Improving of Active Cell Balancing by Equalizing the Cell Energy Instead of the Cell Voltage

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1 Page4 EVS25 Shenzhen, China, Nov 5-9, 21 Improving of Acive Cell Balancing by Equalizing he Cell Energy Insead of he Cell Volage Markus Einhorn 1, Fiorenino Valerio Cone 1, Juergen Fleig 2 1 Mobiliy Deparmen, Elecric Drive Technologies, AIT Ausrian Insiue of Technology Giefinggasse 2, 121 Vienna, Ausria, markus.einhorn@ai.ac.a 2 Insiue of Chemical Technologies and Analyics, Vienna Universiy of Technology Gereidemark 9/164ec 16 Vienna, Ausria Absrac This aricle presens how acive charge balancing of energy sorage devices such as baeries and supercaps can be improved by using he capaciy and he sae of charge insead of he cell volage as balancing crierion. Boh for charging and discharging an improvemen of performance is gained when using he sae of charge and he capaciy of he cells as informaion. A baery sack is modeled and a realisic driving cycle is applied o compare he difference beween boh mehods in erms of usable energy. Finally, he simulaion is validaed by measuremens. Keywords: Li-ion baery, acive charge balancing, capaciy balancing, baery managemen 1 Inroducion Energy sorage devices such as baeries and supercaps are usually conneced in series o achieve a higher volage and o provide enough energy e.g. for elecric vehicles bu also for several oher applicaions. In elecrical vehicles, he lihium ion (Li-ion) baery is he mos promising energy sorage due o is high energy and power densiy and herefore his aricle is focused on Li-ion baeries [1, 2]. A ypical baery sack consiss of welve serially conneced single cells which yields a volage of 5.4V when fully charged and 32.4V when compleely discharged. During he discharging process he open circui volage (OCV ) of each cell follows he shape of he curve shown in figure 1. If several serially conneced and fully charged cells wih differen capaciies C become discharged, he cell wih he lowes C is he firs which reaches he discharging volage limi DV L (ypically 2.7V) as shown in figure 2. Alhough he cells are no all compleely discharged, he discharging process mus sop immediaely o avoid damage on he weakes cell [3]. By shifing he charge from he no compleely discharged cells o he discharged cells, he performance of he baery sack can be improved a lo. This process is called acive charge balancing as described in lieraure [4 8]. The curren approach is ransferring charge from he cell wih he larges volage o he cell wih he lowes volage (volage balancing) and here are basically wo mehods. The charge can be ransferred using eiher a capacior or an inducor as a shor-ime energy sorage. A very promising srucure is shown in [1] and [11]. A flyback converer is used o ransfer energy eiher from one cell o he whole sack (op balancing) or from he whole sack o one cell (boom balancing) as shown in figure 3. In Figure 4a he cell volages of hree serially conneced cells during one charging and discharging period are shown. The ypical charge ransfer wih volage balancing is illusraed wih arrows. Since only he cell wih he lowes volage and he cell wih he highes volage are essenial, a scenario wih hree serially EVS25 World Baery,Hybrid and Fuel Cell ElecricVehicle Symposium 1

2 Page Cell n Cell n 4 OCV (V) 3.5 Cell 1 Cell 1 3 (a) (b) SOC Figure 1: Linear inerpolaed OCV -curve for differen sae of charge (SOC) gained from ess wih he EIG eplb C2B lihium ion polymer cell [9]. VCell (V) Ah 4 Ah 45 Ah 1 2 (s) Figure 2: Simulaion of he cell volages of 3 serially conneced cells wih capaciies beween 35 Ah and 45 Ah when applying a 4 A consan discharging curren conneced cells is sufficien o analyze he mode of operaion. Cell 1 (e.g. 35 Ah) has he lowes, (e.g. 4 Ah) an inermediae and (e.g. 45 Ah) he highes capaciy. When he charging process sars in phase I, has he highes volage and herefore energy is aken and ransferred o and. Indeed, he energy from is ransferred o he whole baery sack and since only hree cells are presen, he energy is spli ino hree equal pars and spread o, and. The ne charge ransfer hough, is from o and o. Cell 1 has he lowes capaciy and is cell volage exceeds he ohers in phase II. Therefore, energy is now ransferred from o and o. When cell Figure 3: Simplified charge ransfer in a baery sack wih boom balancing (a) and op balancing (b). 1 reaches DV L, he charging process mus sop immediaely o avoid overcharging of hough and are no ye compleely charged. Wih an acive balancing sysem, he charging process could be coninued wih a severely reduced charging curren unil all cells are fully charged. This would ake much more ime and is no considered. During discharging in phase III, charge is ransferred o because i has he lowes cell volage hough i has he larges amoun of sored energy. In phase IV, is suppored because of he lowes cell volage. Boh for charging and discharging, limis he performance of he baery sack. In phase I and III he wrong cells are balanced because he charge ransferred in hese wo phases mus be parially reransferred in phase II and IV. Therefore volage balancing can be improved. The drawback of volage balancing in phase I and III can be eliminaed by using he SOC and he acual raed capaciy as balancing crierion (capaciy balancing) as shown in figure 4b. While charging, energy from he cell wih he lowes energy o full charge (cell 1) is aken. During discharging he cell wih he lowes amoun of usable energy is suppored (). In he nex secion, a specific discharging scenario is simulaed and measured o compare he performance of volage balancing and capaciy balancing in erms of usable energy of he baery sack. 2 Modeling and simulaed scenario The simulaion environmen used for he approach is Modelica/Dymola because of he simple and objecoriened possibiliy o model inerdisciplinary relaions [12]. The baery model as well as he parameerizaion procedure is described and validaed in [13]. All EVS25 World Baery,Hybrid and Fuel Cell ElecricVehicle Symposium 2

3 Page42 CVL V Cell CHARGING op balancing DISCHARGING boom balancing CVL V Cell CHARGING op balancing DISCHARGING boom balancing DVL I II III IV DVL I II III IV C C C 2 C 2 C 1 (a) (b) Figure 4: Balancing wih respec o cell volages (a) and cell capaciies (b). The arrows indicae he charge ransfer beween he cells. C N / Ah C / Ah SOC ini Table 1: Raed cell capaciies C N, measured capaciies C and iniial SOC for he simulaed scenario. Figure 5: Simulaion circui generaed wih Modelica/Dymola 6.1. measuremens for he parameerizaion are performed on he EIG eplb C2B lihium ion polymer cell [9]. An overview of he model used for he simulaion is shown in figure 5. Three single cell models of he EIG eplb C2B lihium ion polymer cell wih differen capaciies and SOC as shown in able 1 are serially conneced o a baery sack. This baery sack is geing discharged unil one cell reaches DV L (usually he one wih he lowes capaciy) wih a curren profile gained from he FTP72 driving cycle as shown in figure 6 [14]. There is an acive balancing sysem conneced o he baery sack wih a balancing curren of 3A (single cell side of he dcdc converer) and he energy over he whole discharging process is calculaed. The simulaion is hen validaed wih he circui from figure 7. During he whole es he cells are in a climae chamber o minimize emperaure effecs. 3 Resuls and discussion The baery sack wih he configuraion from able 1 has a heoreical sored energy of Wh. The available discharging energy for differen balancing scenarios is shown in figure 8. Wihou any balanc- EVS25 World Baery,Hybrid and Fuel Cell ElecricVehicle Symposium 3

4 Page43 velociy (km/h) power (kw) curren (A) (s) Figure 6: Definiion of he FTP72 driving cycle, power consumpion of a ypical compac elecrical vehicle and curren requiremen from a baery sack wih 1 serially conneced single cells wih a cell volage of 3.6V respecively [14]. I cycle V Ba V 1 V 2 2 Ah 4 Ah I ch P I dch Wdischarge (Wh) simulaed measured no balancing v balancing c balancing Figure 8: Measured and simulaed discharging energy wihou balancing, wih volage balancing and wih capaciy balancing. and herefore he balanced cells are he same. There is also no difference beween volage and capaciy balancing during charging if all cells are compleely discharged before saring he charging process (he cell wih he highes volage is also he cell wih he lowes energy o full charge). When he cells are no all compleely charged before discharging, capaciy balancing improves he amoun of usable energy. When he cells are no all compleely discharged before charging, he baery sack can be charged in a shorer ime and more energy can be loaded ino he baery sack when using capaciy balancing. V 3 6 Ah 4 Conclusion and oulook Figure 7: Tes circui o validae he simulaion. ing, he baery sack is as weak as he smalles cell. In his case, he baery sack has a maximum capaciy of 21.9 Ah which correlaes wih an usable energy of 24 Wh. Volage balancing increases he capaciy by 27% o 28.3 Ah or 36 Wh. The bes performance is accomplished when balancing he capaciy. The capaciy of he baery sack can be increased by 32% o 29.1 Ah or 318 Wh. Even wih capaciy balancing, he usable energy is jus around 79% of he heoreical value (46.17 Wh). Therefore he balancing curren can be increased. There is no difference beween volage and capaciy balancing during discharging when all cells are fully charged before discharging. In his case he cell wih he lowes volage has also he lowes usable energy Alhough acive balancing wih respec o he cell volages is already a grea advance compared o baery sacks wih passive or no balancing sysems here is sill room lef for furher improvemen. The cell wih he lowes volage (during discharging) in a baery sack is no always he cell which has he lowes amoun of energy sored. When having he capaciy and he SOC from all cells as balancing crierion he cell which has he leas amoun of energy sored can be suppored. I has been shown ha he usable energy of he baery sack can be increased in his case. One problem could be he exac esimaion of he acual capaciy of each cell, which varies due o aging and emperaure influence. Especially when he baery sack has no been used for a long ime he sored values in he baery managemen sysem can be inaccurae. Therefore, furher work will focus on mehods for an accurae esimaion of capaciy, SOC and he OCV EVS25 World Baery,Hybrid and Fuel Cell ElecricVehicle Symposium 4

5 Page44 vs. SOC curve during baery operaion and how inac- [12] Peer Frizson. Principles of Objec-Oriened Modeling and Simulaion wih Modelica 2.1. IEEE Press, curacies in hese parameers influence he acive capacwiley-inerscience, 24. iy balancing. Acknowledgmen [13] Markus Einhorn, Fiorenino V. Cone, Chrisian Kral, Juergen Fleig, and Rober Permann. Paramerizaion of an elecrical baery model for dynamic sysem simulaion in elecric vehicles. IEEE Vehicle Power and Propulsion Conference, Sepember 21. The auhors graefully acknowledge he suppor of he Ausrian Research Promoion Agency (Oeserreichis- [14] U.S. Environmenal Proecion Agency. FTP72 Urban Dynamomeer Driving Schedule (UDDS). che Forschungsfoerderungsgesellschaf mbh, FFG) for he research projec Acive Balancing fuer Lihium-Ionen-Baerien in AuomobilanwendunAuhors gen (BALI). References [1] Chrisian Rosenkranz, Chrisian Kupfer, and Uwe Koehler. Baery challenges. 21h Inernaional AVL Conference Engine and Environmen, Sepember 29. [2] Srdjan M. Lukic and Ali Emadi. Charging ahead. IEEE Indusrial Elecronics Magazine, December 28. [3] Andreas Jossen and Wolfgang Weydanz. Moderne Akkumulaoren richig einsezen. Reichhard Verlag, 1 ediion, 26. [4] Jian Cao, Nigel Schofield, and Ali Emadi. Baery balancing mehods: A comprehensive review. IEEE Vehicle Power and Propulsion Conference, Sepember 28. [5] Sephen W. Moore and Peer J. Schneider. A review of cell equalizaion mehods for lihium ion and lihium polymer baery sysems. Sociey of Auomoive Engineers, January 21. [6] Xuezhe Wei and Bing Zhu. The research of vehicle power li-ion baery pack balancing mehod. IEEE Ninh Inernaional Conference on Elecronic Measuremen and Insrumens, Augus 29. [7] Michael Kulgen and Linear Technology. Managing high-volage lihium-ion baeries in hevs. EDN: Informaion, News, and Business Sraegy for Elecronics Design Engineers, April 29. [8] Sihua Wen and Texas Insrumens. Cell balancing buys exra run ime and baery life. Analog Applicaions Journal, 1Q 29. [9] EIG. EIG eplb C2B lihium ion polymer cell Daashee, 21. [1] Werner Roessler and Infineon Technologies. Akiver ladungsausgleich fuer lihium-ionen baerien. ATZelekronik, 3, February 28. [11] Werner Roessler and Infineon Technologies. Boos baery performance wih acive charge-balancing. EE Times-India, July 28. Markus Einhorn was born in Vienna, Ausria in 1984 and received he BSc. degree as well as he Dipl.-Ing. degree in elecrical engineering and he PhD degree in echnical chemisry all from he Vienna Universiy of Technology in 28, 29 and 211, respecively. He is currenly a Scienis a he Mobiliy Deparmen, Elecric Drive Technologies a he AIT Ausrian Insiue of Technology in Vienna, Ausria. His recen work is focused on design and modeling of power elecronics and energy sorages wih emphasis on baery managemen sysems and aging phenomena of Li-ion baery cells. Dr. Einhorn is a member of he Insiue of Elecrical and Elecronics Engineers (IEEE), he Ausrian Elecroechnical Associaion (OVE) and he Modelica Associaion. Fiorenino Valerio Cone received his PhD in ransporaion a he Universiy of Pisa in 23. He joined he AIT Ausrian Insiue of Technology in 23 afer working in a German R&D deparmen. Dr. Cone is energy sorage group leader wihin AIT. He leads projecs dealing wih energy sorage sysems for HEVs as well as EVs and he has over 1 years of experience in he research of advanced powerrains. Believing in he imporance of he disseminaion and he neworking he is involved wihin he aciviies of he Inernaional Energy Agency. Juergen Fleig received his Diploma degree in Physics in 1991 (Universiy of Tuebingen, Germany) and his PhD in Chemisry in 1995 (Max-Planck-Insiue of Solid Sae Research, Sugar, Germany). Afer working as a researcher a he same insiue for several furher years he acceped a posiion as professor of elecrochemisry a Vienna Universiy of Technology in 25. His main research subjecs are elecroceramics and maerials for elecrochemical energy conversion devices including basic invesigaions on he physical and chemical processes deermining he cell efficiencies. EVS25 World Baery,Hybrid and Fuel Cell ElecricVehicle Symposium 5