Active Battery Balancing for Battery Packs

Transcription

1 do: /ecce Actve Battery Balancng for Battery Packs Javer Gallardo-Lozano (PhD student, Unversty of Extremadura UE), Abdul Lateef (PhD student, McMaster Unversty), Enrque Romero-Cadaval (Professor, UE), M. Isabel Mlanés-Montero (Professor, UE) Abstract In electrc vehcle applcatons, t s necessary to use seres strngs of batteres snce the requred voltage s hgher than the one that can be obtaned from a sngle battery. Due to several factors, mbalance of batteres n these battery systems s usual and an mportant factor that has to be taken nto account. Many balancng methods have been developed wth a lot of dfferent advantages, but all of them also have a lot of dsadvantages such as complexty and/or hgh cost, whch are the common problems that can be found n most of these equalzaton methods. In the present work, a low cost and very smple equalzaton method s proposed, n whch a novel control s appled to a shuntng transstor topology. It allows the transstors to regulate the amount of current that goes through each battery cell n the strng dependng on ther State of Charge (SOC), durng the chargng process. hs control ensures that the least charged cells to be charged faster, and the most charged ones to be charged more slowly. Desgn crtera are dscussed and smulaton results are carred out n a generc battery low power applcaton whch proves the control method. Fast equalzaton wth a low complexty and cost s obtaned. Keywords Batteres, current control, power MOSFE, power dsspaton. I. INRODUCION Along the last years, the research on electrc vehcles has become of specal nterest for many reasons such as envronmental awareness (whch leads us to a more effcent transportaton vehcles and Government ncentves), contnuous ncrease of the ol prce due to the natural resource exhauston and the nstablty n the Mddle East, etc. Hybrd electrc vehcles (HEVs) were the frst proposed soluton to the trend toward more electrc vehcles [1], and nowadays BEVs (Battery Electrc Vehcles) are comng out more and more. Dfferent battery technologes have been studed and Resstor Balancng Capactor Balancng Inductor Balancng Energy Converter Balancng utlzed for the EVs (Electrc Vehcles), but new researches are developng the Lthum-on batteres whch are becomng the most vable opton for portable and moble energy storage applcatons, especally for EVs [2]-[4]. Most of applcatons for batteres need hgher voltage than can be obtaned from a sngle electrochemcal battery and seres strngs of cells (each battery that makes up the whole battery pack s called cell hereafter unless otherwse noted) are used to meet voltage requrements n EVs [5]-[6]. Imbalance of cells n battery systems s very usual and an mportant matter n the battery system lfe. A battery system wthout a balancng technque can lead ts cells to be overcharged, undercharged, or even overdscharged, and t takes specal mportance n L-on cells [7]-[9]. he reasons of ths mbalance may be of two categores [10]: nternal sources (manufacturng varance n physcal volume, nternal mpedance and self-dscharge rate) and external sources such as thermal dfference across the pack. here are many dfferent balancng methods [11]-[13]. Balancng methods can be classfed nto passve balancng methods and actve balancng ones. In passve equalzaton an external overcharge s enforced to allow weak cells to reach the less weak ones. At hgh SOC (Battery State-of-Charge), the charge process for lead-acd and nckel cells becomes neffcent snce ts water begns to dssocate and energy goes nto electrolyss rather than the charge process. However, Lon cells do not have the aqueous component, and so do not have the gassng overcharge process that allows a passve equalzaton n other chemstres. Snce the L-on battery s the most mportant one, accordng to these prevous lnes, ths paper s focused on an actve balancng method. he dfferent actve methods are summarzed n able I [11]-[12]. ABLE I CLASSIFICAION OF ACIVE BALANCING MEHODS Shuntng Method Shuttlng Method Energy Converter Method Complete shuntng Dsspatve resstor Sngle nductor Mult-nductor Boost shuntng Sngle swtched capactor Swtched capactor Double-tered capactor Sngle-wndngs transformer Mult-wndngs transformer Multple-wndngs transformer Cûk converter Buck or/and Boost converter Flyback converter Ramp converter Full-brdge converter Resonant converter Quas-resonant converter 40

2 2013 / 2 Shuntng actve balancng methods remove the excess energy from the hgher voltage cell (or cells) wth the am to let them wat for the lower voltage cell (or cells) to catch up wth them. Shuttlng actve balancng methods utlze external energy storage devces (usually capactors) to shuttle the energy among cells n order to balance them. Energy converters are featured by fully control of balancng process. All of these balancng methods have been developed wth a lot of advantages, but all of them also have a lot of dsadvantages such as complexty and/or hgh cost, whch are some of the common features that can be found n most of these equalzaton methods. In the present work a low cost and very smple method s proposed, presentng a novel control for a shuntng transstor balancng method n a generc battery chargng process. In Secton II the proposed equalzaton system s presented and n Secton III the dfferent smulaton results are shown, valdatng ths proposed control balancng method. Fnally, Secton IV remarks the dfferent conclusons. II. EQUALIZAION SYSEM DESIGN he man dea of ths equalzaton system s to use a MOSFE workng n saturaton mode as a varable resstance, nstead of usng a resstance, whose value s fxed. he am of ths new control s to regulate the amount of current that goes through the cells that make up the battery n order to balance the whole battery system. A large number of advantages are obtaned by usng ths equalzaton system together wth the novel control. In addton to the advantages of the shunt resstor -for example relablty, effectveness, low cost, smple to mplement, sutablty n modular desgn, fast equalzaton durng chargng, etc.- t reduces dsadvantages such as energy losses, snce the bypassed current through the MOSFE decreases when ts correspondng cell s beng closer to be balanced or even t stops beng bypassed under some condtons. he bypassed current through the MOSFE s regulated varyng ts gate-source voltage accordng to Fg.1. Fg.1. Regulaton of the bypassed current though the MOSFE. Example of the SS452DN MOSFE curve. Fg.2. Confguraton of the equalzaton system for two cells. Dstrbuton of the currents n battery chargng mode. he hgher the gate-source voltage of the MOSFE s, the hgher the bypassed current through the MOSFE s, and therefore the less the correspondng cell s charged, and vce versa. In order to be able to equalze durng the chargng process or durng the regeneratve brakng, a MOSFE s set n parallel to each cell of the strng. he MOSFE s an n- channel devce. he topology s shown n Fg.2. he man dea of the equalzer s to measure the voltage of every cell that makes up the battery, set the average voltage of the aforementoned measurements whch s consdered the reference one, and extract the dfference between every cell voltage and the cells average value. Durng the battery chargng process, cells wth lower voltage than the average one need to be charged as much as possble n order to reach the average value as soon as they can. hese cells receve all the current that comes to the battery from the charger, and therefore ther correspondng MOSFEs (n-channel one) are set to the off-state and they do not bypass the cell. On the other hand, cells wth hgher voltage than the average one need to stop beng charged, n order to approxmate to the average value more and more snce ths mentoned average value s ncreasng as the lower voltage cells are beng charged. Once all the cells reach the reference the battery can be consdered balanced. Wth the am to optmze the chargng process, nstead of bypassng all of the current of the hgher voltage cells untl the battery s balanced and then keepng chargng all the cells together, the hgher voltage cells are charged n a lower rate of current, bypassng the remanng current. he closer to the average value the hgher cell voltage s, the lower the amount of current s bypassed by the correspondng MOSFE. Once the cell voltage reaches the average one, the MOSFE s set to the off-state and all of the current that comes to the battery goes through the cell. It allows the system to decrease the energy losses. 41

3 v average Cell 1 Cell 2 t t t t t 0 stop t 1 t t 2 chargng t 0 t t 1 balancng t 1 t t 2 balanced chargng t t charger Cell 1 Cell 2 = = = 0 charger Cell,1 Cell,2 charger = Cell,2 charger= Cell,1= Cell,2 Cell,1 Fg.3. Schematc of the batteres and average voltages n an a) unbalance system. b) chargng equalzaton process. he aforementoned process of equalzng under the charge condtons s depcted n Fg.3. In order to decde how much current s bypassed when a cell s unbalanced, a maxmum dfference dff max between the cell voltage and the reference (the average value) s set. he dfference of the -cell dff s defned as dff V V where: V s the voltage of the battery (cell) number and bat bat bat, (1) V bat s the average voltage of the whole battery system, obtaned as N 1 V bat V, (2) N 1 bat where N s the number of batteres connected n seres that bulds the whole battery system. When the dfference of an unbalanced cell s equal or hgher than dff max, all of the current s bypassed through the correspondng MOSFE, and therefore the system s watng for the cell voltage to approach to the average one. When the dfference s equal or lower than dff max, the current s partally bypassed to the cell, and the dfference keeps decreasng, untl the error (the dfference between the cell voltage and the reference one) s zero, when all of the current must go through the cell, so the MOSFE s set to ts off-state. As t has been mentoned and also shown n Fg.1, the amount of bypassed current s set by the gate-source voltage of the MOSFE (nchannel). Accordng to the prevous lnes, the gate-source voltage s obtaned as a lnear functon of the dfference between the cell voltage and the average one. Fg.4 represents such relatonshp. Fg.4. Lnear relaton between the gate-source MOSFE voltage and the dfference between the cell voltage and ts reference one. For an n-channel MOSFE, the equaton of the dran current n saturaton mode s K 2 ID VGS V 1 VDS, (3) 2 where I D s the MOSFE current, K s the conducton parameter, V GS s the gate-source voltage, V s the threshold voltage, λ s the lambda parameter and V DS s the dran-source voltage. he MOSFE works n the saturaton regon whenever t s kept workng fulfllng VGS V, (4) and VDS VGS V VGS VDS V. (5) Accordng to (4), V GS,mn from Fg.4 s obtaned as V GS,mn V. (6) V GS,max from Fg.4 s found from (3) accordng to V GS,max 2I K 1 V bat,max he equaton of the lne n Fg.4, s obtaned as whch leads to and therefore DS V V V V dff 0 dff 0 GS GS,mn GS,max GS,mn max 2I bat,max V V V K1 VDS GS V dff dff max max V. (7), (8), (9) 2Ibat,max K1 VDS. (10) VGS dff V dff 42

4 2013 / 2 he lower the dfference s, the lower the bypassed current s, and the less the equalzaton s beng carred out. It means that the reference would be lke an asymptote that s reached n the nfnte tme. Once the cell voltage s nsde a very narrow margn of error (very small dfference wth the reference) the bypassed current s neglgble, but before reachng ths margn the speed of the equalzaton process was slower and slower, that ncreases the energy losses, and after havng reached ths cted margn, the system keeps equalzng untl the nfnte tme that s when the reference s reached also decreasng the effcency. In order to decrease the energy losses even more, and therefore optmze the control more, an ntermedate level of V GS (and dff) s set. When an unbalanced cell starts the equalzaton process from a hgh dfference, the control s kept accordng to Fg.4, but when the dfference reaches the mentoned ntermedate level, from that moment ths dfference s consdered fxed (although t really keeps decreasng), and so the bypassed current s fxed nto an ntermedate value that allows the cell voltage to get zero error (zero dfference between the cell and reference voltage). Once the cell voltage comes to zero error, and ts correspondng MOSFE s set to off-state, the cell equalzaton process s stopped. Consderng that there can be manufacturng and thermal varances between cells, ts voltage could be oscllatng around the reference that could cause the actvaton and deactvaton of the equalzaton process, and a consderable ncrease n the energy losses. he way ths problem s solved conssts of the utlzaton of the aforementoned ntermedate dfference level. When the cell voltage gets the zero error, the equalzaton process s deactvated untl the dfference comes to the ntermedate level that s when the equalzaton s actvated agan. hs dea avods energy losses whch can decrease the effcency of the system. he algorthm that decdes whether the n-channel MOSFE are actvated or deactvated s shown n Fg.5. he algorthm that explans how the gate-source MOSFE voltage s calculated s shown n Fg.6. Fg.5. Algorthm of the actvaton/deactvaton of the dfferent MOSFEs. Fg.6. Algorthm explanng how to obtan the gate-source voltage of the actvated MOSFEs. III. SIMULAION RESULS he modeled system conssts of 3 cells, each of them n parallel to an n-channel MOSFE, the equalzer control system and fnally the nput of the whole system, whch s the current that s njected nto the whole battery system or delvered by ths one. he cells utlzed are the batteres Batterst PQ5400 LP2, whch have been modeled [14]-[15]. he n-mosfes (modeled n ther saturaton regon) are the Vshay Slconx SS452DN. he Matlab/Smulnk model of the proposed system s gven n Fg.7. Below the chargng process s smulated and studed wth a help of ths model. he amount of current that s njected nto the battery s set to 7A. he SOC of the cells s set to 50%, 60% and 70% for the cell number 1, the cell number 2 and the cell number 3, respectvely. Fg.8 s dvded nto three subplots. he frst subplot shows the cells voltages. It can be seen that after some hundreds of seconds, the system can be consdered balanced. he second subplot shows the cells currents. wo of the cells (the ones whch started wth a SOC of 50% and 60%) are beng njected all the current snce ther voltage s lower than the average one. One of these cells wth lower SOC suffers a small devaton that s solved placng ts control n the ntermedate level, and recoverng the balanced state after a short tme. he remanng cell (the one whch started wth a SOC of 70%) s not njected current at the begnnng, then t s njected more and more gradually, untl t reaches a level (the ntermedate level aforementoned) n whch the amount of njected current keeps constant n an ntermedate value, and fnally the cell s njected all of the current (7A). 43

5 Fg.7. General block dagram of n Matlab/Smulnk me [s] Fg.8. Matlab/Smulnk smulaton of dschargng mode - subplots (from top to bottom): cells voltages behavor, cells currents behavor, MOSFEs currents behavor. 44

6 2013 / 2 he thrd subplot depcts the MOSFEs currents, whch are the ones bypassed to the cells. It represents the equalzaton process behavor. As t can be seen, the MOSFEs that are n parallel to the lower voltage cells bypass zero current snce all of the current s njected nto the correspondng cells, and the effect of the small devaton of one of them s shown as the MOSFE bypasses the ntermedate level of current. hs ntermedate level s manly enough to brng the cell to ts balanced state, whch allows the system to decrease the energy losses. he remanng cell (the one that started wth a hgher voltage than the average one, that s to say, wth a hgher SOC) s fully bypassed at the begnnng, and therefore t can be seen that all of the current goes through ts correspondng MOSFE. hen, the cell s beng balanced more and more, thus, the MOSFE s bypassng less and less current. When the ntermedate level s reached, the MOSFE starts to bypass a constant current untl the cell s consdered balanced, and then the MOSFE s set to ts off-state and the cell s njected wth all of the current. Once t has been checked that the dfferent cells n a strng can be balanced, Fg.9 shows the lost energy as a functon of the tme. It has to be remnded that the dfferent cells SOCs are set n order to smulate a huge unbalance (from 50% of SOC the least charged cell to 70% the most charged one). After a chargng process of the cells (takng nto account that the mnmum consdered SOC was 50%), t can be checked that at the begnnng, the amount of losses s hgh snce there s a bg unbalance and the equalzer has to work hard, but at the end t can be seen that the relaton of lost energy n the equalzaton process over the total energy delvered by the battery to the charger s really low and s 10%. Fg.9. Relaton of lost energy over total energy delvered by the battery (%). IV. CONCLUSIONS Battery system equalzer s a very mportant ssue n an applcaton desgn whch ncludes energy storage. A well balanced battery system can manly ncrease ts lfe, ts safety, and optmze the capacty of the battery. he equalzer system has to be able to carry out a proper balance mnmzng the lost energy durng ts operaton, obtanng the proposed am n the shortest possble tme, and mnmzng ts complexty and cost. In ths paper, based on the shuntng transstor equalzaton topology, a novel control s presented, n whch all the aforementoned requrements can be satsfed. hs smple topology and novel control allow lower losses and low cost equalzaton, obtanng good results n a really short tme. hs equalzaton can be carred out n the battery chargng process, whch allows the battery system to be balanced, and therefore all the cells n the seres strng are workng under the same condtons along the whole battery lfe. ACKNOWLEDGEMEN hs research was supported by Mnstero de Economía y Compettvdad, Fondos FEDER, Junta de Extremadura, Consejería de Empleo, Empresa e Innovacón and Fondo Socal Europeo. REFERENCES [1] Sveum, P.; Kzlel, R.; Khader, M.; Al-Hallaj, S.; "II Plug-n Converson Project wth the Cty of Chcago", Vehcle Power and Propulson Conference VPPC IEEE, pp , 9-12 Sept [2] Uno, M.; anaka, K.; "Influence of Hgh-Frequency Charge Dscharge Cyclng Induced by Cell Voltage Equalzers on the Lfe Performance of Lthum-Ion Cells", IEEE ransactons on Vehcular echnology, vol.60, no.4, pp , May 2011 [3] Kuhn, B..; Ptel, G.E.; Kren, P..; "Electrcal propertes and equalzaton of lthum-on cells n automotve applcatons", IEEE Vehcle Power and Propulson Conference, 2005, pp. 5, 7-9 Sept [4] Broussely, M.; Perelle, M.; McDowall, J.; Sarre, G.; Martaeng, J; "Lthum on: the next generaton of long lfe batteres characterstcs, lfe predctons, and ntegraton nto telecommuncaton systems", elecommuncatons Energy Conference, INELEC. wentysecond Internatonal, pp , [5] Gottwald,.; Ye, Z.; Stuart,.; "Equalzaton of EV and HEV batteres wth a ramp converter", IEEE ransactons on Aerospace and Electronc Systems, vol.33, no.1, pp , Jan [6] Kutkut, N.H.; Wegman, H.L.N.; Dvan, D.M.; Novotny, D.W.; "Desgn consderatons for charge equalzaton of an electrc vehcle battery system", IEEE ransactons on Industry Applcatons, vol.35, no.1, pp.28-35, Jan/Feb [7] Hung, S..; Hopkns, D.C.; Moslng, C.R.; "Extenson of battery lfe va charge equalzaton control", IEEE ransactons on Industral Electroncs, vol.40, no.1, pp , Feb [8] Kren, P..; Balog, R.S.; "Lfe extenson through charge equalzaton of lead-acd batteres", elecommuncatons Energy Conference, INELEC. 24th Annual Internatonal, pp , [9] Hopkns, D.C.; Moslng, C.R.; Hung, S..; "Dynamc equalzaton durng chargng of seral energy storage elements", IEEE ransactons on Industry Applcatons, vol.29, no.2, pp , Mar/Apr [10] Bentley, W.F.; "Cell balancng consderatons for lthum-on battery systems", Battery Conference on Applcatons and Advances, 1997., welfth Annual, pp , Jan [11] Jan Cao; Schofeld, N.; Emad, A.; "Battery balancng methods: A comprehensve revew", Vehcle Power and Propulson Conference, VPPC '08 IEEE, vol., no., pp.1-6, 3-5 Sept [12] Daowd, M.; Omar, N.; Van Den Bossche, P.; Van Merlo, J.; "Passve and actve battery balancng comparson based on MALAB smulaton" IEEE Vehcle Power and Propulson Conference (VPPC) 2011, pp.1-7, 6-9 Sept [13] Kong Zh-Guo; Zhu Chun-Bo; Lu Ren-Gu; Cheng Shu-Kang; "Comparson and Evaluaton of Charge Equalzaton echnque for Seres Connected Batteres", 37th IEEE Power Electroncs Specalsts Conference, PESC '06, pp.1-6, June [14] Gallardo-Lozano, J.; Mlanes-Montero, M.I.; Guerrero-Martnez, M.A.; Romero-Cadaval, E.; "hree-phase bdrectonal battery charger for smart electrc vehcles", 7th Internatonal Conference-Workshop Compatblty and Power Electroncs (CPE) 2011, pp , 1-3 June [15] Gallardo Lozano, J; Mlanés Montero, María Isabel; Guerrero Martínez, Mguel A.; Romero Cadaval, Enrque; "Electrc Vehcle Battery Charger for Smart Grds". Electrc Power Systems Research. Vol. 90, pp , ISSN Elsever. Septembre

7 Javer Gallardo-Lozano was born n Badajoz, Span, n He receved the B.Sc. and the M.Sc. degrees electronc engneerng from Unversty of Extremadura, Span, n 2007 and 2009, respectvely, where he s currently workng toward the Ph.D. degree. Hs Ph.D. thess s devoted to the research and development of energy storage n electrc vehcle applcatons. He s currently wth the Power Electrcal and Electronc Systems (PE&ES) Research Group ( Hs research nterests are power electroncs n the power system, battery chargers, battery equalzaton and electrc vehcles. Postal address: Unversdad de Extremadura, Campus unverstaro, Escuela de Ingenerías Industrales, Laboratoro C2.7, Avda. de Elvas, s/n 06006, Badajoz, Span. Abdul Lateef receved hs B.E. (Elec. & Instr.) from Gulbarga Unversty n 2000 and hs M.S.E.E. from Illnos Insttute of echnology (II) n From 2004 to 2008, he worked at II on safety and thermal management of lthum on battery systems for hgh power and hgh energy applcatons. From 2008 to 2011, he worked at echnovalutons, Duba on feasblty of lthum on batteres for PV applcatons. Snce 2012 he has been pursung hs graduate studes at the Department of Electrcal & Computer Engneerng, McMaster Unversty. Hs man research area s power converson and battery safety. Postal address: 1280 Man Street West, IB A102, Hamlton, Ontaro, Canada L8S 4K1. Enrque Romero-Cadaval was born n Vllafranca de los Barros, Badajoz, Span, n He receved the M.Sc. degree n electronc ndustral engneerng from ICAI, Unversdad Pontfca de Comllas, Madrd, Span, n 1992 and the Ph. D. degree from the Unversdad de Extremadura, Badajoz, Span, n He s a full Professor n power electroncs at the Unversty of Extremadura, Badajoz, Span. He s currently wth the Power Electrcal and Electronc Systems (PE&ES) Research Group ( Hs research nterests are power electroncs n the power system, power qualty, electromagnetc nterferences, actve power flters, electrc vehcle and renewable energy sources control. Postal address: Unversdad de Extremadura, Campus unverstaro, Escuela de Ingenerías Industrales, Laboratoro C2.7, Avda. de Elvas, s/n 06006, Badajoz, Span. María Isabel Mlanés-Montero (S 03 M 06) was born n Badajoz, Span, n She receved the M.Sc. degree n ndustral engneerng and the Ph.D. degree from the Unversty of Extremadura, Badajoz, n 1997 and 2005, respectvely. Snce November 1998, she has been wth the Unversty of Extremadura, where she was frst an Assstant Professor at the Electrcal, Electroncs and Automaton Engneerng Department and s currently at the Power Electrcal and Electronc Systems Research Group, School of Industral Engneerng. Her major felds of research nterest nclude sold-state power converter desgn and control, electromagnetc nterferences, power qualty, renewable energy sources control, and electrcal machne drves. Postal address: Unversdad de Extremadura, Campus unverstaro, Escuela de Ingenerías Industrales, Laboratoro C2.7, Avda. de Elvas, s/n 06006, Badajoz, Span. 46