Individual Cell Equalizaion for Series Conneced Lihium-Ion Baeries * Yuang-Shung Lee, a) Ming-Wang Cheng, 2,3b) Shen-Ching Yang, Co-Lin Hsu Deparmen of Elecronic Engineering Fu-Jen Caholic Universiy 50 Chung-Cheng Rd., Hsin-Chuang, Taipei 24205, Taiwan Tel: +886-2-2903-379 Fax: +886-2-29042638 2 Graduae Insiue of Applied Science and Engineering, Fu-Jen Caholic Universiy 3 Indusrial Technologies Research Insiue, Blog77,95-5 Chung Hsing Rd. Secion 4, Chuung, Hsinchu, 305, Taiwan a) lee@ee.fju.edu.w b) MWCheng@iri.org.w Absrac: A sysemaic approach o he analysis and design of a bi-direcional Cûk converer for he cell volage balancing conrol of a series-conneced lihium-ion baery sring is presened in his paper. The proposed individual cell equalizers (ICE) are designed o operae a disconinuous-capacior-volage mode (DCVM) o achieve he zero-volage swiching (ZVS) for reducing he swiching loss of he bi-direcional DC/DC converers. Simulaion and experimenal resuls show ha he proposed baery equalizaion scheme can no only enhance he bi-direcional baery equalizaion performance, bu also can reduce he swiching loss during he equalizaion period. Two designed examples are demonsraed, he swich power losses are significanly reduced by 52.8% from he MOSFETs and he equalizaion efficiency can be improved by 68~86.9% using he proposed DCVM ZVS baery equalizer under he specified cell equalizaion process. The charged/discharged capaciy of he lihium-ion baery sring is increased by using he proposed ICEs equipped in he baery sring. Keywords: Bi-direcional converer, DCVM Cûk converer, Lihium-ion baery, Individual cell equalizer, ZVS References. H. V. Venkaasey, and Y. U. Jeong, Recen Advanced in Lihium-Ion and Lihium-Polymer Baeries, Proceeding of Baery Conference on Applicaions and Advances, The Seveneenh Annual, Jan. 2002, pp. 73-78. 2. J. McDowell, A. Brenier, M. Broussely, and P. Lavaur, Indusrial Lihium-Ion Baeries: From The Laboraory o Real Telecom Applicaion, Proceeding of Telecommunicaions Energy Conference, INTELEC 24h Annual Inernaional, Sep. 2002, pp. 373-378. 3. N. H. Kuku, A Modular Nondissipaive Curren Diverer for EV Baery Charge Equalizaion, Proceeding of Applied Power Elecronics Conference and Exposiion, APEC'98, Thireenh Annual, Vol. 2, 998, pp. 686-690. 4. K. Nishijima, H. Sakamoo, and K. Harada, A PWM Conrolled Simple and High Performance Baery Balancing Sysem, Proceeding of IEEE Power Elecronics Specialiss Conference, PESC 00, IEEE 3s Annual, Vol., 2000, pp 57-520. 5. Z. Zhang, and S. Cûk, A High Efficiency.8 kw Baery Equalizer, Proceeding of IEEE Applied Power Elecronics Conference and Exposiion, APEC '93, Eighh Annual, 993, pp. 22 227.
6. Toshio Masushima, Shinya Takagi and Seiichi Muroyama, Rack-Mouned DC Power Supply Uilizing Li-Ion Baeries for Backup, IEICE Transacions on Communicaions, Vol. E88-B, No., 2005, pp. 4353-4359. 7. W. F. Benley, Cell Balancing Consideraions for Lihium-Ion Baery Sysems, Proceeding of he 997 Baery Conference on Applicaions and Advances, Twelfh Annual, 4-7, Jane, 997, pp. 223-226. 8. S. W. Moore, and P. J. Schneider, A Review of Cell Equalizaion Mehods for Lihium Ion and Lihium Polymer Baery Sysems, SAE Technical Paper Series, 200-0-0959, March, 5-8, 200, pp. -5. 9. Y. S. Lee and C. W. Jao, Fuzzy Conrolled Lihium-Ion Baery Equalizaion wih Sae-of-Charge Esimaor, Proceeding of 2003 IEEE Inernaional Conference on Sysem, Man and Cyberneics, IEEE CSMC2003, pp. 443-4438. 0. J. Chazakis, K. Kalaizakis, N. C. Voulgaris, and S. N. Manlas, Designing A New Generalized Baery Managemen Sysem, IEEE Transacion on Indusrial Elecronics, Vol. 50, No. 5, 2003, pp. 990-999.. Tosiba Masushima, Shinya Takagi, Seiichi Muroyama and Toshio Horie, Fundamenal Characerisics of Saionary Lihium-Ion Secondary Cells and A Cell-Volage-Equalizing Circui, IEICE Transacions on Communicaions, Vol. E88-B, No. 8, 2005, pp. 3436-3442. 2. P. Melin and O. Casillo, Inelligen Conrol of Complex Elecrochemical Sysem wih A Neuro-Fuzzy-Geneic Approach, IEEE Transacion on Indusrial Elecronics, Vol. 48, No. 5, 200, pp. 95-955. 3. D. Maksimovic and S. Cûk, A Unified Analysis of PWM Converers in Disconinuous Modes, IEEE Transacions on Power Elecronics, Vol.6 No.3, July 99, pp. 476 490. 4. M. Brkovic and S. Cûk, Auomaic Curren Shaper wih Fas Oupu Regulaion and Sof-Swiching, Telecom Energy Conference, INTELEC 93, 5h Inernaional Conference, Vol., Sep. 993, pp. 379-386. 5. K. K. Tse, M. T. Ho, H. S. H. Chung and S. Y. Hui, A Novel Maximum Power Poin Tracker for PV Panels Using Swiching Frequency Modulaion, IEEE Transacion on Power Elecronics, Vol. 7, No. 6, 2002, pp. 980-989. 6. Bo-Tao Lim and Yim-Shu Lee, Power-Facor Correcion Using Cûk Converer in Disconinuous Capacior Volage Mode Operaion, IEEE Transacion on Indusrial Elecronics, Vol. 44, No. 5, 997, pp. 648-653. 7. D. S. L. Simonei, J. Sebasian and J. Uceda, The Disconinuous Conducion Mode Sepic and Cûk Converer Power Facor Preregulaors: Analysis and Design, IEEE Transacion on Indusrial Elecronics, Vol. 44, No. 5, 997, pp. 630-637. 8. G. Spiazzi, P. Maavelli, L. Rosseo and S. Buso, High-Qualiy Recifier Based on Cûk Converer in Disconinuous Capacior Volage Mode, European Power Elecronics Conference, Vol. 2, Sep. 995, pp. 385-390. 9. H. S. H. Chung, K. K. Tse, S. Y. Ron Hui, C. M. Mok and M. T. Ho, A Novel Maximum Power Poin Tracking for Solar Panels Using a Sepic and Cûk Converer, IEEE Transacion on Power Elecronics, Vol. 8, No. 3, 2003, pp. 77-724. 20.Yan-Fei Liu, Requiremens and Technologies in Telecom Power Sysem, Inernaional Power Elecronics and Moion Conrol Conference, PIEMC2000, Vol. 3, Augus 2000, pp. 478-482. 2.Yuang-Shung Lee and Jiun-Yi Duh, Fuzzy Conrolled Individual-Cell Equalizer Using Disconiunous Inducor Curren Mode Cûk Converer for Lihium-Ion Chemisries, IEE Proceedings Elecric Power Applicaions, Vol. 52, No. 5, Sepember 2005, pp. 27-282. * To whom all correspondence should be addressed. 2
. Inroducion Because a single baery cell volage is limied due o he acive maerials chemisry, baery cells conneced in series are usually employed in many applicaions, such as elecric vehicles (EV), hybrid elecric vehicles (HEV), or elecom baery energy sysems. Imbalanced cell volage wihin a series sring can be aribued o he differences in he cell s inernal resisance, imbalanced sae-of-charge (SOC) beween cells, degradaion and he ambien emperaure gradiens of he baery pack during charging and discharging. Volage monioring and curren diversion equalizaion schemes and baery managemen sysems (BMS) have been presened in he lieraure o preven imbalances during charging and discharging in a series conneced baery cells []-[3]. Inegraed individual cell equalizaion schemes (ICE) for baery pack applicaions have been proposed o equalize baery srings [3]-[6]. The bidirecional baery equalizaion scheme has many advanages such as higher equalizaion efficiency for non-dissipaive curren diverers, and a modular design approach [3]. The disadvanage of his equalizaion scheme is ha he sored energy in he inducor is ransferred o he weaker cell only in he (-D)T s duy cycle. The equalizaion ime and efficiency of his equalizaion scheme are herefore poor for pracical baery equalizaion applicaions in a smar baery managemen sysem (SBMS) [6]-[]. Baery equalizaion conrol should be implemened o resric he charge-discharge curren o he allowable cell limiaions in he baery sring. Cell balancing conrol is designed o obain he maximum usable capaciy from he baery sring. However, baery sring charging and discharging are limied by any single cell reaching is end-of-charge volage and by low volage hreshold, respecively. Cell balancing algorihms search o efficienly remove energy from a sronger cell and ransfer ha energy ino a weaker one unil he cell volage is equalized across all cells. This enables addiional charging capaciy for he enire baery sring [4]. Complee cell volage balancing is performed using a bi-direcional dc-dc converer based on he Cûk converer [9], []. This uni can be designed o operae a he DICM or DCVM o obain sof swiching in MOSFET swiches []-[5]. The disconinuous conducing mode of a Cûk converer is generally of used in applicaions of he power facor correcion echnology [6]-[8] and he maximum power poin racker of PV panels [5],[9]. The analysis and design of he uni-direcional power flow conrol scheme Cûk converer in disconinuous capacior volage mode (DCVM) was already discussed in [5],[6],[8]. The aforemenioned analysis and design mehod of he uni-direcional Cûk converer operaed a DCVM can no be oally adoped for he baery equalizaion 3
applicaion due o he alernaing characerisics of he cell volage balancing process in he charge/discharge of baery sring [3],[4],[7]-[9]. The coninuous and disconinuous inducor curren modes of he bi-direcional Cûk converer applicaion for cell volage balancing conrol of lihium-ion baery srings were invesigaed in lieraures [] and [20,2], respecively. The main conribuion of his paper is he firs applicaion of he bi-direcional Cûk converer operaing a DCVM in he design and analysis of he individual cell equalizaion conrol of he lihium-ion baery srings. Two designed cases are used o demonsrae he performance in he proposed ICEs for reducing he swiching power losses of he MOSFET swiches and increasing he equalizaion efficiency and baery sring capaciy. The consideraion and experience for he cell balancing conrol sysem of a lihium ion baery sring are summarized as follows [8,,2]: The equalizaion algorihm will be sared when he volage difference beween wo adjoining cells exceeds 0.096 (V) (hardware resoluion limi of A/D converer, ADC0804) o minimize he cell-o-cell imbalance. During he charged equalizaion sae, he cell volage can no exceed is end-of-charge volage (abou 4.V/cell) o preven overcharging o damage he acive maerials. During he discharged equalizaion sae, he cell volage can no go below is low volage hreshold (abou 2.8 V/cell) o preven overdischarge and damaging he cell capaciy and life. When he volage difference beween adjoining cells is large hen i needs a higher equalizaion curren o speed up he ime required o execue a balancing algorihm, he maximum equalizaion curren limi is 2.5 A. When he volage difference beween adjoining cells is small hen i needs a small equalizaion curren o preven low volage cell overdischarge, he minimal equalizaion curren limi is 0.5 A. If eiher a cell volage in he baery sring exceeds is end-of-charge volage during charge equalizaion sae, or one cell volage in he baery sring reaches is low volage hreshold during discharge equalizaion, he BMS will send a command o sop he cell volage balancing process. 4
Fig. Sudied baery charging sysem wih ICE and microprocessor based BMS 2. Topologies descripion of he ICEs The sudied baery charging sysem wih he proposed ICEs and he microprocessor based BMS is shown in Fig.. The sysem is composed of N baery cells and (N-) ICEs. The jh module is comprised of wo inducors L j and L j+, an energy ransfer capacior C j, and wo power MOSFETs wih body diodes as he baery cell-balancing swiches. The single module of he ICE is redrawn and simplified in Fig. 2. The cell volage balancing conrol algorihm for his equalizaion scheme is insruced by a microprocessor-based BMS. The energy beween he adjoining baery cells is ransformed hrough he energy ransferring capacior for cell 5
volage balancing. The energy ransfer direcion is deermined by he cell volage and/or SOC difference in he baery sring and conducion from he conrolled power MOSFET swiches [9]. The wo adjoining cells volages are balanced by swiching he MOSFETs on/off according o he PWM signals generaed from he BMS. The PWM signals correspond o he respecive cell volage hrough he microprocessor-based BMS, which conrols he swiches Q j and Q j+. The iniial capacior volage V Cj equals V Bj +V Bj+. For example, he PWM conrol signal urns on/off he Q j o ransfer some of he sronger cell volage, V Bj, o he weaker cell, V Bj+. The sronger cell energy is ransferred from cell V Bj o cell V Bj+. Conversely, if he cell V Bj+ is sronger han cell V Bj, he sored chemical energy is ransferred from cell V Bj+ o cell V Bj by conrolling he Q j+. The equalizaion process will be uninerruped unil he volages in he remaining cells are all equalized o he same end-of-charge or end-of-discharge level. The proposed bidirecional baery equalizer is designed o operae a DCVM for achieving he zero volage swiching o reduce he MOSFETs swiching losses. The DCVM operaion principle of proposed ICE is described in he following secion. Fig. 2 Single sage of he proposed ICE 6
Fig. 3 Equivalen circui of DCVM for V Bj > V Bj+ (a) Q j urn-on, (b) Q j and D j+ urn-on, and V Cj = 0, (c) Q j urn-off and D j+ urn-on 7
Fig. 4 Equivalen circui of DCVM for V Bj < V Bj+ (a) Q j+ urn-on, (b) Q j+ and D j urn-on and V Cj = 0, (c) Q j+ urn-off and D j urn-on 8
0 2 3 0 2 3 V cc V GSj V Bj V Lj V Cj - V Bj + V cp V Bj V Lj+ I P i Lj - V Bj + I P i DSj I O * I P i Lj+ * I P i DSj+ * I O 0 0 D T D T T D s s T DT T s s s s 5. (a) 0 2 3 0 2 3 V cc V GSj+ V Bj + V Lj+ -V Bj V cp V Cj V Bj + V Lj I P I O -V Bj i Lj+ I P i DSj+ * I P * I O 0 D T s D T s T s i Lj * I P 0 D Ts DTs Ts i DSj 5. (b) Fig. 5 Typical swiching waveforms of ICE j for (a) V Bj >V Bj+, (b) V Bj <V Bj+ 3. Circui analysis of he j h ICE The inducors L j and L j+ are assumed o be large enough o operae in coninuous inducor curren. In addiion, he capacior is also sufficienly small so i can be fully discharged during he swiching period. Where T s is he converer swiching period, he deail equivalen circui of he proposed baery equalizaion schemes are shown 9
in Figs. 3 and 4 during he various ime inervals for he differen cell volage, V Bj > V Bj+ and V Bj < V Bj+, respecively. The corresponding ypical swiching waveforms for various operaing saes are depiced in Figs. 5 (a) and 5 (b), respecively. Referring o he capacior volage waveform and he dynamic sae equaions of he ICE in Fig. 5 (a) for V Bj > V Bj+ can be explained as follows: Assume he capacior volage υ cj has reached a maximum before he main swich Q j is urned on. From he duy cycle 0 o = D T s, he swich Q j is urned on and diode D j+ is urned off a he beginning of he swiching cycle 0 = 0, as shown as Fig. 3 (a). The inducor L j is charged by inpu volage V Bj and he curren i Lj+ hrough inducor L j+ is discharged by capacior C j. The energy sored in C j is compleely ransferred o he cell V Bj+, and υ cj becomes o zero a = D T s. From he duy cycle = D T s o 2 = DT s, he swich Q j was sill on and D j+ sars conducing o allow i Lj+ o flow since υ cj is equal o zero during his inerval, as shown as Fig. 3 (b), V Bj coninues o charge L j and he sored energy in L j+ is sill discharged o V Bj+ for cell volage balancing conrol. From duy cycle 2 = DT s o 3 = T s, he swich Q j is urned off a 2 = DT s, and D j+ is sill on for cell volage balancing. Capacior C j is charged from zero volage by i Lj. The capacior volage υ cj reaches maximum value a 3 = T s, as shown in Fig. 3 (c). The proposed ICE has more han one sage, in conras o a Cûk converer operaing in CICM, where boh he MOSFET swich Q j and flywheel diode D j+ are conducing in he DCVM operaion, as shown in Fig. 3(b). The dynamic equaions of he equivalen circui for V Bj > V Bj+ in Fig.3 can be expressed by he following: Sage ( 0 < ), Fig.3 (a) shows ha Q j is urned on and D j+ is urn-off, and denoed he swiching variable u = (HIGH sae): L di Lj j S Lj Bj Bj d = ( I i ) R + V, ilj ( 0) = I 0 () dilj+ L j+ = ( IS + ilj+ ) RBj+ + vcj VBj+, d C j dv d Cj = ilj+, Cj 0 * Lj ( 0) 0 i + = I (2) v ( ) = V (3) CP Sage 2 ( < 2 ), Fig. 3 (b) shows ha Q j is sill urned on and D j+ is forced o sar urn-on when υ cj = 0, and denoed he swiching variable u = 0.5 (FLOATING sae): L di Lj j S Lj Bj Bj d = ( I i ) R + V, [ DI P + ( D D) I0] ilj ( ) = (4) D 0
dilj+ L j+ = ( IS + ilj+ ) RBj+ VBj+, i d Lj+ * * DIP + ( D D) I 0 ( ) = (5) D v = 0 (6) Cj Sage 3 ( 2 < 3 ), Q j is urned off and D j+ is sill urned on, and denoed he swiching variable u = 0 (LOW sae): dilj L j = ( IS + ilj+ ) RBj vcj + VBj, ilj ( 2) = IP (7) d dilj+ L j+ = ( IS + ilj+ ) RBj+ VBj+, d Lj 2 * P i + ( ) = I (8) dvcj C j = il j, vcj( 2 ) = 0 (9) d The compac sae equaion for describing of he hree saes menioned above can be combined and simplified as: R ( u)( u) 2 Bj 0 2 VB + I j srb di j Lj Lj Lj d Lj 2 uu ( ) i di R V + I R Lj Lj+ B 0 2 B s B i Lj d = + Lj+ L + j+ L j+ vcj dv Cj d j+ j+ j+ 2( u)( u) 2 u( u ) 2 2 0 0 Cj C j (0) where u is a ri-sae swiching conrol variable. I is also suggesed ha u = (HIGH sae) denoes Q j urn-on and D j+ urned off shown as Fig. 3 (a), u = 0 (LOW sae) denoes Q j is urned off and D j+ is urned on shown as Fig. 3 (c), and u = 0.5 (FLOATING sae) denoes Q j and D j+ are boh urned on and υ cj = 0 shown as Fig. 3 (b). The inernal resisance of baery cell R B is negleced o simplify he seady sae circui analysis, and he charging/discharging source effec is absen in he principle operaion of he converer. If L j and L j+ are large enough, he ripples in i Lj and i Lj+ are small. The ime average values of i Lj and i Lj+ are denoed as I Lj and I Lj+, respecively. From Fig. 5(a), he condiions of DCVM operaion can be derived as follows. The insananeous capacior volage in he full duy cycle of he DCVM Cûk converer can be expressed by
ν Cj ILj ( D) TS ILj+, for 0 < < DT S C j = 0, for D T < < DT I j( DTS), for DT S < < T S C j S S () The power MOSFETs of he modified QRZVS baery equalizer are urned off a he zero curren. The sub-duy raio D in erms of he duy raio D can be governed as: D = ( D ) I L j I L j + (2) are denoed as V cj and V Dj+, respecively. This can be The ime-average of volages of υ cj and υ D j + deermined from () as Ts Vcj = I ( D)( D + D ) Lj (3) 2 C j T s V = I ( D) D Dj+ Lj (4) 2 C j Therefore, he erminal volages of he baery cells and he volage conversion raio are T s V = V = I ( D) D Bj+ Dj+ Lj (5) 2C j Ts 2 V = V + V = I ( D) Bj Dj+ Cj Lj (6) 2C j V Bj+ V Bj = D D Combining (2) and (5) and subsiuing ino (7) o yield (7) I Lj = 2 f s C jv B j ( D ) V 2 Bj+ = 2 s j (9) I Lj+ D f C To operae he proposed ICE in DCVM, he inequaliy should be saisfied, or equivalen (8) V I Bj + 2 (20) S j j + D f C 2 V Bj+ ( D ) L j (2) 2 Df I j + S 2
L j + V Bj+ ( D ) (22) 2 f I S j + The swiching boundary surface of he converer o operae beween DCVM and coninuous-capacior-volage mode (CCVM) is depiced in Fig. 6. However, he proposed converer may no operae in DCVM when V Bj is small because he cell volages do no have consan dc volage and depending on he equalizaion curren of he ICE, herefore, he inequaliies (20), (2) and (22) are used o design he proposed ICE ha can be guaraneed o operae in DCVM [5], [6]. Duy cycle (D) DCVM CCVM Curren (I) Volage (V) Fig. 6 Swiching boundary surface beween DCVM and CCVM 3
Fig. 7 Configuraion of MATLAB/SimuLink model 4
(V) (A) (V) (A) Time (ms) Fig. 8 Simulaion resuls (V) (s) 9. (a) 5
Vc(V) Vgs(V) Vgs Vc 5V/div A/div 20us/div Time(sec) 9. (b) (V) (s) V T (V) Vgs(V) Vgs 9. (c) V T 5V/div 5V/div 0us/div Time(sec) 9. (d) Fig. 9 Simulaion and experimenal resuls of MOSFET conrol signal V gs and drain-source volage V T for V B >V B2 >V B3, (a)(c) Simulaions, (b)(d) Experimens 4. Simulaion and experimenal resuls 4. Three lihium-ion cells module (n=3) In order o validae he performance of he proposed bi-direcional baery equalizer, a Malab/Simulink simulaion and an experimen were carried ou for a hree cells baery module (n=3) wih he wo proposed baery equalizers (2 ICEs). Malab/Simulink simulaion was performed for mahemaical model of ICEs. A hree-modular baery sack wih wo proposed equalizaion schemes was used o verify he analysis resuls menioned above. The simple signal flow graph is defined using Malab/Simulink simulaion for he proposed 6
ICE Malab/Simulink model for a hree baery-cell, he ICE (composed of L, L 2, C, Q and Q 2 ) and he ICE 2 (composed of L 3, L 4, C 2, Q 3 and Q 4 ) comprise he block diagram shown in Fig. 7. The baery sorage elemens were simply assumed for he baery charge/discharge model, which was esablished by a baery charge/discharge profile wih equivalen series resisor (ESR) from he library of he Malab/simulink block model. The baery iniial volages, inducors and energy ransferring capacior wih ESR were se as V B = 4.0 (V), V B2 = 3.9 (V), V B3 = 3.6 (V), L = L 2 = L 3 = L 4 = 230µH and C = C 2 = 0.66µF wih ESR = 0.00Ω, respecively. The swiching frequency was 6.67 khz and he duy raio was D = 0.53 for boh V B >V B2 >V B3 and V B <V B2 <V B3 o ensure ha he proposed bi-direcional dc-o-dc converer could be designed o operae in DCVM. This can obain a zero volage swiching (ZVS) o reduce he MOSFET swiching loss in he proposed ICEs. Fig. 8 shows he simulaion volages and currens for he capacior and inducor in ICE, respecively. The MOSFET conrol signal V gs and he corresponding drain-source volage V T for V B >V B2 >V B3 are shown in Figs. 9 (a) and 9 (c). The simulaion resuls of he cell volage rajecories under saic sae, added A charging and discharging curren saes of he proposed baery equalizer are illusraed in he Figs. 0 (a), (c) and (e), respecively. The cell balancing process is sopped when he cell volage is equalized o he same end-of-charge or end-of-discharge sae. The experimenal insallaion of a hree-modular lihium ion baery sack wih he proposed equalizaion scheme is used o verify he equalizaion performance of he hree cells baery sack wih he proposed ICEs. The driving signals for he equalizaion schemes are conrolled using a microprocessor-based baery managemen sysem according o each cell volage. The driving signals are consruced using a logical swiching algorihm, and insruced by he BMS processor of AT89C52. Cell volages were balanced wihin 0.096 (V), due o he hardware resoluion, which was limied by he analogue o digial converer (ADC0804). The volage balancing process is sopped when he BMS sends an execuable command o cu off he MOSFET. The experimenal parameers of he baeries and he designed ICEs are lised as follows: The iniial volages of he hree lihium ion baeries MRL/ITRT 0AH are 4.0 (V), 3.5 (V), and 3.0 (V), respecively. The MOSFETs wih body diodes are IRF530. The inducances are L ~L 4 = 230µH, C = C 2 = 0.66µF. The swiching frequency and he duy raio of he baery equalizer are 6.67 khz and 0.53, respecively. 7
Figs. 9 (b) and 9 (d) show he measured volages of V gs, V c, and MOSFET drain-source volage V T, respecively. The ransien oscillaions in he drain-source volage of MOSFET due o he fas swiching ransien effec can be suppressed by well designed urned off DRC snubber circuis in he swiching devices. Figs. 0 (b), (d) and (f) show he experimenal resuls of he cell volage rajecories under saic sae, added A charging and discharging curren saes of he proposed baery equalizers. Therefore, he equalizaion mehod can balance all he adjoining cell volages of he baery sring o he same volage level. Consequenly, each cell can be simulaneously charged o he end-of-charge volage, so he oal charging capaciy of he baery sring would be increased. Fig. shows he waveform and he corresponding FFT specrum of MOSFET swich power losses, P T = V T * i T. The experimenal resuls of he equalizaion efficiency of ICE under various operaing modes for he specified equalizaion processing are shown in Fig. 2 (a). The average equalizaion efficiency of he ICE operaed as DICM can be improved from 52% o 60% compared wih he equalizer operaed as CICM, he maximum equalizaion efficiency can achieved 62% for his designed es sample. When i is designed as DCVM, he oal power losses of he MOSFETs in he baery equalizer can be significanly reduced from 33.5% o 52.8% compared wih he same equalizer operaed a CICM. The average equalizaion efficiency can be improved from 52% o 68% compared wih he equalizer operaing as CICM, and he maximum equalizaion efficiency can achieve above 70% for his designed case. Using opimally designed passive elemens and acive devices he equalizaion efficiency can reasonably improve. The experimenal insallaion of he proposed equalizer is redesigned by he following devices: The MOSFET is chosen as a SBL040, Schoky diode is seleced AM20N06-90D, and he low ESR inducor is wound by a sranded-wires PVF (0.4mm 4) around a SENDUST core. Fig. 2 (b) shows he equalizaion efficiency of he reformed equalizaion scheme under a specified equalizaion process, he average and he maximum equalizaion efficiency can achieve 86.9% and 89.8%, respecively. The alernaing sof swiching echnology in he ICE design for a fuure sudy can significanly improve efficiency. Table shows he differenially designed resuls and performance comparison of he ICEs operaed a CICM, DICM and DCVM under he specified equalizaion procession and equalizing curren, respecively. Several observaions and comparison abou he proposed baery equalizer can be summarized as in he following secion. 8
VB VB2 VB3 (V) (ms) 0(a) 0(b) VB VB2 VB3 (V) (ms) 0(c) 0(d) VB2 VB VB3 (V) (ms) 0(e) 0(f) Fig. 0 Simulaion and experimenal resuls of cell volage rajecories for V B >V B2 >V B3, (a) (b) saic sae, (c) (d) added A charging curren, (e) (f) added A discharging curren 9
. (a). (b) Fig. (a) Waveform and, (b) FFT specrum of MOSFET swich power losses Fig. 2 (a) 20
Fig. 2 (b) Fig. 2 Equalizaion efficiency of ICE under various operaing modes (a) Original equalizaion scheme (b) Reformed equalizaion scheme 2
Table Comparisons beween coninuous and disconinuous modes CICM DICM DCVM Inducor (L j ) 98.3µH 00.5µH 229.2µH Inducor (L j+ ) 00.7µH 0.2µH 230.3µH Capacior (C j ) 470µF 470µF 0.66µF Swiching Frequency (f s ) 8. khz 6.67 khz 6.67 khz Duy Cycle (D) 0.53 0.5 0.5 Boundary Condiion L L j f j+ s f V > s j+ V > ( D) 2D I j+ 2I j+ j+ 2 ( D) L L j f j+ s f V < s j+ V < ( D) 2D I j+ 2I j+ j+ 2 ( D) D > 2 f C s j V I j+ j+ Volage Sress Eq. (23) Eq. (23) Eq. (23) Curren Sress Eq. (24) Eq. (24) Eq. (24) Sof Swiching Characerisic Applicaions Equalizaion ime NO ZCS ZVS Low power Low curren, High volage High curren, Low volage Shor Long Long 4.2 Twelve lihium-ion cells module (n=2) The oher experimenal insallaion of a welve lihium ion baeries sack wih he eleven proposed equalizaion schemes ( ICEs) is used o verify he equalizaion performance of he proposed ICEs. The driving signals for he equalizaion schemes are from a microprocessor-based baery managemen sysem according o each cell volage. The experimenal parameers of he baeries and he designed ICEs are lised as follows: The 22
iniial volages of he welve lihium ion baeries MRL/ITRT 50AH in charging and discharging es are shown in he below of Figs. 3, 4, 5 and 6, respecively. The MOSFETs wih body diodes are IRF530. The design parameers, swiching frequency and he duy raio of he eleven ICEs are he same as he aforemenioned esing case. Figs. 7 (a) and 7 (b) show he phoography of he 2-cells lihium-ion baery pack and he prooype of he proposed ICEs. Two schemes, one is baery sring wihou ICEs and he oher is wih ICEs are used o demonsrae he performance of he proposed equalizaion mehod. The auomaic baery esing equipmen is MACCOR 4000, he charging and discharging esing cycles are se as follows: CHARGING STEP_ I=25A A: Volage>50.4V NEXT STEP B: Cell Volage>4.2V NEXT STEP C: T>50 STOP STEP_2 I=0A A: Volage>50.4V NEXT STEP B: Cell Volage>4.2V NEXT STEP C: T>50 STOP STEP_3 I=5A A: Volage>50.4V NEXT STEP B: Cell Volage>4.2V NEXT STEP C: T>50 STOP STEP_4 I=2.5A A: Volage>50.4V STOP B: Cell Volage>4.2V STOP C: T>50 STOP DISCHARGING I=50A A: Volage<33V STOP B: Cell Volage<2.75V STOP C: T>50 STOP 23
Figs. 3 and 4 show he charging/discharging cell volages and he modular capaciy under he charging es for he sring wihou and wih he proposed ICEs. The final saes of he baery sring are: he maximal cell volage deviaion is 52mV, he oal charging capaciy is 2440 Whr and he charging ime for reaching he end-of-charge sae is 3.45 minues for he sring wihou ICEs. When he sysem is equipped wih he proposed ICEs, he maximal cell volage deviaion is decreased o 20 mv, he oal charging capaciy is increased o 2480 Whr and he charging ime for reaching he end-of-charge sae is exended o 43.36 minues. Figs. 5 and 6 show he discharging cell volages and he modular capaciy under discharging es for he same esing schemes. The final saes of he baery sring are: he maximal cell volage deviaion is 0.63V, he oal discharging capaciy is 2343 Whr and he discharging ime for reaching he end-of-discharge sae is 6.46 minues under he sring wihou ICEs. When he sysem is equipped wih he proposed ICEs, he maximal cell volage deviaion is decreased o 37 mv, he oal discharging capaciy is increased o 2379 Whr and he discharging ime for reaching he end-of-discharge sae is exended o 65.43 minues. By using he proposed ICEs for he lihium-ion baery sring, each cell can be simulaneously charged/discharged o he end-of-charge/discharge sae. The oal charging/discharging capaciy of he baery sring is improved under he safe operaion specificaions. 24
4.20 2500.00 4.00 2000.00 Cell Legend Volage Tile 3.80 Cell Cell 2 500.00 Ceii 3 Cell 4 Cell Volage (V) 3.60 3.40 Module Legend Capaciy Tile Whr Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 0 Cell Cell 2 000.00 500.00 Module Capaciy (Whr) 3.20 0.00 0.00 2000.00 4000.00 6000.00 8000.00 0000.00 Time(sec) Iniial and final cell volage for charging es Module Daa Cell Volage Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Iniial 60 42.30 0.4 7 3.493 3.534 3.536 3.49 3.529 3.550 3.540 3.540 3.529 3.535 3.533 3.539 Final 8487 50.06 50.8 2440 4.200 4.86 4.82 4.97 4.83 4.82 4.48 4.80 4.83 4.57 4.58 4.55 Fig. 3 Charging curves of 2 cells wihou ICEs 25
4.20 2500.00 4.00 2000.00 Cell Legend Volage Tile 3.80 Cell Cell 2 500.00 Ceii 3 Cell Volage (V) 3.60 3.40 Module Capaciy Legend Tile Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 0 Cell 000.00 500.00 Module Capaciy (Whr) Whr Cell 2 3.20 0.00 0.00 2000.00 4000.00 6000.00 8000.00 0000.00 Time(sec) Iniial and final cell volage for charging es Module Daa Cell Volage Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Iniial 60 42.24 0.4 7 3.400 3.378 3.390 3.357 3.379 3.400 3.40 3.43 3.405 3.44 3.42 3.42 Final 8602 50.03 5.0 2480 4.95 4.94 4.92 4.95 4.92 4.88 4.92 4.89 4.89 4.90 4.90 4.90 Fig. 4 Charging curves of 2 cells wih ICEs 26
4.40 0.00 4.00-500.00 Cell Legend Volage Tile 3.60 Cell Cell 2-000.00 Cell Volage (V) 3.20 2.80 Ceii 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 0 Cell Cell 2 Module Capaciy Legend Tile Whr -500.00-2000.00 Module Capaciy (Whr) 2.40-2500.00 0.00 000.00 2000.00 3000.00 4000.00 Time(sec) Iniial and final cell volage for discharging es Module Daa Cell Volage Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Iniial 60 48.36-0.8-40 4.042 4.034 4.036 4.044 4.039 4.047 4.020 4.037 4.045 4.025 4.07 4.028 Final 3688 37.29-5.2-2343 2.727 3.47 3.86 2.73 3.57 3.262 3.28 3.97 3.88 3.77 3.36 3.206 Fig. 5 Discharging curves of 2 cells wihou ICEs 27
4.40 0.00 4.00-500.00 3.60 Legend Tile Cell Volage Cell Cell 2-000.00 Cell Volage (V) 3.20 2.80 Ceii 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 0 Cell Cell 2 Module Capaciy Legend Tile Whr -500.00-2000.00 Module Capaciy (Whr) 2.40-2500.00 0.00 000.00 2000.00 3000.00 4000.00 Time(sec) Iniial and final cell volage for discharging es Module Daa Cell Volage Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Time Volage Ahr Whr Cell_ Cell_2 Cell_3 Cell_4 Cell_5 Cell_6 Cell_7 Cell_8 Cell_9 Cell_0 Cell_ Cell_2 Iniial 60 48.36-0.8-40 4.034 4.038 4.037 4.04 4.040 4.043 4.032 4.034 4.040 4.032 4.025 4.026 Final 3926 32.849-52.2-2379 2.74 2.749 2.749 2.758 2.756 2.758 2.739 2.740 2.747 2.736 2.72 2.730 Fig. 6 Discharging curves of 2 cells wih ICEs 28
Fig. 7 (a) ICEs module Fig. 7 (b) Baery pack module Fig. 7 Phoography of 2-cells lihium-ion module 5. Comparison of baery equalizer in DICM and DCVM In order o obain a more complee comparison abou he use of bi-direcional converers operaing in coninuous and disconinuous modes for he baery equalizer, design resuls and performance will be furher evaluaed in deail, based on he same equalizaion condiions as in he Table. The proposed ICE schemes operaing in CICM, DICM and DCVM can perform he cell volage equalizaion, selecing a suiable operaing mode is based on various sysem desired feaures [5], [2]. The curren ripple in he CICM and DCVM are smaller han ha in he DICM. Consequenly, he equalizaion curren in he DCVM is smaller, i needs a compensaing conroller o improve he equalizaion ime during cell balancing process. For he inrinsic characerisics, he maximum volage sress on a MOSFET swich, V ds max, occurs in he ime inerval when he swich is urn-off and he diode is urned on. The maximum volage sress on a diode, V D max, occurs when swich is urned on and he diode is urned off. The volage sress can be expressed as 29
V Cj VBj + VBj+ for CICM V ds max = VD max = V Cj VBj + VBj+ for DICM (23) 2V Bj for DCVM D The volage sress in DCVM is higher han ha in he CICM and DICM under he same erminal and specified equalizaion condiions. The maximum curren sress on a MOSFET swich and diode, I ds max and I D max a he specified ime duraion, i can be shown as L j I Pk ( + ) for CICM L j + 2VBjLj+ Ids max = I D max = I Lj ( ) for DICM VBj+ LjD VBj I Pk ( + ) for DCVM VBj+ (24) where I Pk = V Bj DT s /L j and I Lj = V Bj (D+ )DT s /2L j, and denoes he duy raio when a swich is urned on in DICM. The curren sress in DICM is higher han ha in CICM and DCVM under he same equalizaion condiions. Table shows a comparison of ICE characerisics in CICM, DICM and DCVM, respecively. Deailed illusraion and observaion from Figs. 8-2, 3-6, and Table show several feaures of he proposed baery equalizers ha are summarized and revealed as follows: The maximum volage sresses in he swich and he diode in he DCVM are higher han in he oher wo modes. The maximum curren sress is significanly reduced compared wih he equalizer designed o operae a DICM. The sresses are compared and illusraed in (23) and (24). The power MOSFET swiches of he proposed baery equalizer are urned off in he zero volage sae. The oal power losses of he MOSFETs in he baery equalizer can be significanly reduced from 33.5% o 52.8% compared wih he same equalizer operaed a CICM. The average equalizaion efficiency can be improved from 52% o 68~86.9% compared wih he equalizer operaed a CICM. The maximum equalizaion efficiency of 72~89.8% can be achieved for he DCVM designed sample. 30
The charged and discharged capaciies in he 2-cells lihium-ion baery-sack module are increased.64% and.54% compared wih he baery sring wihou equipped he proposed ICEs, respecively. The DCVM ZVS and DICM ZCS Cûk converer have spen slighly more equalizaion ime o balance he cell volage o reach he end-of-charge sae. Therefore, as a fuure sudied of a smar lihium-ion baery managemen sysem, i is necessary o design an equalizaion conroller, which speeds up he equalizaion processing. 6. Conclusion An ICE for he ZVS sof-swiching of DC/DC converers has been proposed. The zero-volage-swiching echnique can grealy reduce he power losses of MOSFET swich was implemened. The proposed ICE s MOSFET is urned off and he body diode is urned on a zero volage of he capacior in DCVM. When he capacior volage approaches zero hen he body diode of he MOSFET is urned on unil he capacior energy is compleely ransferred o a weaker baery cell. Therefore, he MOSFET swich power losses are reduced by abou 52.8% more han in CICM. The MOSFET swich power losses and he corresponding FFT frequency specrums of he proposed baery equalizer in he DVCM are reduced han hey are in he DICM and CICM. The energy harmonic specrum is concenraed in he low frequency for CICM, and is dispersed low o higher frequencies in DCVM. Hence, he high frequency EMI emission is improved in a series-conneced baery energy sysem wih DCVM designed ICEs. The performance and capaciy of he series conneced lihium-ion baery sring are improved by using he proposed baery equalizaion echnology. Acknowledgmens This work was financially suppored by he Naional Science Council of Taiwan, under gran NSC 92-223-E-030-020 and NSC 93-2745-E-030-002-URD. The auhors would like o hank he MRL, ITRI, Taiwan for supplied he MRL/ITRI 0AH and 50AH lihium-ion baeries and esing. 3