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BatteryStackManagementMakesanotherLeapForward ByGregZimmer Sr.ProductMarketingEngineer,SignalConditioningProducts LinearTechnologyCorp. Anydoubtsabouttheviabilityofelectricvehicleshavelongbeenputtorest.Theprimaryquestion nowis“howfar,howwideandhowdeepwillthisnewhigh-powerbatterytechnologypenetrate?”Perhaps notsurprisingly,noonereallyknowsforsure.Butitisinterestingtoconsidertheevolutionofthebattery managementsystem(BMS)electronics,andspecificallythemulticellbatterymonitorelectronicsatitsheart. Doingsomayprovidecluestotheadoptionrateofhighvoltagebatterypacksinapplicationsrangingfrom batterybackupsystemstoexoskeletons.Let’sconsidertheadvancesmadeinonefamilyofproducts,Linear Technology’sLTC68xx,intermsofsafety,accuracy,functionalityanddevelopmenttoolsupport. In2008,LinearTechnologyannouncedthefirsthighperformancemulticellbatterymonitor,the LTC6802.Amongitskeyfeatures,thispartincludedtheabilitytomeasureupto12Li-Ioncellswith0.25% maximumtotalmeasurementerrorwithin13ms.Thekeycapabilityofthesemulticellbatterymonitorsisthat manycanbeconnectedinseries,enablingsynchronizedmonitoringofeverycellinverylong,highvoltage batterystrings(Figure1).Sincethattime,LinearTechnologyhasfollowedupwiththeLTC6803,LTC6804and now,itsmostadvancedmulticellbatterymonitor,theLTC6811.Allfourofthesepartsservethesamebasic function:theymeasurethevoltageoneachbatterycellof12series-connectedcells.Theevolutionofthis familytrackscontinuousimprovementsinfunctionalsafety,measurementaccuracyandintegrated functionality. Figure1.SimplifiedDescriptionoftheMulticellBatteryMonitor S59, EN Themostdramaticprogressofthebatterymonitorelectronicsisthedevelopmentoffunctionalsafety, asdefinedbytheISO26262standard.Inessence,ISO26262systematicallyaddressespotentialhazardsinan automobilethatcouldbecausedbythemalfunctioningbehaviorofelectronicandelectricalsystems. AlthoughtheISO26262standardinvolvesnearlyeveryphaseofproductdevelopmentanduse,thesystem designermustfocusonhowtocontinuouslyconfirmtheproperoperationofeveryelementthatcouldimpact safety.Themulticellbatterymonitorplaysacentralroleinthistask,sinceincorrectvoltagesonthebattery cellsarethefirstindicatorsofapotentialproblem.Thispresentsasignificantdesignchallenge. BuiltintotheDNAofLinearTechnologyisthedeterminationtopursuetoughanalogelectronic problems,andautomotiveelectronicsarenoexception.Themulticellbatterymonitorsillustratethe achievementofhighreliability,highstability,andhighmeasurementaccuracy,withtheexpectationofyears ofoperationinanenvironmentofhighvoltages,extremetemperatures,hotpluggingandelectricalnoise.The ISO26262standardgoesonestepfurtherbyrequiring,amongotherthings,ananalysisofpotentialfailures andtheirremedies.Acommonmethodwithinelectronicstorecognizeandremedypotentialfailuresisto includeself-testcapabilityandredundancy.EvenbeforethepublicationoftheISO26262,LinearTechnology recognizedtheimportanceoffunctionalsafetyandincludedself-testcapabilityandinternalredundancyinthe LTC6802.Eachnewgenerationofmulticellbatterymonitorsaddedandrefinedthiscapability.Thelatest device,theLTC6811,continuesthisprogression,withimprovementstoincreaseitsinternaldiagnostic coverage.Thesefunctionsincludeadditionalredundantmeasurementpaths,improvedsynchronization betweeninputsignals,andimprovedself-testaccuracy.Theresultisquicker,simplerandmoreefficientselfteststohelpdesignersachievetheirISO26262goals.Evenfornon-automotiveapplications,thesefeatures givethedesignerconfidenceforwhateverhighreliabilityapplicationsthattheytarget. TheLinearTechnologypartshavesteadilyimprovedandinnovatedincellmeasurementaccuracy. Pursuingoutstandingaccuracyhasalwaysbeenakeydesigngoalbecausepotentialmeasurementerror translatestolesseffectivebatterymanagement,andultimatelylesspackcapacity,reliabilityand/orlife. Significanteffortwasmadetooptimizethebuilt-involtagereference,sinceitistheprimarydeterminantof measurementerror.ThefirstLinearTechnologymulticellbatterymonitorsincorporatedabandgapvoltage reference.Thiswastheconventionalchoice,sincebandgapreferencesaresmall,lowpowerandlowdropout. However,abandgapreferencecanbehavelikeastraingauge,convertingmechanicalstressfromPCB assembly,thermalvariations,humidity,andlong-termdriftintomeasurableerror.Toavoidthislimitation, Linearinitiatedauniqueapproachbyaddingadedicatedsub-surfaceZenervoltagereferencetothedesign. Thesereferencesofferoutstandingstability,overtemperature,timeandotheroperatingconditions.The result,availabletodayintheLTC6811,istheabilitytomeasureeverybatterycelltoworst-caseaccuracyof betterthan1.2mV(Figure2). Figure2.OutstandingTemperatureDriftPerformanceofthe BuriedZenerVoltageReference Furthermore,theLinearTechnologypartsensureoutstandingmeasurementaccuracy,eveninthe presenceofnoise,byfilteringthenoiseoneachbatterycellvoltage.Thisisaccomplishedbyusingdelta-sigma analog-to-digitalconvertersratherthanthefastSARconverters,commonlyusedinalternativeapproaches. Onceagain,LinearTechnologychosetobreakfromthepack,despitetheclearspeedadvantageofSAR converterswhenmeasuringhundredsofindividualbatterycells.Thischoicewasmadebecausethe automotiveenvironmentischock-fullofnoiseandtransientsfrommotors,solenoids,powerinverters,etc.All ofthisnoiseaffectsmeasurementaccuracy.Withadelta-sigmaconverter,theinputissampledmanytimes overthecourseofaconversionandthenaveraged.Theresultisbuiltinlowpassfilteringtoeliminatenoise asasourceofmeasurementerror,wherethecutofffrequencyisestablishedbythesamplerate.Forexample, theLTC6802usesa2ndorderdelta-sigmathatoperatesatafixed1ksamplepersecond.Theresultis36dBof rejectionto10kHzswitchingnoise(Figure3).Thetrade-offhowever,isthatmeasuring12cellswiththe LTC6802requires13msec,whichistooslowforsomeapplications.1Nonetheless,usingadelta-sigma converterhasremainedthemostpracticalmethodforachievingaccuratecellmeasurementinthenoisyreal world.Forthisreason,Linearhascontinuouslyimproveditsdelta-sigmaapproach.Today,theLTC6811usesa muchfaster3rdorderdelta-sigmaADCwithprogrammablesampleratesand8selectablecutofffrequencies. Theresultisoutstandingnoisereductionand8programmablemeasurementrates(Figure4),enabling measurementofall12batterycellsasfastas290µsec. 1 Although the SAR ADC topology allows for faster data conversions, the usefulness of the resulting data is questionable in a noisy system. For the same amount of 10kHz noise rejection as the LTC6802, a 1M sample per sec SAR converter would require a single-pole RC filter on each cell, with a corner frequency of 160Hz (Figure 3). The 12-bit settling time of the RC filter is 8.4msec. Therefore, even though a SAR converter can sequence through 10 channels in 10µsec, scanning more than once every 8.4msec is pointless because of the response of the filter. Figure3.LTC6802Delta-sigmaConvertervs.SAR ConverterwithRCCircuit. Figure4.LTC6811DeltaSigmaConverter Finally,itisinterestingtonotehowthefunctionalityofthemulticellbatterymonitorhasexpanded.As previouslymentioned,theprimarytaskofamulticellbatterymonitoristoaccuratelymeasurecellvoltages andcommunicatethisinformationtoahostprocessor.Furthermore,itisbestthatthemulticellbattery monitornotincludeinternalsoftware,asthiscouldpresentapotentialconflictwiththesystem-levelbattery management;thetaskofcollectingdatafromallofthecellsanddeterminingthestateofchargeorstateof healthshouldbeaccomplishedbythemainBMSprocessor.However,themulticellbatterymonitorresidesat themostcriticallocationinthebatterysystem,directlyconnectedtothecells.Here,itisinanidealposition tomonitorotherbatterysensors,suchascurrentortemperature,andcloselycorrelatethesevaluestocell measurements.Forthisreason,themulticellbatterymonitorcanactasacentralhubbetweentheBMS microprocessorandperipheraldevices. Forexample,theLTC6811offersveryflexiblegeneralpurposeI/Othatcanoperateasdigitalinputs, digitaloutputs,orasanaloginputs.Whenoperatedasanaloginputs,theLTC6811canmeasureanyvoltage fromV-to5Vwiththesamemeasurementaccuracyasthecellmeasurements.TheLTC6811canthen synchronizetheseexternalsignals,orthefull12-cellstackvoltage,tothecellvoltagemeasurements. Alternatively,thegeneralpurposeI/OcanbeusedinadigitalmodetocontrolI2CorSPIslavedevices.This enablestheLTC6811tocontrolmorecomplexfunctions,suchasmultiplexersforexpandedanaloginputsor EEPROMtostorecalibrationinformation. TheLTC6811hasadvancedcellbalancingcapability.UsingtheSPImasterfeature,theLTC6811can controlLinearTechnology’sSPI-basedactivebalancingIC,theLTC3300.TheLTC6811includesinternalpassive balancingFETSthatcandischargeindividualcells,ordirectlycontrollargerhigh-powerexternalFETs.The LTC6811hastheabilitytoconfigureeachcelldischargepintooperatewithanindependentperiod.This enablesbalancingeachcelluniquelyoverlongperiods,whilethemulticellbatterymonitorisnotactive. Finally,eachpassivebalancingpincanbeusedasaserialinterface.Thisisparticularlyusefulforinterfacingto LinearTechnology’sLT8584monolithicactivecellbalancers,whereactivebalancingcanbecontrolledand currentandtemperaturecanbemonitoredforeachindividualbatterycell. Tointegrateallofthisfunctionalityandshortendevelopmenttime,theLTC6811isfullysupportedby LinearTechnology’sLinduino™One(Figure5).TheLinduinoOneisanArduinoUnocompatible microcontrollerboardthatisfullyUSBisolatedandconnectsdirectlytoanLTC6811demonstrationboard. Withanonboardbootloaderforquickin-circuitfirmwareupdates,thisplatformprovidesasimple,stable hardwaredevelopmentplatform.SinceArduinoisanopensourceplatform,BMSdesignershaveeasyaccess tothesimpleandpowerfulArduinointegrateddevelopmentenvironment(IDE).Alibraryofcode,calledthe bmsSketchbook,providesLTC6811samplecodedesignedtocompileinanystandardCcompiler.Forexample, thebmsSketchbookincludesroutinestoreadandwriteconfiguration,readandwritecellvoltages,runselftests,redundancytestsandcontrolthepassivebalancing. Figure5.LinduinoDevelopmentSystem Conclusion Since2008,LinearTechnologyhasreleasedfourgenerationsofmulticellbatterymonitors.Thesafety features,accuracyandfunctionalityofthesedeviceshaveevolvedsignificantlyoverthisperiod,illustratingthe growingimportanceoftheseICsinhighperformancebatterymanagement.Inaddition,newtoolssimplify andstandardizetheirintegrationintobatterymanagementsystems,irrespectiveoftheendapplication. LinearTechnology’smostadvancedmulticellbatterymonitor,theLTC6811,asshowninFigure6,providesan impressivesetoffeaturesforpracticallyanyhighvoltage,highpoweredbatterysystem. th Figure6.LinearTechnology’sLTC6811:A4 GenerationMulticellBatteryMonitor