Parallel Balancing Converter for Serially Connected Batteries String

Transcription

1 Parallel Balancing Converter for Serially Connected Batteries String Or Kirshenboi, Student Meber, IEEE, Yoav Dickstein, Alex Shvarchov, Mor Mordechai Peretz, Meber, IEEE The Center for Power Electronics and Mixed-Signal IC Departent of Electrical and Coputer Engineering Ben-Gurion University of the Negev P.O. Box 653, Beer-Sheva 845, Israel. Abstract This paper introduces a new isolated converter topology for parallel balancing of serially connected batteries string. The syste uses a low voltage capacitor as an energy buffer that is coon for all the cells and balancing of the string is achieved by voltage equalizing of the cells. Each converter is in charge of balancing three adjacent cells, reducing the coponents count of the syste. The converter operates in DCM and the current that flows between the cells and the is a function of their voltage difference. As a result, the quiescent power loss is inial since no energy circulates in the syste when the cells are balanced. Furtherore, no voltage or current sensors are required, aking the ipleentation of the syste siple and cost-effective. Theoretical analysis as well as design guidelines for the construction of the new topology are detailed and validated by experiental results that deonstrate the syste s balancing capability. I. INTRODUCTION Batteries have been widely used in various applications as an energy storage eleent and a power source. To achieve the required high voltage and high power for applications such as electric vehicle (E) and its derivatives, and due to low cell voltage, large nuber of batteries cells are connected in series []-[5]. Batteries suffer fro anufacturing and environental variances, degradation with aging, charging and discharging, theral conditions and internal ipedance differences. Each of these potential flaws or a cobination of the ay lead to ibalances in the stored energy of the cells and as a result reduce their lifetie, efficiency and reliability [6]. Therefore, serially connected batteries strings ust be assisted by a balancing circuit to iniize ibalances and iprove the overall perforance [7], [8]. The passive balancing approach is predoinant in the ajority of coercial batteries balancing applications, priarily due to low cost and siplicity [9]. There, excess energy of each cell is dissipated either through a resistor or transistor, but fro an energy efficiency perspective it less attractive due to the inherent power loss. An alternative concept that has been extensively investigated in recent years is the active balancing approach []-[7]. Here, power converters are used to evenly distribute the stored energy along the string. Typically, energy is transferred fro cells with higher voltage to the lower ones, or in ore sophisticated designs by cell s State-of-Charge (SoC) [8], [9]. Active Ilya Zeltser, Meber, IEEE Power Electronics Departent Rafael Advanced Defense Systes td. P.O. Box 5, Haifa 3, Israel. ilyaz@rafael.co.il balancing can be realized in a variety of ways, for exaple using switched-capacitors converters []-[], switchedinductor converters [3]-[4], or ulti-winding transforer based converters [5]-[7]. Balancing circuits are further distinguished by power flow architecture, i.e., series balancing and parallel balancing. In series balancing, e.g. as in [], energy is transferred fro one neighboring cell to another using power converters that link between each two adjacent cells. The converters act as a local bypass to the energy flow in case a cell is daaged or has lower energy, but in any cases their operation requires synchronization between the circuits. Parallel balancing is assisted by a sall energy storage coponent, typically a capacitor, and often referred as energy buffer which is used as a link to transfer energy fro a charged cell to the cell that needs to be charged. It eans that when this type of balancing is used, the energy does not need to be processed through the whole batteries string [3]-[3]. Therefore, an apparent advantage of the parallel balancing approach is the fewer aount of conversions to balance the string, and as a result faster balancing with higher efficiency, especially in large arrays. However, the penalty often coes with large nuber of coponents in each balancing circuit and coplex control. Practical ipleentation of active balancing systes is a challenge, ainly due the large nuber of cells in the string that contribute to the coplexity of the syste in ters of large coponent count, high stress coponents, ultiwinding transforers or sophisticated control algoriths. This forces the individual balancing circuit and its controller to be as siple and cost-effective as possible in order to be a copetitive solution. The objective of this study is to introduce a new isolated parallel balancing syste with low coponents count and very siple control. The new syste, depicted in Fig., utilizes each of the odules for balancing three adjacent cells. A coon capacitor for all odules in the string is used as the energy buffer to link between odules. The energy transfer is proportional to the voltage difference between a cell and the, resulting in low quiescent power losses since no energy circulates in the syste when the cells are balanced. The operation of the balancing odules does not require sensors, neither for current nor for the voltage. In addition, asynchronous operation of the odules is allowed, aking the solution siple and cost-effective. The paper is organized as follows: Section II describes the topology, its principle of operation and the ajor features of /7/$3. 7 IEEE 9

2 Module i kg cell kg 5 7 priary secondary cell, 3 C cell, Fig.. oltage equalizing circuit of a battery cell and the capacitor. i cell Module cell i cell, cell, i kg cell,n- cell,n- S kg, S 3 S 5 S 7 i cell,3 cell,3 S S 3 C cell,n Fig.. Batteries balancing syste for n serially-connected batteries. it. Section III delineates the syste s ipleentation and provides design guidelines. Experiental results are then provided in Sections I, Section concludes the paper. II. PRINCIPE OF OPERATION The balancing syste, shown in Fig., is divided into =n/3 balancing odules, where n is the nuber of cells in the string. A single odule is in charge of balancing three adjacent cells and the capacitor that is coon for all the odules. Each odule consists of two half-bridge transistors asseblies at the cells side (transforer s priary side), a transforer that provides isolation of the cells fro the capacitor, and a full-bridge transistors assebly at the capacitor s side (transforer s secondary side). The core concept of the balancing syste is based on charge equalization by equalizing the voltages of the cells using a converter operating in DCM, where the balancing current is a function of the voltage difference between the cell and the. Balancing between the cells is achieved by a capacitor that operates as an energy buffer to create a parallel energy path, and each of the cells voltages is equalized to the capacitor voltage. The basic equalization circuit, shown in Fig., coprises an isolation transforer, with unity transforer ratio (in this study), which is connected between the battery cell and the capacitor. The leakage of the transforer is used as a ain inductance. The current that flows through this inductance is deterined by the voltage difference between the battery cell and the voltage, which is priary secondary, In this arrangeent, the current naturally flows toward the source with the lower voltage and charges it. For this circuit the current s slew rate is given by di dt priary secondary, () kg Fig. 3. Balancing odule of three adjacent cells. where kg is the transforer s leakage inductance. Eploying this balancing approach for all the cells in the string, the capacitor voltage converges to the cells voltages average value, given by n cell, k n k () where cell,k is the voltage of a single cell in the string (cell nuber k) and is the voltage of the capacitor. Since the capacitor is coon for the entire string, all the cells are eventually balanced and their voltages are equal to (). A. Switching Sequence and Current Paths A balancing odule for three adjacent cells is depicted in Fig. 3. In every switching period T s a different cell is being balanced. The balancing sequence is exeplified in Fig. 4 for cell no. with the current paths highlighted in red. For this case, switches, S 3, S 5 and S 8 are turned on for a predefined on tie T on (further details on setting the on tie duration are provided in Section III), the transforer s priary side voltage is cell and its secondary side voltage is (see Fig. 4(a),(c)). For the tie only switch S 3 reains on whereas, S 5 and S 8 are turned. Two possible states are recognized: cell< and cell>, as depicted in Fig. 4(b) and Fig. 4(d), respectively. The two states are distinguished by the current direction during the on tie and as a consequence, by the current continuity, the body diodes that conduct during the tie. In case that cell< the body diodes of, S 6 and S 7 are forward biased, the priary side s voltage is priary= cell and the secondary side s voltage is secondary=-. In case that cell>, the body diodes of S, S 5 and S 8 are forward biased, the priary side s voltage is priary=- cell, and the secondary side s voltage is secondary=. Fig. 5 shows an idealized current wavefor for the case that cell no. is being balanced and cell >. As can be seen, the positive voltage difference between the cell and the is applied on the inductance during the on tie whereas during the tie the applied voltage is negative with a agnitude that equals the su of the voltages. This negative voltage raps the current back to zero. 9

3 On tie Off tie cell kg cell kg cell < cell, S S 3 cell, C S S 3 C cell,3 cell,3 (a) (b) cell kg cell kg cell > cell, S S 3 cell, C S S 3 C cell,3 cell,3 (c) (d) Fig. 4. Balancing operation of cell no. for the case that cell< : (a) on tie, (b) tie, and for the case that cell> : (c) on tie, (d) tie. Red arrows ark actual current direction. TABE I SWITCHING SEQUENCE OF A SINGE BAANCING MODUE On tie (<t<ton) Off tie (Ton<t<Ts) Active Cell Active Active Switches priary secondary State Switches priary Cell, S 3, S 5, S 8 cell cell< cell> S 3 S 3 cell - cell, - Cell S, S 3, S 6, S 7 - cell, - cell,< cell,> 3/S S 3/S cell, cell/ cell,3 - S - Cell 3 S,, S 5, S 8 cell,3 cell,3< S cell,3 - cell,3> S - cell, secondary i cell kg The coplete switching sequence of the odule for balancing the 3 cells is detailed in Table I. It should be noted that balancing of cell no. has two switching possibilities as explained in the next subsection. B. Balancing Cycle Sequence T on Two switching sequences are possible during the tie while cell no. is being balanced (see Table I). That is, during the on tie switches S, S 3, S 6, and S 7 are on, but during the tie it is possible to keep either switch S or S 3 on, which deterines priary for this period. In case that S 3 is on then priary= cell, and for case that S is on then priary= cell,3. T s cell, Fig. 5. Idealized current wavefor of a single switching cycle for balancing cell no. for the case that cell>. kg This iplies that during the tie, for the case that cell,>, cell no. or 3 is being charged. To avoid any undesired charging of only one of cells no. or 3 and to evenly distribute both the stored energy between the cells and the processed power between the switches, cell no. is balanced twice in each balancing cycle. Therefore, a balancing cycle consists of four balancing phases of the cells. In each balancing phase only one cell is being balanced, and the cell s balancing order is 3, as deonstrated in Fig. 6. Of particular interest is a balancing cycle of cell no. 3 (T s<t<3t s), shown in Fig. 6 for cell,3>. In this case, the inductor current flows through cell no. 3 during the on tie and through the cell no. during the tie, so the voltage at transforer s priary side is claped to cell,.. III. IMPEMENTATION AND DESIGN CONSIDERATIONS To realize the balancing syste of the architecture described in Fig., several practical design challenges need to be addressed. This includes setting of the transistors sequence, design the allowed range of the balancing current to satisfy DCM operation, selection of the capacitor, and transforer s design; All need to be defined to ensure proper operation of the syste. 93

4 i cell i cell, i cell,3 cell cell, cell,3 cell, ON OFF () () i - - i Clap by cell, cell < (3) (4) (5) (6) i kg (7) i - - A. Transforer s eakage Inductance, Bus Capacitor and Balancing Current Design The transforer s leakage inductance in this balancing syste is utilized as the ain inductor. The current that flows through the transforer in every switching cycle is governed by the on tie of the switches T on, the voltage difference between the cell and the capacitor Δ and the inductance value kg. Since the circuit is operated in DCM, the peak inductor current I pk and an inductor current ripple ΔI are equal and given by I I T. (3) pk on kg After turning the switches, the tie it takes for the current to rap down back to zero can be expressed as T ' priary secondary T on, (4) where priary and secondary during the tie are given in Table I. The average current in a single switching cycle is given by I kg T on T T s T s 3T s 4T s Fig. 6. Currents of the cells (top) and the transforer (botto) during four switching cycles for the case that cell< cell,< < cell,3. priary secondary Ton. (5) Ts As can be seen fro (5), in case that no voltage difference exists (eaning that the cells are balanced), i.e. Δ=, the current is zero and no energy is processed by the syste. To assure that both leakage and agnetizing currents are zero at the end of every switching period, a sufficiently long tie needs to be allowed. Considering operation at constant switching frequency, this iposes a liitation on the axiu allowed on tie. On the other hand, to achieve fast convergence of the cells voltages, T on should be set as high as possible. Idealized currents wavefors of the agnetization inductance, the leakage inductance and the capacitor are shown in Fig. 7. The currents slew-rates when cell no. is being balanced are detailed in Table II. Under the di dikg di cell > TABE II CURRENTS SEW-RATES OF FIG. 7 cell < () () (3) (4) (5) (6) (7) cell kg aboveentioned constraints, the iniu tie that is required for the agnetizing current to rap back to zero is T s/, and therefore the axiu allowed on tie, i.e. T on,ax, is also T s/. The value of T on,ax for balancing cells no. or 3 is the sae. Sizing of the capacitor can affect the proper balancing operation. Its voltage should converge to the cells voltages average value relatively fast and therefore its capacitance should be relatively low. However, it ust have sufficiently low voltage ripple so that it can be considered as a near ideal voltage source for each of the balancing phases, which require high capacitance. Therefore, the iniu capacitance ust satisfy the condition, (6) ripple di where ripple is the voltage ripple of the capacitor. Using (5) and after soe anipulations, condition (6) translates into Ton,ax C. (7) 4 f kg kg s cell > () () (3) cell dikg (4) kg kg cell (5) cell i kg (3) (4) di i kg (6) (5) (7) () () i Fig. 7. Idealized currents wavefors of the agnetization inductance, the leakage inductance and the capacitor. The currents slew-rates are detailed in Table II. (6) cell cell kg kg lkg cell cell lkg cell kg (7) cell kg 94

5 3 v cell TABE III EXPERIMENTA PROTOTYPE AUES v cell, v v cell,3 Coponent Batteries (eulated by large capacitors) Transforer s leakage inductance kg Transforer s agnetizing inductance MOSFETs -S 8 Bus capacitor C Switching frequency f s alue/type 6 F.5 μh 7 μh Si468DY, 3, 5.7Ω 47 μf 5 khz.5 i cell, = cell = cell,3 = Tie (s) i Fig. 8. Siulation results for 4 batteries cells eulated by large capacitors. i cell,3 -icell, cell = 3.4 cell, =. cell,3 =.9 cell = 9.7 cell, =. cell,3 = 3.6 i cell i Fig. 9. Transforer s current during unbalanced steady-state operation. C4 Transforer s current i (A/ div), Tie scale is 5µs/div. To deonstrate the balancing operation of the syste, a siulation case study has been carried out and the results are shown in Fig. 8. It depicts the convergence of three cells (eulated by large capacitors to allow tiely convergence), each set with different initial voltage, to the cells average voltage, validating the balancing capability of the syste. As can be seen, the current agnitude decays to zero as the cells voltages converge, in agreeent with the theoretical prediction in (5). I. EXPERIMENTA ERIFICATION In order to deonstrate the balancing operation and verify the theoretical analysis and siulation results, several experients have been carried out using three cells connected in series, built using large capacitors. Table III shows the coponents types and values of the experiental setup. Fig. 9 shows the steady-state operation current wavefor of the transforer for unbalanced cells with T on=t s/. The cells voltages are cell=9.7, cell,=. and cell,3=3.6. The differences between the cells voltages and the voltage result in current flowing through the transforer. For this case the voltage relations are cell,3> > cell,> cell, and therefore i cell,3 >, i cell, < and i cell <. Fig.. Convergence of the cells voltages for the cases that they are precharged to different values. C cell (.5/div), C cell, (.5/div), C3 - cell,3 (.5/div), C4 Transforer s current i (A/div). Tie scale is s/div. Fig. shows the cells voltages and the transforer s current over a long period of tie for a case that the cells are pre-charged to different voltages. As can be observed, the voltages of the three cells converge to their average value, in agreeent with the theoretical analysis, and the voltage difference between the balanced cells is negligibly sall. Also, the current in the transforer decreases as the cells voltages converge and Δ decreases, as predicted in (5).. CONCUSION In this work, a new isolated balancing topology for serially connected batteries strings has been introduced. Fast convergence of the cells is achieved by using a parallel balancing approach with low voltage capacitor that is coon for all the cells. Each balancing odule is in charge of balancing three adjacent cells and therefore the nuber of coponents in the syste is low. The DCM operation and the fact that no energy circulates in the syste when the cells are balanced result in extreely low quiescent power loss. Control of the odules is very siple and does not require current nor voltage sensors to regulate the operation of the syste. The theoretical analysis and the results of the experiental prototype deonstrated fast convergence of the cells to a negligibly sall voltage difference. 95

6 ACKNOWEDGMENTS This research was supported by the Pazi foundation. REFERENCES [] A. Eadi,. Young Joo, and K. Rajashekara, Power electronics and otor drives in electric, hybrid electric, and plug-in hybrid electric vehicles, IEEE Trans. Ind. Electron., vol. 55, no. 6, pp , Jun. 8. [] M. Ehsani, G. Yiin, J. M. Miller, Hybrid electric vehicles: architecture and otor drives, Proceedings of the IEEE, vol. 95, no. 4, pp , Apr. 7. [3] A. Y. Saber and G. K. enayagaoorthy, Plug-in vehicles and renewable energy sources for cost and eission reductions, IEEE Trans. Ind. Electron., vol. 58, no. 4, pp. 9 38, Apr.. [4] H. Qian, J. Zhang, J. S. ai, and W. Yu, A high-efficiency grid-tie battery energy storage syste, IEEE Trans. Power Electron., vol. 6, no. 3, pp , Mar.. [5] B. Gu, J. Doinic, B. Chen, and J. S. ai, A high-efficiency single-phase bidirectional AC-DC converter with iniized coon ode voltages for battery energy storage systes, in Proc. IEEE Energy Convers. Congr. Expo. 3, pp , Sep. 3. [6] B. T. Kuhn, G. E. Pitel, and P. T. Krein, Electrical properties and equalization of lithiu-ion cells in autootive applications, in Proc. IEEE ehicle Power Propuls. Conf., pp , Sep. 5. [7] P. T. Krein and R. S. Balog, ife extension through charge equalization of lead acid batteries, in Proc. Int. Telecoun. Energy Conf. (INTEEC), pp ,. [8] M. Uno and K. Tanaka, Influence of high-frequency charge discharge cycling induced by cell voltage equalizers on the life perforance of lithiu-ion cells, IEEE Trans. eh. Technol., vol. 6, no. 4, pp , May. [9] J. Cao, M. Schofield and A. Eadi, Battery balancing ethods: A coprehensive review, IEEE ehicle Power and Propulsion Conference, PPC 8. pp.,6, Sep. 8. [] A. C. Baughan and M. Ferdowsi, Double-tiered switched-capacitor battery charge equalization technique, IEEE Trans. Ind. Electron., vol. 55, no. 6, pp , Jun. 8. [] C. Pascual and P. T. Krein, Switched capacitor syste for autoatic series battery equalization in Proc. IEEE Appl. Power Electron. Conf. Expo. 997, pp , Feb [] M. W. Cheng, Y. S. ee, R. H. Chen, and W. T. Sie, Cell voltage equalization using ZCS SC bidirectional converters, in Proc. Int. Telecoun. Energy Conf., pp. 6, Oct. 9. [3] F. Mestrallet,. Kerachev, J. C. Crebier and A. Collet, Multiphase interleaved converter for lithiu battery active balancing, IEEE Trans. Power Electron., vol. 9, no. 6, pp , Jun. 4. [4]. Wang,. Wang, C. iao, and J. iu, Research on battery balance syste applied on HE, PPC 9, pp , Sep. 9. [5] Z. Nie and C. Mi, Fast battery equalization with isolated bidirectional DC-DC converter for PHE applications, IEEE ehicle Power and Propulsion Conference, PPC 8, pp. 78-8, Sep. 9. [6] Y. S. ee and G. T. Cheng, Quasi-resonant zero-current-switching bidirectional converter for battery equalization applications, IEEE Trans. Power Electron., vol., no. 5, pp. 3-4, Sep. 6. [7] M. Uno and K. Tanaka, Single-switch cell voltage equalizer using ultistacked buck-boost converters operating in discontinuous conduction ode for series-connected energy storage cells, IEEE Trans. ehicular Technology, vol. 6, no. 8, pp , Oct.. [8] M. Uno and K. Tanaka, Single-switch ulti-output charger using voltage ultiplier for series-connected lithiu-ion battery/supercapacitor equalization, IEEE Trans. Ind. Electron., vol. 6, no. 8, pp , Aug. 3. [9] G. Oriti, A.. Julian and P. Norgaard, Battery anageent syste with cell equalizer for ulti-cell battery packs in Proc. IEEE Energy Convers. Congr. Expo. 4, pp. 9-95, Sep. 4. [] M. Uno and K. Tanaka, Single-switch cell voltage equalizer using voltage ultipliers for series-connected supercapacitors, in Proc. IEEE Appl. Power Electron. Conf. Expo, pp. 66-7, Feb.. [] Y. Yuanao, K. W. E. Cheng, and Y. P. B. Yeung, Zero-current switching switched-capacitor zero-voltage-gap autoatic equalization syste for series battery string, IEEE Trans. Power Electron., vol. 7, no. 7, pp , Jul.. [] C. H. Sung, K. ee, and B. Kang, oltage equalizer for li-ion battery string using C series resonance, IECON 3-39th Annual Conference of the IEEE, pp.44-49, Nov. 3. [3] A.. Julian, G. Oriti, M. E. Pfender, SR converter design for ulticell battery charging, in Proc. IEEE Energy Convers. Congr. Expo., pp , Sep. 3. [4] D. Costinett, K. Hathaway, M. U. Rehan, M. Evzelan, R. Zane., Y. evron, and D. Maksiovic, Active balancing syste for electric vehicles with incorporated low voltage, in Proc. IEEE Appl. Power Electron. Conf. Expo. 4, pp , Mar. 4. [5] S. i, C. C. Mi and M. Zhang, A high-efficiency active batterybalancing circuit using ultiwinding transforer, IEEE Trans. Ind. Applications, vol. 49, no., pp. 98-7, Jan. 3. [6] S. H. Park, K. B. Park, H. S. Ki, G. W. Moon, M. J. Youn, Singleagnetic cell-to-cell charge equalization converter with reduced nuber of transforer windings, IEEE Trans. Power Electron., vol. 7, no. 6, pp. 9-9, Jun.. [7] M. Y Ki, J. H. Ki, G. W. Moon, Center-cell concentration structure of a cell-to-cell balancing circuit with a reduced nuber of switches, IEEE Trans. Power Electron., vol. 9, no., pp , Oct. 4. [8] Y. S. ee and M. W. Cheng, Intelligent control battery equalization for series connected lithiu-ion battery strings, IEEE Trans. Ind. Electron., vol. 5, no. 5, pp , Oct. 5. [9] M. U. Rehan, F. Zhane, M. Evzelan, R. Zane, and D. Maksiovic, Control of a series-input, parallel-output cell balancing syste for electric vehicle battery packs, IEEE 6th Workshop on Control and Modeling for Power Electronics 5, Jul. 5. [3] B. Dong, Y. i and Y. Han, Parallel architecture for battery charge equalization, IEEE Trans. Power Electron., vol. 3, no. 9, pp , Sep. 5. [3] M. Evzelan, M. U. Rehan, K. Hathaway, R. Zane, D. Costinett, and D. Maksiovic, Active balancing syste for electric vehicles with incorporated low voltage," IEEE Trans. Power Electron., vol. 3, no., pp , Nov. 6. [3] I. Zeltser, O. Kirshenboi, N. Dahan, and M. M. Peretz, ZCS resonant converter based parallel balancing of serially connected batteries string, in Proc. IEEE Appl. Power Electron. Conf. Expo., pp. 8-89, Mar