Researching on Discharge Efficiency of Battery Pack with Redundant Cells

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1 Iteratioal Joural of Research i Egieerig ad Sciece (IJRES) ISSN (Olie): , ISSN (Prit): Volume 4 Issue 7 ǁ July. 216 ǁ PP. 1-5 Researchig o Discharge Effiecy of Battery Pack with Redudat Cells Yig Wag 1, Wekui Li 2, Wei Yu 2*, Xiao Ya 1, Youju Zhu 1, Shhao Zhag 1, Bixiog Huag 1, Miao Liu 1 1 (College Of Automotive Egieerig,Shaghai Uiversity Of Egieerig Sciece, Shaghai, Chia,) 2 (Departmet Of Iterdiscipliary Research, Suzhou Istitute Of Nao-Tech Ad Nao-Bios (SINANO), Chiese Academy Of Scieces, Suzhou, Chia, ) Abstract: Redudat Balacig Method Is A Effiet Battery Maagemet System (BMS) Strategy, Whh Ca Make The Effective Capacity Of The Battery Group Close To The Sum Of Battery Capacity. I This Paper, The Discharge Switchig Strategy Of The Battery Pack With Redudat Equalizatio Is Aalyzed, Ad The Effect Of The Strategy O The Discharge Effiecy Of The Battery Pack Is Aalyzed I Detail From Three Agles Such As Number Of Redudat, Switchig Duratio Ad Battery Iitial Capacity Variace. Fially, The Simulatio Results Are Verified By Experimets. The Results Show That The Simulatio Data Is Cosistet With The Actual Situatio. Keywords: Redudat Cell; Maagemet Strategy; Battery Maagemet System (BMS) I. INTRODUCTION For high eergy, high specif power, small size, log cyclig life, ad wide workig temperature rage, etc. [1], lithium io battery has become mai eergy storage for carryig ew eergy. Due to the differeces amog productio materials, maufacturig process, ad workig eviromet, sigle lithium io battery usually shows o-uiformity i iteral resistace, voltage ad capacity. I order to overcome short board effect caused by No-uiformity, batteries i series geerally have to be equipped with battery maagemet system (BMS). At preset, there are several typal strategies for battery maagemet system, such as active equalizatio, passive equalizatio, redudacy equalizatio ad so o. The strategy of redudat equalizatio is a kid of maagemet method that ca maximizes the effective capacity of the battery pack to the amout that close to the total capacity summed up by each uit. I 24, Li Hogya [2] put forward a strategy addig more tha oe redudat battery cell to a lithium io battery pack, whh oly works whe o stadby batteries depleted or damaged. It has othig to do with the improvemet i eergy utilizatio of the battery pack. I 27, Xu Jiamig [3] also metioed the redudat backup ad equal time iterval switchig algorithm to make the battery pack to achieve a balaced dischargig. [4] [5] [6] I 21,Yu Wei team at the Suzhou Istitute of Nao-Tech ad Nao-Bios(SINANO),Chiese Academy of Scieces, brought the cocept of redudat backup ito the field of power battery. Istead of usig relay switch, they achieved redudacy by usig semoductor deve, whh created a ew digitized era i power battery field. I 211, Atoio Maeti [7] ad his group came up with the idea of redudat equalizatio ad carried out experimetal verifatio. It is proved that this method ca greatly improve the Balacig effiecy. I 212, a algorithm to miimize the switchig times i completig equalizatio was give by Fag Biyu ad Xu Luxiog [8]. I this paper, the strategy applied to the process of discharge by switch shiftig for the battery pack with redudat batteries is aalyzed, ad from multiple perspectives such as the umber of redudat batteries ad switches, the stadard deviatio of the iitial battery capacity, the paper aalysis the effect of the strategy o dischargig effiecy of the battery pack i detail by umeral simulatio method ad the results will be testified by experimets at the ed of this paper. II. REDUNDANT EQUALIZATION METHOD No-uiformity of the power batteries cosists of o-uiformity of voltage, o-uiformity of resistace ad o-uiformity of capacity. Therefore, i order to get a better performace with the battery pack, a battery maagemet system (BMS) eeds to be built based o the cotrol of voltage ad capacity of the battery pack. A lot of experimets showed that the failure of the battery pack is usually caused by oly oe or two batteries withi it. Ad the icosistecy of the battery ( ) is a major cause whh ca lead to sever 1 Page

2 Researchig O Discharge Effiecy Of Battery Pack With Redudat Cells damage to the whole battery pack. Thus, the itelliget charge ad discharge maagemet strategy based o the structure with redudat uit ca effectively solve the problems above ad icrease the effective capacity of the battery pack. Figure 1 is a schemat diagram of a typal redudat balacig battery maagemet system, the umber of batteries i a geeral group will be more tha the actual requiremet umber m, The system ca idepedetly cotrol the usage of each battery i the pack, the specif switchig cotrol strategy affects the performace of the pack. At preset, there are three kids of commo methods, whh are as follows: simple switchig method, Numeral optimizatio method, ad equal eergy chage / time switchig method. Figure 1. Schemat diagram of a typal redudat balace battery maagemet system (BMS) 1. Simple Switchig Method The method is applied i the dischargig process, the backup battery works oly whe the o-stadby battery depletio or damage happes. So the effiecy is ot high. 2 Numeral Optimizatio Method This method ca optimize the switchig times accordig to the iitial state of battery. However, for there is o steady iitial stat due to the chagig workig coditios, the calculatio will get complated, ad the iitial state give by this method ca be very differet form the reality. 3 Equal Eergy Chage / Time Switchig Method The method is i costat chage of eergy or time redudacy cell switchig, the method i the discharge process try to keep the battery optimizatio combiatio discharge, is the best way to deal with the discharge process may occur at ay time of emergecy situatios. The specif switchig eergy or time iterval is related to the maximum eergy ad discharge curret of the battery pack. III. EQUAL ENERGY CHANGE/TIME SWITCHING METHOD Assumig there are cells, the subscript i i i stads for the battery uder workig coditio, we defie the situatio with switch K 1 closed ad K 2 ope as state 1, i cotrast of that is state. The r stads for redudat umber, uder coditio that the dischargig curret I is give, the system will make a reselectio after a duratio time to sort by size of umber. mi 1 2 m m1 max (1) Set m batteries with lowest SOC to state ad the other -m batteries to workig coditio. Defiitio: i i1 i1 i1 i stads for the remaiig capacity of the battery labeled i. (2) IV. SIMULATION Based o the priciple above, we developed a simulatio software by VB, it studies the effect of three factors (the variace distributio of iitial state, umber of redudat batteries ad switchig time / eergy) to the effiecy i discharge with the method of redudat balacig. The umber of cells i the simulatio is 2 Page

3 Discharge Effiecy(%) Researchig O Discharge Effiecy Of Battery Pack With Redudat Cells 1, the mea value of is 7, ad the data i the simulatio is geerated radomly through the Normal distributio algorithm. 1 The ifluece by the umber of redudat batteries Set variace of the pack to1, switchig iterval to 1s, discharge rate to C=.1c. Make the redudat cell umber m to m =, 1, 2, 3, 4, 5, 1, 2, the simulate the effiecy i discharge of the battery pack. 1 4 Discharge Effiecy (% ) Defied rate of retur as: Discharge Effiecy(%) Rate of Retur(%) Number of Redudat Figure 2. Effect of the umber of redudat cells o discharge effiecy r i * r ri r i (3) We ca see from figure 2, the discharge effiecy ca be improved sigifatly by addig the umber of redudat battery to the pack. It ca be see, with five redudat batteries, the dischargig effiecy ca reach 93%; but as the umber icreases, the improvemet of dischargig effiecy becomes slow. Therefore, we eed to choose the redudacy umber ratioally i practal applatio. 2. The ifluece of switchig time / eergy differece Whe switchig, set a fixed dischargig curret value, chage the switchig time that is to chage the shiftig eergy differece. Set battery to =1, discharge rate to C=.1c, back-up umber m=5, switchig iterval to t=1~5s respectively. The simulatio is as follow: Rate of Retur (%) Discharge Effiecy Liear fittig "Discharge Effiecy" Switchig Duratio(s) Figure 3. Effect of switchig iterval o effective eergy release rate As ca be see from figure 3, the greater the switchig iterval, the less times it eed to shift ad the less the eergy that releases. But too high switchig frequecy will itroduce harmos i the circuit. This will affects the quality of the power supply, ad will brig certai switchig losses. 3 Page

4 SOC(%) Discharge Effiecy(%) SOC(%) Researchig O Discharge Effiecy Of Battery Pack With Redudat Cells 3 The effect of iitial variace o discharge effiecy Set battery stadard variace =2, 5, 8, 1, the redudat battery umber m=1, 3, 5, switchig iterval time for t=1s, discharge rate settig for C=1C, the variace of the iitial simulatio o discharge effiecy ifluece r=1 r=3 r=5 Poliomial Fit of "r=1" Poliomial Fit of "r=3" Poliomial Fit of "r=5" Stadrad Variace Figure 4. Sigma factors ifluecig the release rate of available eergy As ca be see from figure 4, the greater the iitial stadard variace of the battery pack to achieve the same discharge effiecy, the more redudat batteries is eeded. It also shows that the redudat battery method ca sigifatly improve the discharge effiecy of the battery pack. But with a give umber of redudat battery, simple selectio of the iitial distributio of the battery pack is ecessary. Cosistecy should ot be too large, so as to maximize the effiecy of the battery pack. V. EXPERIMENTAL VERIFICATION I order to verify the simulatio data, this paper tries to build a itelliget battery maagemet system cotais 18 batteries accordig to figure 1, as show i figure 5. The battery capacity i the group were 68, 86, 77, 89, 85, 85, 67, 73, 71, 6, 83, 9, 85, 79, 74, 69, 88, 76. Figure 5. Balaced redudat module Battery discharge parameters are as follows: discharge cut-off voltage: 2.V, discharge rate:.5 c, the umber of redudat batteries is seve. That is to say, discharge voltage is 36V. I the discharge process, the chage of the serial umber for the four sectio of the 1, 8, 1, 11, 12 is show i figure Before discharge After discharge i=1 i=8 i=1 i=11 i= Time/mi Serial umber Embedded image is the iitial capacity compared with the after discharge of the battery diagram. Figure 6. Discharge of SOC chages before ad after Page

5 Researchig O Discharge Effiecy Of Battery Pack With Redudat Cells Amog them, i i i1 i1 i1 i1 18 i i i1 i1 (145 12) / % (4) As show i figure 6, with the discharge, the serial umber for the four sectio of the 1, 8, 1, 11, 12 battery gradually ted to be cosistet. Discharge capacity of the iitial icosistecy has bee greatly improved after dischargig. By calculatio, the discharge effiecy of the battery is 99.35%, so the full discharge of the battery is achieved. Therefore, the redudat battery ca effectively improve the discharge effiecy of the battery pack. VI. CONCLUTION I this paper, the battery discharge switchig strategy with redudat balacig method is aalyzed from three respects, redudat umber, switchig time iterval ad iitial capacity battery stadard variace. This paper studies the ifluece of the strategy o the battery discharge effiecy by umeral simulatio method. The results are as follows: First of all, icreasig the umber of redudat batteries ca improve the discharge effiecy of the battery pack, but there is a optimal value. Secod, icreasig the switchig iterval times ca icrease the discharge effiecy of the battery pack, but too high the frequecy ca lead to harmo i the circuit, ad a icreasig switchig eergy loss. Fially, the greater the iitial stadard variace of the battery pack to achieve the same discharge effiecy, the more redudat batteries is eeded. Therefore, i practal applatio, it is recommeded that the variace of the group battery should be selected ratioally, so as to save the quatity of redudat battery. At the ed of this paper, we built a system with redudat battery uits to verify the simulatio data. But limited by the experimetal coditios ad with the diffulty of buildig a itelliget battery maagemet system cotais 1 batteries.the simulatio results are simply verified by 18-battery system, whh ca oly prove the ratioality ad reliability of the redudat simulatio data, ad the further work is to be carried out i the future. ACKNOWLEDGEMENTS This project is sposored by Shaghai Uiversity of Egieerig Sciece Iovatio Fud for Graduate Studet (No.15KY66) ad the key project of Shaghai Sciece ad Techology Commissio ( ) ;Characterizatio, Cotrol ad Evaluatio of Power Battery i Electr Vehle. REFERENCES [1]. Lei Zhou, Hao Zhag, Wejua Xi. The Research Progress of electrochemal Hybrid Capacitor [J]. Power techology, 213, v.37;no.27811: [2]. BOE Techology Group Limited by Share Ltd. Battery Pack with Redudat Cells [P]. Chia: zl , 24, 1, 13. [3]. Jiamig Xu. Itelliget Power Lithium Battery Chargig ad Dischargig fie Maager [P]. Chia: , 27. [4]. Wei Yu. Bas Uit for Itelliget Battery Maagemet System [P]. Chia: , X. [5]. Wei Yu. A itelliget Battery Maagemet System [P]. Chia: X zl [6]. Wei Yu. A Itelliget Chargig System [P]. Chia: , 7, 1, 213. [7]. Atoio Maeti, Adrea Abba, Alessadro Merati et al. A New BMS Architecture Based o Cellredudacy. [J].IEEE Trasactios o Idustrial Electros, 211, 58 (9). [8]. Biyu Fag, Luxiog Xu. Study o the algorithm of redudacy uit battery maagemet [J]. Joural of Fuqig Brach of Fujia Normal Uiversity, Page