize any performance attenuation and sustain a safe operation of the tested battery. This section EVS28 International Electric Vehicle Symposium and Exhibition 2

provides an overview of the most commonly used charging techniques in LIBs. 2.2.1 Constant current-constant voltage method (CC-CV) The most common process to charge LIBs is through a consecutive two-step constant currentconstant voltage process (i.e. CC CV), as seen in Fig.1 The current rate of the CC stage and the predefined cut-off voltage strongly depend on the LIB chemistry; these parameters are specified by the battery manufacturer to operate the battery under standard charging conditions. Similarly, most LIB manufacturers recommend a current below C/2 in the CV stage to reach full charge. Since the CC CV method is easy to implement, performs efficient charges and is reliable at mid to low charging rates, it has often been denoted as the standard charging method for LIB batteries. CC CV Figure 1: Charging CC CV protocol at.5c (currentblue, cell voltage-red and state of charge in green). Adapted from [13] Despite its advantages, one major drawback of the CC CV method is that it takes long time to fully charge the battery. Charging times of over 2 hours are common using the CC CV method [14], [13]. The CV stage prolongs the charging time, as current decays, but is necessary to prevent overcharging, achieve full charge and reduce temperatures. To reduce the charging time, high charging rates are required. However, high currents decrease the charging efficiency due to Joule heating, and may cause Li deposition at the end of the CC stage [13]. Additionally, increasing the charging rate above a certain level does not reduce the charging time. This is because the increase of polarization reduces the charged capacity during the CC stage, which is the quickest charging stage (see SOC evolution in Fig. 1). 2.2.2 Constant voltage method (CV) The constant voltage (CV) charging method was developed as the simplest strategy to reduce the charging time of the CC CV standard method. The CV method starts charging the battery directly at CV, to terminate the charging process when the current drops below a certain level. Regardless of its simplicity, the main disadvantage is that demands very high initial charging currents. Moreover, charging continuously at the cut off voltage is highly detrimental to the battery due to electrolyte oxidation [1]. Therefore, CV method rapidly decreases the life of the battery [15]. Limiting the charging current and charging at the pre-determined cut-off voltage may help to reduce both the charging time and aging effects [14]. Still, this charging protocol is not recommended to be carried out on a regular basis. 2.2.3 Multistage charging method (MCM) The multistage charging method (MCM) proposes different charging stages, with stepwise descending and/or ascending currents, taking into account characteristics of the selected battery. Ideally, parameters such as the internal resistance (IR) evolution, temperature evolution and current rate dependencies shall be considered to design an optimal MCM. Similarly, currents over the maximum rates or voltages out of the stablished limits cannot be exceeded. Situations which lead to premature capacity fade must be avoided, mainly large currents at high SOCs which may lead to Li plating [13]. To avoid electrolyte oxidation, long time CV stages at high SOCs shall be reduced. Several MCM strategies have been proposed [13], [14]. Recently, a new multistage charging strategy was proposed by Vo et al. [16]. The battery technology selected was lithium manganese oxides (LMO) that permits maximum charging rates of 2C. The proposed strategy is based on a four stage constant current (FSCC) pattern, where each charging stage is set to 25% SOC (see Fig. 2). A mathematical approach was developed to select the optimal current rates and to estimate the SOC of the cell to set the boundaries of the steps. The results shows that the FSCC has better performance than to standard CCCV, with a reduction of charging time of ~2%, and a slightly higher energy efficiency (1.5%). EVS28 International Electric Vehicle Symposium and Exhibition 3

Figure 2: Four stage constant current (FSCC) charging methods [16] To sum up, the MCM has important advantages to achieve both healthy and quick recharges, when is properly designed. Maximum charging rates and IR evolution with the SOC are key parameters in the design. Similarly, the current rate and length of the charging steps has to be determined through either experimental approaches [13], [14] or mathematical modeling [17]. 2.2.4 Varying current method (VCM) The varying current method (VCM) consists in continuously modifying the charging current as the battery is being charged, based on several assumptions to optimize the charging process, including the lithium concentration levels in the electrolyte and impedance evolution. Proper design is required to avoid risk of overvoltage [15] and/or the long charging periods within the cut-off voltage value, which lead to premature capacity fade and impedance increase. Recently, Guo et al. [18] proposed a strategy based on impedance evolution with SOC, coupled with a current decaying profile to accommodate the polarization variations with SOC (see Fig. 3). The charging parameters were derived from equivalent circuit modelling and algorithm optimization, showing better charging efficiency and cycle life than standard charging. In this type of methods, the main problem is that complex charging profiles are difficult to implement in practical chargers. Advanced techniques can be used to improve charging capabilities. Battery chargers may include on-line IR and SOC calculations to evaluate the SOH, and automatically modify the charging profiles as the cell ages. However, this adaptive method can be achieved at the expenses of adding complexity to the LIB system, and increasing previous research time and experimental resources. Figure 3: Charging current as a function of time using a modified varying current method [18] 2.2.5 Pulse charging The pulse charging method consists in using short rest periods in the charging process, which can be accompanied by short discharge pulses. This method is intended to eliminate concentration polarization at the electrode/electrolyte interface, to reduce the risk of Li deposition while fast charging [19]. Due to its potential advantages, several experiments have been carried out [2]. It was found that the pulse charging method only induces better results in some cases, depending on the battery technology and the profile of charging pulse. Hence, although this method can reduce charging time and energy losses, improvements can be only achieved by trial-and-error or by empirical methodologies, which may not always provide the expected results and can be time consuming and complex. 2.3 Fast-Charge Strategy As shown in previous section, not every charging protocol with the selected LIB success to meet the aims of optimal fast charging. Some techniques shortened the charging time, but failed to maintain safe operation. Here is presented a set of key points to achieve fast charging, based on the analysis of both LIB architecture designs and fast charging techniques: Battery selection: High power (HP) designed batteries accommodate best for fast charging. Various high power cell technologies are available on the market, including LFP and LTO. Similarly, targets to achieve high energy density may limit the power capabilities, as both high energy and high power battery systems compete with each other. In that regard, system designer shall consider which option is optimal for the final application. EVS28 International Electric Vehicle Symposium and Exhibition 4

Internal resistance (mω) Battery evaluation tests: this stage is needed to provide experimental results on the selected cells and evaluate its charging capabilities under the expected temperature working conditions. The tests shall include charges under several current rates, ranging from nominal to maximum. Similarly, the evolution of battery temperature and voltage with SOC has to be studied. Temperature, voltage and current charging limits must be established and never exceeded. Other parameters such as charging time, charging capability and energy efficiency have to be evaluated. The measurement of the IR as a function of the SOC plays a key role in the design of the protocol: increase charging C-rates at lower IR values, and decrease the C-rates at higher IR values. This strategy aims to reduce Joule heating, hence providing better charging efficiency in the process. Multistage charging method (MCM) development: The MCM charging method is considered as a high-quality charging pattern with the advantages of long cycle life, high energy efficiency and short charging time. The VCM technique has similar strengths and sometimes similar current profiles, but previous research is more complex and the implementation in battery chargers can be difficult. The rest of the charging methods did not show many advantages. Hence, a fast charging technique based on the MCM is selected. The combination of different levels of charging currents is adopted to achieve optimal fast-charge: high currents at low to mid SOCs, and lower currents at high SOCs to reduce risk of Li plating. The combination of current steps also helps to reduce the cell s internal temperature. An end CV stage help to prevent overcharging and reduce cell temperature, but have to be limited in time to decrease risk of accelerated aging. Additional considerations: extreme working temperatures are critical when designing fast charging techniques, as accelerate battery aging. Hence, the necessity of using a thermal management system (TMS) to keep the battery operating at safe temperatures under extreme conditions is required. 3 Fast Charging Technique Implementation Here is presented a practical example to implement a fast charging technique. A complete analysis of the charging technique and the experimental conditions can be found in our previous work [21]. Here however are highlighted and further analyzed the main findings regarding the charging technique development. Following the charging strategies, the selected battery was a HP, nanophosphate based LFP cell. Material enhancement in these cells considerably improves their conductivity and rate capability in comparison to standard LFP cells [11]. According to the manufacturer (A123 Systems), these cells allow fast charging and high discharging rates, and they are suitable for portable high power devices, commercial trucks and bus hybrid electric vehicles (HEVs) [22]. The datasheet shows that these cells weigh an average of 7 g and have a capacity of 2.3 Ah. The selected cell was subsequently tested. Under nominal charge/discharge conditions (1C), the cell exhibited a capacity of 2.27 Ah, energy density of 98.8 Wh/kg and energy efficiency of 94%. The cell achieved maximum charges up to 1 A (4.3C), as stated by the manufacturer. Similarly, the voltage limits were 3.6 V and 2 V for the end of charge and discharge, respectively. To develop the charging strategy, we carried out IR measurements. Fig. 4 shows the results of the IR evolution versus the SOC for charging at 4C. The IR reaches its minimum immediately after starting the charging process. As the cell reaches approximately 1% SOC, the internal resistance remains practically constant at 15-16 mω until 7% SOC is achieved, after which the IR value increases rapidly. This IR behavior has been reported for LFP power cells from the same manufacturer [23]. 3 25 2 15 1 5 % 2% 4% 6% 8% 1% SOC (%) Figure 4: Internal resistance evolution profile versus the state of charge of the tested cell at 4C charging current Following the fast charge strategy proposed, a MCM technique was developed. The charging process is split into three different stages, referred as CC-I, CC-II and CV-I (see Fig. 5a-d). The first EVS28 International Electric Vehicle Symposium and Exhibition 5

Cell temperature (ºC) Normalized charged capacity (%) Cell voltage (V) State of charge (%) Cell current (A) 1 8 6 4 2 1 5 1 15 2 25 8 6 4 2 5 1 15 2 25 4, 3,5 3, 2,5 5 1 15 2 25 27 26 25 24 23 CC-I CC-II CV-I 22 5 1 15 2 25 Time (min) Figure 5: (a) current, (b) state of charge, (c) voltage and (d) temperature profiles for the proposed fast charging technique stage (CC-I) starts with a constant current at 4C to the charging cut-off voltage (3.6 V). The second stage (CC-II) is a constant current charge at 1C. Since the current in CC-II is lower than in CC-I, the cell voltage drops below 3.6 V (see Fig. 5b) allowing the charge to be extended, until the cell reaches again the charging cut-off voltage. The last stage (CV-I) is performed at a constant voltage of 3.6 V for a duration of 5 min. The SOC is shown in Fig. 5c; it increases linearly as the cell is charged at constant current and asymptotically during the constant voltage stage. Fig. 5d shows the cell temperature evolution. The temperature is reminiscent of a symmetric bell curve over time: it starts at an ambient temperature of 23ºC, increases by 2ºC to its maximum at approximately half the charging time, and ends back at the ambient temperature. The multistage fast charging technique is based on the evolution of the IR during the charging sequence. The highest charging current (4C) is applied when the cell s IR is at the lower values; a) b) c) d) this charging rate was selected because it is close to the maximum recommended by the manufacturer (the security margin reduces the likelihood of overheating). The last two stages of the charging process are applied as the cell s IR increases rapidly; this procedure helps to decrease the cell s temperature to its initial value (see Fig. 5d). The last stage (CV-I) is necessary to achieve full recharge, but it is only applied 5 min., avoiding an acceleration of aging process. As seen in Fig. 6, recharging can be performed very quickly with the proposed fast charging technique. The cell is charged from % to 4% of its capacity in approximately 6 min and fully recharged in approximately 2 min. Thus, the fast charging technique allows the cell recharge in 1/3 rd of the standard charging time. 1 8 6 4 2 Multistage fast charge (4C-1C-CV) Standard charge (CC-CV) 1 2 3 4 5 6 7 Charging time (min) Figure 6: Comparison of the normalized charged capacity versus charging time for the proposed fast charging technique and the standard charge 3.1 Aging Study Here is presented a comparative study to evaluate the aging effects that the proposed fast charging technique may cause on the selected cell. To do so, another identical cell is long-term cycled at standard charging (CC CV), and discharge at 1C rate. The intention of this section is not to decipher the aging modes, but to compare the aging patterns of both tested cells under the different protocols. The capacity retention of the tested cells as a function of cycle number is shown in Fig. 7. The discharged capacity is normalized to the adopted standard capacity measured during conditioning. The initial cycles with the fast charging technique procedure coupled with 4C discharges achieved over 99% of the standard discharge capacity. The capacity then decreases slightly through cycling. The two cells show similar capacity degradation patterns, following a linear trend which remained consistent throughout the testing: approximately EVS28 International Electric Vehicle Symposium and Exhibition 6

Incremental capacity (Ah/V) Specific energy (Wh/kg) Incremental capacity (Ah/V) Normalized capacity (%) 1 95 9 85 8 75 7 1 2 3 4 Cycle # Cell 1 - Cycling 1C Cell 2 - Cycling 4C Figure 7: Evolution of the normalized capacity for the tested cells cycle aging 4% of the capacity is lost after every 1, cycles. As it can be seen, the cell would reach its end-of-life (8% of its nominal capacity) after approximately 5, cycles. Fig. 8 shows the Ragone plot, which reveals the specific energy (Wh kg -1 ) versus specific power (W kg -1 ) delivered by the cells as cycling progresses. Since the capacity decreases with cycling, the Ragone curves follow a downward linear trend in terms of specific energy. In contrast, the specific power capability remains unaltered with cycling. Despite the specific energy decrease, the cells can deliver over 75 Wh kg -1 at power demands over 5 W/kg. Most importantly, both cells share the same evolution pattern, only showing small differences within the ~1% range. 11 15 1 95 9 85 8 75 Cell 1 Cell 2 Cycle 1, 9, 18, 3 7 1 1 1 1 Specific power (W/kg) Figure 8: Evolution with cycling of the Ragone plot for the tested cells The diagnostic analyses proposed here are carried out using the incremental capacity (IC) technique. The IC is an electrochemical technique that can detect gradual changes with great sensitivity in cell behavior during life-cycle tests, by analyzing the evolution of the resulting IC peaks [24], [25]. Fig. 9 compares the kinetic IC curves for Cell 1 (standard cycling) and Cell 2 (fast charge) at the beginning of cycling (solid lines) and when aged, at cycle ~38 (dashed lines) at 1C discharge. These kinetic curves of both cells show the same voltage potential at where the first reaction is taking place. After 38 cycles, both the slope and the voltages remain similar as at the beginning of cycling, which indicates that the polarization resistance is barely affected by aging. Hence, both cells do not show kinetic degradation. The evolution of the IC signatures of Cell 1 and Cell 2 from the initial state of cycling to cycle ~38, carried out at thermodynamic (C/25) discharges is shown in Fig. 1. The IC curves show great resemblance between Cell 1 and Cell 2, both at fresh and aged stages. Under thermodynamic tests the reactions occur very slowly, and the resulting IC curves have the largest sensitivity. -2-4 -6-8 -1-12 -14-16 -18 1C Cell 1 - Fresh Cell 1 - Aged Cell 2 - Fresh Cell 2 - Aged -2 3 3,1 3,2 3,3 3,4 3,5 Voltage (V) Figure 9: Comparison of IC curves at 1C of the tested cells at beginning of cycling (solid lines) and aged at 38 cycles (dashed) -2-4 -6-8 -1-12 -14-16 -18 C/25 Cell 1 - Fresh Cell 1 - Aged Cell 2 - Fresh ❺ ❹ ❸ Cell 2 - Aged ❷ ❶ -2 3 3,1 3,2 3,3 3,4 3,5 Voltage (V) Figure 1: Comparison of the IC curves at C/25 of the tested cells at beginning of cycling (solid lines) and aged at 38 cycles (dotted) EVS28 International Electric Vehicle Symposium and Exhibition 7

Since both cells show similar IC curve signatures, this suggests that the ongoing aging modes on both cells under such different cycling schemes are virtually the same. A comprehensive analysis of the aging modes is to be published elsewhere. 4 Conclusions This study was carried out due to the increasing interest that fast charging capabilities are taking part in many battery powered systems, including EVs. To achieve effective fast charges that cause no additional aging and are safe, several factors shall be taken into account. Namely, a proper high power lithium ion battery, coupled with a dedicated charging protocol. Here we showed the main features of both topics. From its analysis, we proposed a general strategy to design simple, yet effective fast charging procedures. The experimental results showed that by selecting proper high power cell architecture, coupled with a multistage charging protocol based on the internal resistance evolution, full charging can be achieved within 2 min. To evaluate the proposed strategy, the fast charging tested cell was compared to another identical cell cycled standard charge. We found that both cells degraded similarly in terms of capacity fade, power capabilities and structural changes. Overall, all indication points that the proposed fast charging scheme can be applied to this particular type of high power without causing any further degradation than of standard cycling. Acknowledgments The authors gratefully acknowledge the funding provided by the Spanish Ministry of Economy and Competitiveness under Grant DPI213-46541-R. References [1] J. Vetter, P. Novák, M. R. Wagner, C. Veit, K.-C. Möller, J. O. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, and A. Hammouche, Ageing mechanisms in lithium-ion batteries, J. Power Sources, vol. 147, no. 1 2, pp. 269 281, Sep. 25. [2] J. W. Fergus, Recent developments in cathode materials for lithium ion batteries, J. Power Sources, vol. 195, no. 4, pp. 939 954, Feb. 21. [3] U. S. A. B. Consortium, Electric Vehicle Battery Test Procedures Manual, 1996. [4] D. Anseán, PhD dissertation Electrical Engineering Department, University of Oviedo, Spain. June 215. [5] C.-K. Park, Z. Zhang, Z. Xu, A. Kakirde, K. Kang, C. Chai, G. Au, and L. Cristo, Variables study for the fast charging lithium ion batteries, J. Power Sources, vol. 165, no. 2, pp. 892 896, Mar. 27. [6] K.-J. Lee, K. Smith, A. Pesaran, and G.-H. Kim, Three dimensional thermal-, electrical-, and electrochemical-coupled model for cylindrical wound large format lithium-ion batteries, J. Power Sources, vol. 241, pp. 2 32, Nov. 213. [7] P. Braun, J. Cho, J. Pikul, W. King, and H. Zhang, High power rechargeable batteries, Curr. Opin. Solid State Mater. Sci., vol. 16, pp. 186 198, 212. [8] S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, and C. Capiglia, Review on recent progress of nanostructured anode materials for Li-ion batteries, J. Power Sources, vol. 257, pp. 421 443, Jul. 214. [9] P. G. Bruce, B. Scrosati, and J.-M. Tarascon, Nanomaterials for rechargeable lithium batteries., Angew. Chem. Int. Ed. Engl., vol. 47, no. 16, pp. 293 46, Jan. 28. [1] W. J. Zhang, Structure and performance of LiFePO4 cathode materials: A review, J. Power Sources, vol. 196, no. 6, pp. 2962 297, Mar. 211. [11] S. Y. Chung, J. T. Bloking, and Y. M. Chiang, Electronically conductive phospho-olivines as lithium storage electrodes., Nat. Mater., vol. 1, no. 2, pp. 123 8, Oct. 22. [12] A. Burke and M. Miller, Life cycle testing of lithium batteries for fast charging and seconduse applications, in 213 World Electric Vehicle Symposium and Exhibition (EVS27), 213, pp. 1 1. [13] S. S. Zhang, The effect of the charging protocol on the cycle life of a Li-ion battery, J. Power Sources, vol. 161, no. 2, pp. 1385 1391, Oct. 26. [14] P. H. L. Notten, J. H. G. O. het Veld, and J. R. G. van Beek, Boostcharging Li-ion batteries: A challenging new charging concept, J. Power Sources, vol. 145, no. 1, pp. 89 94, Jul. 25. [15] G. Sikha, P. Ramadass, B. S. Haran, R. E. White, and B. N. Popov, Comparison of the capacity fade of Sony US 1865 cells charged EVS28 International Electric Vehicle Symposium and Exhibition 8

with different protocols, J. Power Sources, vol. 122, no. 1, pp. 67 76, Jul. 23. [16] T. T. Vo, X. Chen, W. Shen, and A. Kapoor, New charging strategy for lithium-ion batteries based on the integration of Taguchi method and state of charge estimation, J. Power Sources, vol. 273, pp. 413 422, Sep. 214. [17] T. Vo, W. Shen, and A. Kapoor, Experimental comparison of charging algorithms for a lithium-ion battery, IPEC, 212 Conf. Power, pp. 27 212, 212. [18] Z. Guo, B. Y. Liaw, X. Qiu, L. Gao, and C. Zhang, Optimal charging method for lithium ion batteries using a universal voltage protocol accommodating aging, J. Power Sources, vol. 274, pp. 957 964, Jan. 215. [19] Z. Li, J. Huang, B. Y. Liaw, V. Metzler, and J. Zhang, A review of lithium deposition in lithium-ion and lithium metal secondary batteries, J. Power Sources, vol. 254, pp. 168 182, May 214. [2] B. K. Purushothaman and U. Landau, Rapid Charging of Lithium-Ion Batteries Using Pulsed Currents, J. Electrochem. Soc., vol. 153, no. 3, p. A533, Mar. 26. [21] 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, 213. [22] C. Vartanian and N. Bentley, A123 systems advanced battery energy storage for renewable integration, 211 IEEE/PES Power Syst. Conf. Expo., pp. 1 6, Mar. 211. [23] M. A. Roscher, J. Vetter, and D. U. Sauer, Characterisation of charge and discharge behaviour of lithium ion batteries with olivine based cathode active material, J. Power Sources, vol. 191, no. 2, pp. 582 59, Jun. 29. [24] J. Barker, Three Electrode Electrochemical Voltage Spectroscopy (TEVS): evaluation of a model lithium ion system, Electrochim. Acta, vol. 4, no. 11, pp. 163 168, Aug. 1995. [25] M. Dubarry, V. Svoboda, R. Hwu, and B. Y. Liaw, Incremental Capacity Analysis and Close-to-Equilibrium OCV Measurements to Quantify Capacity Fade in Commercial Rechargeable Lithium Batteries, Electrochem. Solid-State Lett., vol. 9, no. 1, p. A454, 26. Authors David Anseán received the B.Sc. degree in Electronics Engineering from the University of Granada, Spain, in 27. After gaining industry experience in Basingstoke, UK, and Berkeley, CA, USA, in 21 he joined the University of Oviedo, Spain, where he received the M.Sc. degree in Electrical Engineering in 211, and where he is currently working towards the Ph.D. degree. In 213 he was awarded with a predoctoral research fellow at the Electrochemical Power Systems Laboratory at the University of Hawaii, USA, working under the supervision of Dr. Bor Yann Liaw. His research interests include lithium ion battery testing and characterization, fast charging and the study of the degradation mechanisms via noninvasive methods. Víctor Manuel García is Associate Professor in the area of Physical Chemistry at the University of Oviedo, Spain, with a doctorate in Quantum Chemistry and expertise in Theory of Electronic Separability. For several years his research interest has been focused on applied aspects of electrochemistry, either in batteries or corrosion. His research is coupled with an intense dedication to chemical education. Manuela González received the M.Sc. and the Ph.D. degrees in Electrical Engineering from the University of Oviedo, Spain, in 1992 and 1998, respectively, where she is currently Associate Professor. She is the founder and head of the Battery Research Laboratory in the Department of Electrical and Electronic Engineering, at University of Oviedo. Her research interests include battery management systems for new battery technologies and fast chargers for traction applications. Juan Carlos Viera received the M.Sc. degree in Electrical Engineering from the University of Technology (ISPJAE), Havana, in 1992 and the Ph.D. degree in Electrical Engineering from the University of Oviedo, Spain, in 23. He is currently an Assistant Professor in the Department of Electrical Engineering at the University of Oviedo, Spain. His research interests include battery management systems, battery testing, EVS28 International Electric Vehicle Symposium and Exhibition 9

fast-charging among others. Juan C. Antón was born in Veracruz, Mexico, in 1966. He received the M.Sc. degree in computer engineering from the University of Valladolid, Spain, in 1996 and the Ph.D. degree from the University of Oviedo, Spain, in 27. He is currently an Associate Professor with the Department of Electrical and Electronic Engineering, University of Oviedo. His research interests include lighting, electronic instrumentation systems, and battery modelling. Cecilio Blanco received the M.Sc. and Ph.D. degrees in Electrical Engineering from the University of Oviedo, Spain, in 1989 and 1996 respectively. In 1989 he joined the Department of Electrical Engineering at the University of Oviedo, where he is currently an Associate Professor. His research interests include battery fast charging, battery modelling and discharge lamp modelling. EVS28 International Electric Vehicle Symposium and Exhibition 1