Cell Balancing Using the bq20zxx (Rev. C) - TI.com

1 ApplicationReportSLUA340C November2005 (SOC) ,givenaconfigurationof3 x 2200-mAhcells(QMax),if onecellis dischargedby100mAh(Q1),thesecondby100mAh ,andthethirdby200mAhfroma fullychargedstate,thefirstandsecondcells 'chemicalstateofchargeis (Qmax-Q1)/Qmax= ,butthethirdcellis 91%.Therefore,cell3 is a differentopen-circuitvoltage(OCV)forcell 3 comparedtocells1 and2, specificcell'stotalchemicalcapacity,Qmax ,initiallymaybedifferentfromtheothersin ,evenif allcellsweredischargedbyanequalamountfro ma fullychargedstate, ,if all3 cellsaredischargedby100mAh,butcell3hasdi fferenttotalcapacity( ,2000mAh),theresultingchemicalstatesofch argeis 200-mAhdifferencein Qmaxcausesonlya (cause1).

2 (thatcanbeanapproximate15%rangein thesameproductionbatch)donotcausediffere ncesin ,theycancausedifferencesin ,cellvoltagecanbeapproximatedasV = OCV+ I currentisnegative(discharge),voltageis lowerfora currentis positive(charge),voltageishigherfora ,it significant(< 200mA)currentis flowing,attemptingtousevoltageasa determiningfactorforpassingmorechargethr ougha cellwithhighervoltage,failstodetermineif thevoltagedifferencesis causedbydifferencesin it is causedbyimpedanceimbalance,bypassingmore currentthroughthiscellresultsin theoppositeeffect increasingtheSOCdifferencefromothercells toa largervaluethanit result,theOCVofthiscellattheendofchargei s differentfromtheothercells,andcanreachhi ghlevels, technologytoidentifythechemicalSOCofeach cell, , :(a)DeterminetheinitialSOCforeachseriesc ellbankseparately.

3 (b)Determinethechargeneededforeachcellto reacha a November2005 RevisedJanuary2011 CellBalancingUsingthebq20zxxSubmitDocume ntationFeedback 2005 2011, (c)Findthecellrequiringthemostchargetobe fullycharged,andthenfindthedifferences,d Q,betweenalltheothercellsrequiringcharge andthatoftheonerequiringthemostcharge.(d )Thesedifferencesmustbebypassedforeach excessive ,thebypassFETis turnedONduringchargingforthecalculateddu rationforeachcellbypasstime.(e)Thebypass timeis calculatedbasedonthevalueofbypasscurrent ,whichin turndependsonbypassresistancevalues,R,as time= dQ R / (V duty_cycle).(f)R is calculatedasthesumoftheinternalbypassres istor(340 , typical)andtheseriesfilterresistors(Rx) = Rx 2 + questionareR2,R3,R9, 100 , whichresultsin R = 540.

4 (g)Thebq20zxxstoresthevalueofR( ) (V duty_cycle(ratio))asa (s/mAh).Here, ,V = ,R = 540 anddutycycle= 40%( ),thevalueis 540 ( )= 1350 Thisvaluemustbechangedif valuesofresistorsR2,R3,R9, November2005 RevisedJanuary2011 SubmitDocumentationFeedback 2005 2011, Balancing Methods SCHEMATICH3 SLUA340C November 2005 Revised January 2011 Cell Balancing Using the bq20zxxSubmit Documentation Feedback 2005 2011, Texas Instruments IncorporatedIMPORTANTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,modifications,enhancements,improvement s, s accordancewithTI s ,testingofallparametersofeachproductis , ,eitherexpressorimplied,is grantedunderanyTIpatentright,copyright,m askworkright,orotherTIintellectualproper tyrightrelatingtoanycombination,machine, orprocessin licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, safety-criticalapplications(suchaslifesu pport)

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