电动汽车BMS锂电池均衡电流控制总结

Howmuchbalancingcurrentdoyouneed?--ByDavideAndreaIs100mAsufficienttobalanceaLi-Ionpack?Whatabout1A?10A?OneofthefunctionsofaBMSistobalanceabattery.Thecellsinabatterymaybeunbalancedinmultipleways,including:1.ActualSOC(StateOfCharge)2.Leakage(selfdischargecurrent)3.Internalresistance4.CapacityOnlycellsfrombettermanufacturersarecloselymatched,andbatteriesthatusethemrequireverylittlebalancing.Unfortunately,cellsfrommanymanufacturershavesignificantcell-to-cellvariations.Balancingtakescareonlyofthe1stone:equalizingtheSOCsofthevariouscellsinabattery.Insodoing,itcompensatesalsoforthe2ndone:cell-to-cellvariationsinleakage.Itsjobmaybesomewhathinderedbythe3rdone:variationsincellresistance.Whatbalancingdoesnotdoistakecareofthe4thone.That'sdonebyadifferenttechnique:Redistribution.Redistributionallowsuseofalltheenergyinthebattery;itrequiressignificantlyhighercurrentsthanbalancing.Thepointofbalancingistomaximizethechargethatthebatterycandeliver,limitedonlybythecellwiththelowestcapacity.Withoutbalancing,thecapacityofabatteryislimitedatoneendwhenacellbecomesfullycharged,andattheotherendwhenacell(sameordifferentcell)becomesfullydischargedAfterbalancing,thecapacityofabatteryislimitedatbothendsbythecellwiththelowestcapacity(or,inextremecases,bythecellwiththehighestinternalresistance)Abalancedbatteryisoneinwhich,atsomeStateOfCharge,allthecellsareexactlyatthesameSOC.ThiscanbedoneatanySOClevel.Inbatteriesthatareregularlychargedfully,itisusuallydoneatthe100%level.ABMSbalancesapackbyremovingextrachargefromthemostchargedcells,and/orbyaddingchargetotheleastchargedcells.Balancingcanbedissipativeornondissipative(dissipative:energyiswastedinheat;nondissipative:energyistransferredandthereforeitisnotwasted).Dissipativebalancingisoftencalledpassivebalancing;nondissipativebalancingisoftencalledactivebalancing.Onafirstorder,howmuchcurrentisrequiredtobalanceabatterydependsonwhythebatteryisoutofbalance:Grossbalancing:toremedyagrossimbalancerightaftermanufactureorrepairofapackthatwasbuiltusingmismatchedcellsMaintenancebalancing:keepingthepackinbalanceduringnormaluseGrossbalancingTheproperwaytomanufactureorrepairapackistodosoinawaythatendsupwithabalancedpack,sothattheBMSisnotrequiredtoprovideaninitial,grossbalancing.Thiscanbedonein1of2ways(orboth):Balancebeforebuild:usecellsthatstartatthesameStateOfCharge(SOC)Balanceafterbuild:applyaload(orasupply)totheindividualcellsthatareovercharged(orover-discharged),whilethepackisstillopenandthereisaccesstotheindividualvoltagepointsinthepackIf,ontheotherside,thepackisbuiltorrepairedwithnoconsiderationforinitialtheSOCofindividualcells,theBMSmayexpectedtodogrossbalancing.Inthatcase,themaximumlengthoftimerequiredtobalancethatpackwilldependonthesizeofthepack,andthebalancingcurrent.Thebalancingcurrentrequiredwillbeproportionaltothesizeofthepackandinverselyproportionaltothedesiredbalancingtime:Balancecurrent[A]=Packsize[Ah]/grossbalancingtime[hours]Thisgraphusestheaboveformulatoshowtherequiredbalancingtime.Timerequiredtobalanceagrosslyunbalancedpack,vs.balancecurrent,forvariouspacksizes.Intheabovegraphyoucanseethat:ABMSwitha1Abalancingcurrentwilltakealmost1weektobalancea100Ahpackthathassomecellsfullycharged,andsomecellstotallyemptyAbalancecurrentof10mAwillnotbeabletobalancea1000AhpackwithinthelifetimeofitsownerConversely,abalancecurrentof10Awouldbeanoverkillfora1Ahpack,asitwillbalancethepackinlessthan6minutesTherefore,iftheBMSisexpectedtobalancealarge,grosslyunbalancedpackinareasonabletime,itwillhavetoprovidearelativelyhighbalancecurrent.MaintenancebalanceIfapackstartsbalanced,keepinginbalanceisafareasierjob:alltheBMShastodoistocompensateforthevariationinself-dischargeleakageinthecells.Ifallthecellshaveexactlythesameleakage,thennobalancingisrequired:theSOCofallthecellswillslowlydropexactlythesame,sothepackwillremaininbalance.Ifallthecellshavethesameleakage,exceptforonecellwhoseleakageis1mAmore,thentheBMShastotake,onaverage,1mAfromalltheothercellstomakeupforit(oradd1mAjusttotheonecell)Sofarwelookedattheaveragebalancingcurrent(1mAintheexamplesabove).Inmanyapplications,theBMSisnotabletobalance24/7.Yet,leakagedischargesacell24/7.Inthosecases,thebalancecurrenthastobehigher,ininverseproportiontohowmuchtimeisavailablefortheBMStobalance.thepack.Forexample:IftheBMScanbalanceconstantly,thebalancecurrentcanbejust1mAIftheBMSisonlyabletobalanceforonehoureveryday,thebalancecurrentmustbe24mAtoachieveanaverageof1mAOfcourse,itisOKiftheBMSisabletorunmorebalancecurrentthantherequiredminimum.Inthatcase,theBMSachievestherequiredbalancecurrentbyoneof2methods:Keepingbalancingalwayson,butreducingitsvaluetomatchthecells'leakagedeltacurrentTurningbalancingonandoffwithadutycyclesuchthat,ontheaverage,thecurrentmatchesthecells'leakagedeltacurrentThereisnoreasontospecifyaBMSthatcanhandlemorebalancingcurrentthanisrequiredbythepackintheworstcaseofleakagecurrentdelta.Thebalancingcurrentrequiredisproportionaltothedifferenceintheleakagecurrentandtowhatpercentofthetimeisavailableforbalancing:Balancecurrent[A]=(Maxleakage[A]-Minleakage