S59, EN

BatteryStackManagementMakesanotherLeapForward

ByGregZimmerSr.ProductMarketingEngineer,SignalConditioningProducts

LinearTechnologyCorp.

Anydoubtsabouttheviabilityofelectricvehicleshavelongbeenputtorest.Theprimaryquestionnowis“howfar,howwideandhowdeepwillthisnewhigh-powerbatterytechnologypenetrate?”Perhapsnotsurprisingly,noonereallyknowsforsure.Butitisinterestingtoconsidertheevolutionofthebatterymanagementsystem(BMS)electronics,andspecificallythemulticellbatterymonitorelectronicsatitsheart.Doingsomayprovidecluestotheadoptionrateofhighvoltagebatterypacksinapplicationsrangingfrombatterybackupsystemstoexoskeletons.Let’sconsidertheadvancesmadeinonefamilyofproducts,LinearTechnology’sLTC68xx,intermsofsafety,accuracy,functionalityanddevelopmenttoolsupport.

In2008,LinearTechnologyannouncedthefirsthighperformancemulticellbatterymonitor,theLTC6802.Amongitskeyfeatures,thispartincludedtheabilitytomeasureupto12Li-Ioncellswith0.25%maximumtotalmeasurementerrorwithin13ms.Thekeycapabilityofthesemulticellbatterymonitorsisthatmanycanbeconnectedinseries,enablingsynchronizedmonitoringofeverycellinverylong,highvoltagebatterystrings(Figure1).Sincethattime,LinearTechnologyhasfollowedupwiththeLTC6803,LTC6804andnow,itsmostadvancedmulticellbatterymonitor,theLTC6811.Allfourofthesepartsservethesamebasicfunction:theymeasurethevoltageoneachbatterycellof12series-connectedcells.Theevolutionofthisfamilytrackscontinuousimprovementsinfunctionalsafety,measurementaccuracyandintegratedfunctionality.

Figure1.SimplifiedDescriptionoftheMulticellBatteryMonitor

Themostdramaticprogressofthebatterymonitorelectronicsisthedevelopmentoffunctionalsafety,asdefinedbytheISO26262standard.Inessence,ISO26262systematicallyaddressespotentialhazardsinanautomobilethatcouldbecausedbythemalfunctioningbehaviorofelectronicandelectricalsystems.AlthoughtheISO26262standardinvolvesnearlyeveryphaseofproductdevelopmentanduse,thesystemdesignermustfocusonhowtocontinuouslyconfirmtheproperoperationofeveryelementthatcouldimpactsafety.Themulticellbatterymonitorplaysacentralroleinthistask,sinceincorrectvoltagesonthebatterycellsarethefirstindicatorsofapotentialproblem.Thispresentsasignificantdesignchallenge.

BuiltintotheDNAofLinearTechnologyisthedeterminationtopursuetoughanalogelectronicproblems,andautomotiveelectronicsarenoexception.Themulticellbatterymonitorsillustratetheachievementofhighreliability,highstability,andhighmeasurementaccuracy,withtheexpectationofyearsofoperationinanenvironmentofhighvoltages,extremetemperatures,hotpluggingandelectricalnoise.TheISO26262standardgoesonestepfurtherbyrequiring,amongotherthings,ananalysisofpotentialfailuresandtheirremedies.Acommonmethodwithinelectronicstorecognizeandremedypotentialfailuresistoincludeself-testcapabilityandredundancy.EvenbeforethepublicationoftheISO26262,LinearTechnologyrecognizedtheimportanceoffunctionalsafetyandincludedself-testcapabilityandinternalredundancyintheLTC6802.Eachnewgenerationofmulticellbatterymonitorsaddedandrefinedthiscapability.Thelatestdevice,theLTC6811,continuesthisprogression,withimprovementstoincreaseitsinternaldiagnosticcoverage.Thesefunctionsincludeadditionalredundantmeasurementpaths,improvedsynchronizationbetweeninputsignals,andimprovedself-testaccuracy.Theresultisquicker,simplerandmoreefficientself-teststohelpdesignersachievetheirISO26262goals.Evenfornon-automotiveapplications,thesefeaturesgivethedesignerconfidenceforwhateverhighreliabilityapplicationsthattheytarget.

TheLinearTechnologypartshavesteadilyimprovedandinnovatedincellmeasurementaccuracy.Pursuingoutstandingaccuracyhasalwaysbeenakeydesigngoalbecausepotentialmeasurementerrortranslatestolesseffectivebatterymanagement,andultimatelylesspackcapacity,reliabilityand/orlife.Significanteffortwasmadetooptimizethebuilt-involtagereference,sinceitistheprimarydeterminantofmeasurementerror.ThefirstLinearTechnologymulticellbatterymonitorsincorporatedabandgapvoltagereference.Thiswastheconventionalchoice,sincebandgapreferencesaresmall,lowpowerandlowdropout.However,abandgapreferencecanbehavelikeastraingauge,convertingmechanicalstressfromPCBassembly,thermalvariations,humidity,andlong-termdriftintomeasurableerror.Toavoidthislimitation,Linearinitiatedauniqueapproachbyaddingadedicatedsub-surfaceZenervoltagereferencetothedesign.Thesereferencesofferoutstandingstability,overtemperature,timeandotheroperatingconditions.Theresult,availabletodayintheLTC6811,istheabilitytomeasureeverybatterycelltoworst-caseaccuracyofbetterthan1.2mV(Figure2).

Figure2.OutstandingTemperatureDriftPerformanceoftheBuriedZenerVoltageReference

Furthermore,theLinearTechnologypartsensureoutstandingmeasurementaccuracy,eveninthepresenceofnoise,byfilteringthenoiseoneachbatterycellvoltage.Thisisaccomplishedbyusingdelta-sigmaanalog-to-digitalconvertersratherthanthefastSARconverters,commonlyusedinalternativeapproaches.Onceagain,LinearTechnologychosetobreakfromthepack,despitetheclearspeedadvantageofSARconverterswhenmeasuringhundredsofindividualbatterycells.Thischoicewasmadebecausetheautomotiveenvironmentischock-fullofnoiseandtransientsfrommotors,solenoids,powerinverters,etc.Allofthisnoiseaffectsmeasurementaccuracy.Withadelta-sigmaconverter,theinputissampledmanytimesoverthecourseofaconversionandthenaveraged.Theresultisbuiltinlowpassfilteringtoeliminatenoiseasasourceofmeasurementerror,wherethecutofffrequencyisestablishedbythesamplerate.Forexample,theLTC6802usesa2ndorderdelta-sigmathatoperatesatafixed1ksamplepersecond.Theresultis36dBofrejectionto10kHzswitchingnoise(Figure3).Thetrade-offhowever,isthatmeasuring12cellswiththeLTC6802requires13msec,whichistooslowforsomeapplications.1Nonetheless,usingadelta-sigmaconverterhasremainedthemostpracticalmethodforachievingaccuratecellmeasurementinthenoisyrealworld.Forthisreason,Linearhascontinuouslyimproveditsdelta-sigmaapproach.Today,theLTC6811usesamuchfaster3rdorderdelta-sigmaADCwithprogrammablesampleratesand8selectablecutofffrequencies.Theresultisoutstandingnoisereductionand8programmablemeasurementrates(Figure4),enablingmeasurementofall12batterycellsasfastas290µsec.

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.SARConverterwithRCCircuit.

Figure4.LTC6811DeltaSigmaConverter

Finally,itisinterestingtonotehowthefunctionalityofthemulticellbatterymonitorhasexpanded.Aspreviouslymentioned,theprimarytaskofamulticellbatterymonitoristoaccuratelymeasurecellvoltagesandcommunicatethisinformationtoahostprocessor.Furthermore,itisbestthatthemulticellbatterymonitornotincludeinternalsoftware,asthiscouldpresentapotentialconflictwiththesystem-levelbatterymanagement;thetaskofcollectingdatafromallofthecellsanddeterminingthestateofchargeorstateofhealthshouldbeaccomplishedbythemainBMSprocessor.However,themulticellbatterymonitorresidesatthemostcriticallocationinthebatterysystem,directlyconnectedtothecells.Here,itisinanidealpositiontomonitorotherbatterysensors,suchascurrentortemperature,andcloselycorrelatethesevaluestocellmeasurements.Forthisreason,themulticellbatterymonitorcanactasacentralhubbetweentheBMSmicroprocessorandperipheraldevices.

Forexample,theLTC6811offersveryflexiblegeneralpurposeI/Othatcanoperateasdigitalinputs,digitaloutputs,orasanaloginputs.Whenoperatedasanaloginputs,theLTC6811canmeasureanyvoltagefromV-to5Vwiththesamemeasurementaccuracyasthecellmeasurements.TheLTC6811canthensynchronizetheseexternalsignals,orthefull12-cellstackvoltage,tothecellvoltagemeasurements.Alternatively,thegeneralpurposeI/OcanbeusedinadigitalmodetocontrolI2CorSPIslavedevices.ThisenablestheLTC6811tocontrolmorecomplexfunctions,suchasmultiplexersforexpandedanaloginputsorEEPROMtostorecalibrationinformation.

TheLTC6811hasadvancedcellbalancingcapability.UsingtheSPImasterfeature,theLTC6811cancontrolLinearTechnology’sSPI-basedactivebalancingIC,theLTC3300.TheLTC6811includesinternalpassivebalancingFETSthatcandischargeindividualcells,ordirectlycontrollargerhigh-powerexternalFETs.TheLTC6811hastheabilitytoconfigureeachcelldischargepintooperatewithanindependentperiod.Thisenablesbalancingeachcelluniquelyoverlongperiods,whilethemulticellbatterymonitorisnotactive.Finally,eachpassivebalancingpincanbeusedasaserialinterface.ThisisparticularlyusefulforinterfacingtoLinearTechnology’sLT8584monolithicactivecellbalancers,whereactivebalancingcanbecontrolledandcurrentandtemperaturecanbemonitoredforeachindividualbatterycell.

Tointegrateallofthisfunctionalityandshortendevelopmenttime,theLTC6811isfullysupportedbyLinearTechnology’sLinduino™One(Figure5).TheLinduinoOneisanArduinoUnocompatiblemicrocontrollerboardthatisfullyUSBisolatedandconnectsdirectlytoanLTC6811demonstrationboard.Withanonboardbootloaderforquickin-circuitfirmwareupdates,thisplatformprovidesasimple,stablehardwaredevelopmentplatform.SinceArduinoisanopensourceplatform,BMSdesignershaveeasyaccesstothesimpleandpowerfulArduinointegrateddevelopmentenvironment(IDE).Alibraryofcode,calledthebmsSketchbook,providesLTC6811samplecodedesignedtocompileinanystandardCcompiler.Forexample,thebmsSketchbookincludesroutinestoreadandwriteconfiguration,readandwritecellvoltages,runself-tests,redundancytestsandcontrolthepassivebalancing.

Figure5.LinduinoDevelopmentSystem

Conclusion

Since2008,LinearTechnologyhasreleasedfourgenerationsofmulticellbatterymonitors.Thesafetyfeatures,accuracyandfunctionalityofthesedeviceshaveevolvedsignificantlyoverthisperiod,illustratingthegrowingimportanceoftheseICsinhighperformancebatterymanagement.Inaddition,newtoolssimplifyandstandardizetheirintegrationintobatterymanagementsystems,irrespectiveoftheendapplication.LinearTechnology’smostadvancedmulticellbatterymonitor,theLTC6811,asshowninFigure6,providesanimpressivesetoffeaturesforpracticallyanyhighvoltage,highpoweredbatterysystem.

Figure6.LinearTechnology’sLTC6811:A4thGenerationMulticellBatteryMonitor