Individual Cell Volage Equalizer Usg Selecive Two Curren Pahs for Series Conneced Li-ion Baery Srgs Chol-Ho Kim, Young-Do Kim, Gun-Woo Moon and Hong-sun Park 2 Division of EE, Korea Advanced Insiue of Science and Technology, Daejeon, Republic of Korea 2 Advanced Tech. T/F, Samsung Elecro-Mechanics E-mail: railroads@angel.kais.ac.kr Absrac -- This diges proposes an dividual cell volage equalizer usg selecive wo curren pahs for series conneced lihium-ion baery srgs. In he proposed equalizer, a cenral equalizaion converer is shared by every baery cells hrough he cell selecion swich, sead of a dedicaed charge equalizer for each cell. The cenral converer is uilized as a conrollable curren source by a modified recifier. Wih his configuraion, alhough he proposed equalizer has a cenral dc-dc converer, dividual charge equalizaion can be effecively achieved for he each cell he srgs. Furhermore, grea size reducion and low cos can be obaed for an dusrial baery equalizer. In his diges, an opimal power rag design guide is also employed o oba a mimal balancg size while saisfyg equalizaion requiremens. A prooype for eigh lihium-ion baery cells is opimally designed and implemened. Experimenal resuls verify ha he proposed equalizaion mehod has good cell balancg performance. Index Terms charge equalizaion, baery equalizer, lihium-ion baery. I. OMECLATURE B h cell he baery srg. S h selecion swich. Q under Charge quaniy of he under-charged cell. Q n () Charge quaniy of he nh cell a ime. V n () Volage of he nh cell a ime. V under () Volage of he under-charged cell a ime. V SOC(%) Open circui volage a percenage of sae-ofcharge (SOC (%) ). I Consan pu curren of he equalizer. I ou Consan oupu curren of he equalizer. P () Inpu power of he equalizer a ime. P ou () Oupu power of he equalizer a ime. P,avg Average pu power of he equalizer. P ou,avg Average oupu power of he equalizer. η Overall efficiency of he equalizer. II. ITRODUCTIO Series conneced baery srgs is found o be used many pracical applicaions such as elecric vehicles (EVs), hybrid elecric vehicle (HEV), unerrupable power supplies (UPS) [], [2]. In hese applicaions, he baery havg a long life ime is economically imporan. However, repeiion of baery chargg and dischargg can cause a volage imbalance among he baery cells due o heir differen characerisics. The volage imbalance decreases he oal sorage capaciy and he whole life cycle of he baeries [3], [4]. Hence, he baery equalizaion conrol should be realized o balance he volage baery srgs. When he lihium-ion baery is applied he pracical applicaion as he series conneced baery srgs, he necessiy of he cell volage balance is more imporan. Lihium-ion baery chemisry canno wihsand he overcharge. An overcharge has a high a risk for explosion. Moreover, an overdicharge of he baery cell can reduce he baery cell s lifecycle [5]-[7]. Therefore, a cell volage equalizer mus be employed o preven he over or undervolage sae of each baery cell lihium-ion baery srgs. The cell equalizer has been presened []-[6]. These equalizers can be classified o wo conrol ypes: a cenralized conrol based equalizer []-[] and a cell conrol based equalizer []-[6]. Each ype respecively has good pos abou he equalizaion performance, implemenaion, and cell conrol. However, he cenralized conrol based equalizer canno be direcly applied o he pracical baery equalizer. This is because ha he cenralized equalizer share he equalizaion curren wih overall baery cells. Hence, as he energy densiy of baery creases, he cenralized ypes canno easily expand heir equalizaion curren flowg o he unbalanced volage cell. For example, he swiched capacior mehod shows he prolonged equalizaion ime lihium-ion baery caused by a cell-o-cell energy shif [], [9]. The secondary muliple wdg mehods also clude he difficuly of implemeng a muliple wdg ransformer a sgle core for a large number of baery cells []. One of he cell conrol based equalizer is he resisive curren shun, which consis of a resisor and an acive swich []. This scheme is very popular mehod due o simple and easy implemenaion and low cos. The criical drawbacks of he resisive curren shun are he energy dissipaion and he hea problem. Oher cell conrol based equalizers are discussed [2]- [6]. These mehods allocae he separaed dc-dc converer o each cell, such ha he implemenaion and dividual conrol of equalizaion curren are easily achieved. Unlike he resisive curren shun, hese mehods show high energy efficiency. However, hese schemes also have problems of large size, high complexiy, and high cos for a large number of lihium-ion baeries. To improve hese defecs, his paper proposes he dividual cell equalizer by usg a cenralized curren source and he array of cell selecion swiches. This mehod is compaible equalizer havg boh meris; cenralized and cell conrol ones. In deail, he curren source is realized by he dc-dc converer wih a modified recifier, which can supply 97--4244-293-9/9/$25. 29 IEEE 2

wo oupu currens of he converer by conrollg curren pahs. Two curren pahs apply o balance he charge for even and odd baery cell respecively. In addiion, Individual baery cells can have a shared dc-dc converer acg as a dedicaed charge equalizer for each cell hrough he cell selecion swiches. In proposed mehod, by uilizg he common dc-dc converer and he cell selecion swiches, he size and cos of he baery equalizer can be dramaically reduced. Moreover, wih he selecive curren pah, he equalizaion can be effecively achieved. In his paper, a prooype of lihiumion baery cells employg he proposed mehod is opimally designed and implemened. The experimenal resuls show ha he proposed mehod has excellen cell balancg performance. III. PROPOSED VOLTAGE EQUALIZER A. Circui descripion Fig. shows he block diagram of he proposed charge equalizaion converer applied o baery cells. The proposed equalizer consiss of wo pars; he dc-dc converer, and selecion swiches. The dc-dc converer makes he equalizaion curren from overall baeries. The selecion swiches consis of he bi-direcional swiches for consrucg a curren pah beween dc-dc converer and seleced baery cell. Fig. 2 presens he configuraion of he cell selecion swiches and he dc-dc converer usg selecive wo curren pahs. The cell selecion swiches is consruced each baery. The curren source can be conrollable wih wo curren pahs for odd and even baery as shown Fig. 2(a), (b). Fig. 2(a) and Fig. 2(b) show he clockwise curren pah for chargg B and he counerclockwise curren pah for chargg B 2 respecively. In he proposed circui, he dc-dc converer wih modified recifier is employed o make wo chargg curren; clockwise and counerclockwise pah. B. Operaional prciple In he proposed charge equalizer, he volage balance is archived by ransferrg he equalizaion curren, which is exraced from he overall baery srg o he under-charged cells. To make his process, he proposed equalizer employs a baery managemen conroller (BMC) wih volage sensg circuiry. The baery managemen conroller collecs he sensg daa from he sensg circui and deermes he operag of he charge equalizaion he baery srgs. Then, i drives he charge equalizer wih hree consecuive seps. Before describg hese hree seps, i is assumed ha he hird baery, B 3, is under-charged. Sep : When he baery managemen conroller urns on he bi-direcional swiches, S 3 and S 4, he firs sep sars. In his sep, he curren pah for B 3 is consruced. As shown Fig. 3 (a), he charge curren can flow o B 3 hrough his curren pah. Sep 2: Afer complee urn-on of he bi-direcional swiches, he secondary swich, Q a, is driven by he BMC. As a resul, he secondary swich is conneced wih B 3 as shown Fig.. Block diagram of he proposed volage equalizer. Fig. 2. Operaional prciple of he selecive wo curren pahs. (a) Clockwise curren for odd baery. (b) Counerclockwise curren for even baery Fig. 3(b). In his mode, alhough he secondary swich connecs, he equalizaion curren does no flow o seleced cell. This is because he dc-dc converer is no urned on, such ha he charge curren does no flow ye. Sep 3: In his mode, he ma swich, Q m, of he dc-dc converer is urned on by he BMC. This sage ransfers he equalizaion curren from he baery sack o seleced baery cell. In his mode, he dc-dc converer can provide he equalizaion curren o B 3 as shown Fig. 3(c). The proposed circui needs anoher operaion process for chargg he even baery, B 4. Fig. 3(d), Fig. 3(e), and Fig. 3(f) show hree consecuive seps respecively o balance he fourh baery as like above menioned seps Fig. 3(a), Fig. 3(b), and Fig. 3(c). In hese seps, he baery managemen conroller urns on he bi-direcional swiches, S 4 and S 5 o make a chargg pah for B 4. Afer selecg he swiches, S 4 and S 5, he secondary swich, Q b, and he dc-dc converer are sequenially driven by he BMC. From hese operaion for chargg he hird baery and fourh baery, B 3 and B 4, we can fd wo changeable curren pahs he proposed equalizer. Fig. 4 shows he urn on process of he selecion swiches. 3

Fig. 3. Operaional prciple of he proposed circui. (a), (d) Sep. (b), (e) Sep 2. (c), (f) Sep 3. All of he bi-direcional selecion swiches is employed wih he opo-couplers o oba a gae signal of he MOSFET swich accordg o he on or off commend from BMC. There are wo kds of echniques o urn on he bi-direcional swiches. As shown Fig. 4, an -channel MOSFET swich can be urned on by usg he upper layer baeries, B, B 2, and B 3. The opo-couplers, O 4 and O 5, are execued o urn on he - channel MOSFET swiches. On he oher hand, a P- channel MOSFET swich can be urned on by usg he lower layer baeries, B 2, B 3, and B 4. The opo-couplers, O and O 2, give a gae signal o he MOSFET swich. This P-channel MOSFET swiches are used only for hree cells, B, B 2 and B 3. IV. THE POWER RATIG DESIG This secion presens he opimal power rag design guide for a charge equalizaion converer. The power rag of an equalizaion circui has a close relaion wih he equalizaion ime; ha is, he higher he power rag, and he shorer he equalizaion ime. We have o fd a way of deermg he opimal power rag o achieve he cell balance wih he desired equalizaion ime [5], [6]. Before preseng he power rag design, we should know he relaionship beween he esimaion of he sae-ofcharge (SOC) and he cell volage for he lihium-ion baery. Baery managemen conroller can decide he equalizaion level from SOC esimaion of he baery. However, he esimaion of SOC can be obaed by usg many variables a very complex algorihm [7], []. In his paper, he SOC is represened wih only open circui volage of he baery cell for simpliciy. The prooype design scheme is applied o a lihium-ion baery srg of cells, where only one cell is assumed o be under-charged. To predic he equalizaion ime for he prooype work, he equalizaion curren of he prooype dcdc converer should be calculaed. This equalizaion curren dicaes he oupu curren of he dc-dc converer. The relaion beween he equalizg curren and he equalizaion ime can be found by he followg wo simulaneous equaions. n Q ( ) k Q under P ou, avg P, avg ( ), () η (2) where he lef side of () is he average charge quaniy of he overall baeries a equalizaion ime, and he righ side is he charge quaniy of he under-charged baery cell a equalizaion ime. Equaion (2) dicaes he relaion beween he pu power and he oupu power of he proposed equalizer wih efficiency of η. The pu power means he equalizaion power which is exraced from he whole baery sack, and he oupu power means he equalizaion power which flows o he under-charged cell. Furhermore, amoun of average charge remaed he under-charged cell, Q () and average oupu power of he equalizer, P ou,avg is given by, respecively. Q ( ) Q () + I Q () + ( I ), (3) pou ( τ ) dτ ou avg vunder ( τ ) I, ou P v ou dτ ( I ou ) τ () + I ou dτ C vunder ( ) + ( I ou ) I (4) ou 2C under where C is capaciance of he 7 Ah lihium-ion baery. In (3) 4

Fig. 4. Turn-on process of selecion swiches. (a) In case of -channel MOSFET. (b) In case of P-channel MOSFET. and (4), I ou and I are he oupu curren and he pu curren of he proposed equalizer. The more specific equaion has presened appendix his paper. Fig. 5 shows he simulaion resuls of he power rag design. By usg his simulaion resuls, he power rag of he equalizer can be obaed accordg o he difference of he SOC. In his simulaion, he efficiency of he proposed equalizer is assumed o be %. As shown Fig. 5, he equalizaion ime is ploed relaion o he equalizaion curren of he equalizer. From hese resuls, we know ha he shorer equalizaion ime will be aken for he higher equalizaion curren of equalizer. In addiion, he equalizaion ime also is prolonged as he SOC gap crease. As one design example, o oba charge balance wih 3 mues, he equalizaion curren of abou.5 A is required a % difference of he SOC. Fig. 5. Simulaion resul of he equalizaion ime. V. EXPERIMETAL RESULTS In order o verify he operaional prciples and show he performance of he proposed charge equalizaion converer, a prooype of eigh lihium-ion cells is designed and implemened. As shown Fig. 6, he prooype consiss of a dc-dc converer, 9 cell selecion swiches, S ~ S 9, are esablished. The drivg signals for he dc-dc converer, he secondary swiches and he cell selecion swiches were conrolled usg a microconroller-based BMC accordg o he sensed cell volages. The secondary swiches of he ransformer he dc-dc converer are selecively urned on by he BMS signal. Fig. 7 presens he urn-on process of he secondary MOSFET swiches by uilizg he opo-couplers, O a, O b. As shown Fig. 7 (a) and Fig. 2(b), he urn-on mehod can be divided wo ways for he secondary MOSFET swiches, Q a and Q b respecively. Table I is a comparaive sudy on he proposed charge he number of componen, balancg performance and sensg conrol circui. oe ha he number of baeries is assumed o be eigh cells. In addiion, he sysem size and cos crieria are evaluaed by he number of elecronic componens. As shown Table I, he proposed charge equalizer has he advanages of small size and high balancg performance. From his comparaive work, we conclude ha he proposed equalizer can be used for an equalizaion sysem ha requires a shor equalizaion ime and a good cell balancg performance a he expense of sysem size and cos. Fig. 6. Prooype work for eigh baeries. Fig. 7. Turn-on process of he secondary MOSFET swiches. (a) In case of urn-on process for odd baery. (b) In case of urn-on process for even baery. Table II summarizes he parameer of he proposed balancg circui and he sae of charge (SOC) of he baeries. Fig. shows he key waveform of he dc-dc converer he prooype equalizer. The dc-dc converer is implemened by usg a flyback ype converer. Inpu volage is 3.2 V, which is he sack volage of baery cells. As shown Fig., he oupu curren is regulaed wih.5a, and he maximum volage sress a he MOSFET swiches, V ds, does no exceed 9 V, even he volage spikes. The efficiency of he charge mode is approximaely 2.2%. To verify he cell 5

Fig.. The experimenal key waveform of dc-dc converer. balancg performance of he proposed charge equalizer, we conduced an equalizaion es. The baery SOC disribuion is as follows. One of even baery cell, B 2, discharge wih he % difference of he maximum SOC. Moreover, he odd baery cell, B 7, is lowes volage cell havg abou 5% SOC gap. The SOC of he remaed cells is abou 6%. In his es, he proposed balancg conrol is sequenially execued for he under-charged even and odd baery. The conrol sraegy of he BMC is wo seps. The firs sep is o fd he mos under-charged cell and hen, drives he proposed equalizer durg fixed equalizaion ime based on he simulaion resuls as shown Fig. 5. Fig. 9 shows he equalizer performance of he proposed equalizer. Afer he microconroller deecs he lowes volage cell, B 7, he equalizer drives he curren chargg o B 7 cell durg 42 mues. The nex cell, B 2, is also decreases is volage durg 7 mues by a conrol of he microconroller. From he prooype experimen, he equalizaion ime of 42mue is required o oba he volage balance of he DC-DC converer block Cell selecion swich block Balancg performance sensg circui TABLE I. COMPARISO OF EQUALIZER FOR BATTERY CELLS Fig. QRZCS [3] Selecive Buck-boos [4] Transformer Diode 2 6 Inducor 4 Capacior 2 7 2 MOSFET swich 3 4 2 MOSFET swich Equalizaion efficiency Equalizaion speed Conrol simpliciy 9 G E G E S G S S P E: Excellen, G: Good, S: Saisfacory, P: Poor Fig. 9. The resul of equalizaion es for lihium-ion baery cells. lowes SOC baery. In addiion, he proposed equalizer balances he % difference of he SOC durg 27 mue for even baery, B 2. In he es resuls of he prooype circui, average curren of.5a is designed o flow o he under-charged cell, and abou 7 mues was aken o achieve cell balancg as expeced by he simulaion resul as shown Fig. 5. The volage difference decrease from mv o approximae mv. VI. COCLUSIOS In his paper, dividual cell volage equalizer usg selecive wo curren pahs for lihium-ion baery cells was proposed, and a prooype was implemened. The equalizaion experimen wih he proposed mehod shows ousandg equalizaion performance. In proposed circui, by applyg he concep of selecive wo curren pahs and he selecion swiches, dividual charge equalizaion can be effecively achieved. Moreover, by sharg a dc-dc converer TABLE II. PARAMETER FOR THE PROPOSED CHARGE EQUALIZATIO CIRCUIT Balancg circui Parameers Ma swich Q M DC-DC converer Selecion swich Lihium-ion baery Recifier diode Value FQPF7 2CWQ3 Opocoupler IRF7A Secondary swich Q a (P-ype) FDS943A Secondary swich Q b (-ype) PS7B Core EPC92 Transfor mer : 2 5:6 P-ype L m L kg 25uH.34uH FPS995 -ype FDS9945 Capaciy (Ah) 7Ah Average SOC (%) 56.% Maximum SOC (%) 6% Mimum SOC (%) 45.3% 6

among he baery cells, he proposed equalizer solves a size and cos problem. Therefore, he proposed charge equalizer can be used widely for a high sack of lihium-ion baery cells. APPEDIX The average pu power of he equalizer, P,avg can be also expressed as ( τ ) dτ, avg p ( v ( τ )) P I dτ ( I ou ) τ I τ vunder () + + v () v () + ( I 2C ou C 2 ) I C dτ where C is capaciance of he 7 Ah lihium-ion baery. We can also oba he capaciy of he baery as followg equaion:.4 C 7Ah. V (7%) V SOC(3%) SOC SOC []. H. Kuku, H. L.. Wiegman, D. M. Divan, and D. W. ovony, Design consideraions for charge equalizaion of an elecric vehicle baery sysem, IEEE Trans. Ind. Appl., vol. 35, pp. 2-35, Feb. 999. [] B. Ldemark, Individual cell volage equalizers (ICE) for reliable baery performance, Proc. 3 h Annu. In. Telecommunicaions Energy Conf., Koyo, Japan, ov. 99, pp. 96-2. [2] Y. -S. Lee and M. -W. Cheng, Inelligen Conrol baery equalizaion for series conneced lihium-ion baery srgs, IEEE Trans. Ind. Elecron., vol. 52, pp. 297-37, Oc. 25. [3] Y. -S. Lee and G. -T. Cheng, Quasi-resonan zero-curren-swichg bidirecional converer for baery equalizaion applicaions, IEEE Trans. Power Elecron., vol. 2, pp. 23-224, Sep. 26. [4] M. Tang and T. Suar, Selecive buck-boos equalizer for series baery packs, IEEE Trans. Aerosp. Elecron. Sys., vol. 36, pp. 2-2, Jan. 2. [5] H. S. Park, C. E. Kim, G. W. Moon, J. H. Lee, and J. K. Oh, Two-sage cell balancg scheme for hybrid elecric vehicle Lihiumion baery srgs, Proc. IEEE PESC 7, Orlando, USA, June 27, pp. 273-279. [6] C. -H. Kim, H. -S. Park, C. E. Kim, G. W. Moon, and J. H. Lee, Individual charge equalizaion converer wih parallel primary wdg of ransformer for series conneced lihium-ion baery srgs an HEV, Journal of power elecronics., vol. 9, o. 3, pp. 472-4, May 29. [7] J. Chiasson, and B. Vairamohan, Esimag he sae of charge of a baery, IEEE Trans. Conr. Sys. Technol., vol. 3, pp. 465-47, May. 25. [] S. M. Lukic, J. Cao, R. C. Bansal, F. Rodriguez, and A. Emadi, Energy sorage sysems for auomoive applicaions, IEEE Trans. Ind. Elecron., vol. 55, pp. 225-2267, June. 2. ACKOWLEDGMET This work was suppored by he STSAT-3 program of he Korea Aerospace Research Insiue (KARI) gran funded by he Korea Misry of Educaion, Science and Technology (MEST). REFERECES [] P. T. Kre, and R. Balog, Life exension hrough charge equalizaion of lead-acid baeries, Proc. 24 h Annu. In. Telecommunicaions Energy Conf., VMonral, Canada, Sep. 22. [2] J. Chazakis, K. Kalaizakis,. C. Voulgaris, and. Manias, Designg a new generalized baery managemen sysem, IEEE Trans. Ind. Elecron., vol. 5, o. 5, pp. 99-999, Ocober 23. [3]. Takeda, S. Imai, Y. Horii, and H. Yoshida, Developmen of highperformance lihium-ion Baeries for hybrid elecric vehicles, Misubishi Moors Technical Review, pp. 6-72, Jan. 23. [4] B. T. Kuhn,, G. E. Piel, and P. T. Kre, Elecrical properies and equalizaion of lihium-ion cells auomoive applicaions Proc. 25 IEEE Vehicle power and Propulsion Conf., Chicago, USA, Sep. 25, pp. 55-59. [5] Y. -S. Lee, W. -Y. Wang, and T. Y. Kuo, Sof compug for baery sae-of-charge (BSOC) esimaion baery srg sysem, IEEE Trans. Ind. Elecron., vol. 55, pp. 229-239, Jan. 2. [6] W. F. Benly, Cell balancg consideraions for lihium-ion baery sysems, Proc. 2 h Annu. Baery Conf. Appl. Adv., Jun. 2-7, 997, pp.223-226. [7] S. W. Moore and P. J. Schneider, A review of cell equalizaion mehods for lihium ion and lihium polymer baery sysems, Proc. SAE World Congress, Deroi, USA, Mar. 2, Doc. o 2-- 959. [] C. Pascual and P. T. Kre, Swiched capacior sysem for auomaic series baery equalizaion, Proc. 2 h Annu. Appl. Power Elecron. Conf. and Exp., Alana, USA, Feb. 997, pp. 4-54. [9] A. C. Baughman and M. Ferdowsi, Double-iered swiched-capacior baery charge equalizaion echnique, IEEE Trans. Ind. Elecron., vol. 55, pp. 2277-225, June. 2. 7