M137,EN

GaugingtheHighAccuracy60VGasGauge

ByChristopherGobokSr.ProductMarketingEngineerLinearTechnologyCorporation

IntroductionManyofuscanrelatetobattery-powereddeviceswhichdisplayhowmuchpowerorruntimeadevicecurrentlyhas,especiallybecausewehavebeensurroundedbyamultitudeofgadgetsatthehomefront.Fromelectricshaverstotablets,werelyonallsortsofbatteryindicatorstohelpdetermineifandhowthesedeviceswillbeused.Intime,webecomesomewhatfamiliarwitheachdevice’slevelofaccuracyandknowhowmuchconfidencetoplaceinadevicereporting,forexample,10%powerleft.Inhigherpowermulticellapplications,situationscanbemorecriticalifusersarefoundwithoutadequatepower,suchasthecasewithebikes,batterybackupsystems,powertoolsormedicalinstrumentation.Asparebatterypackmaynotalwaysbeavailableorcontinuousoperationforaspecificlengthoftimemayberequired,whichallowsustoappreciateaccuratebatterygasgauging(akafuelgauging)orassessinghowmuchchargeabatteryorbatterypackhasatanypointintime.Batterygasgaugingisjustoneofmanyfunctionstypicallyfoundinasmartmulticellbatterysysteminadditiontocharging,protectionandcellbalancingcircuitry.Regardlessoffunction,batterysystemspresentauniquesetofdesignchallengessimplybecausebatteriesarealwayschanginginelectricalnature.Forexample,abattery’smaximumcapacity(alsoknownasstate-of-healthorSOH)andself-dischargeratealwaysdecreaseovertime,whilechargeanddischargeratesvaryovertemperature.Well-designedbatterysystemscontinuouslyaddressasmanyoftheseparametricshiftsaspossibleinordertoprovideenduserswithconsistentlyaccuratebatteryperformancecriteria,suchaschargingtime,estimatedpowerorexpectedbatterylifetime(ornumberofchargesleft).Simplyput,accuratebatterygasgaugingrequiresanaccuratebatterygasgaugeICandarelevantbattery-specificmodeltoultimatelyprovidesystemswiththemostcovetedparameterinbatterygasgauging–state-of-charge(SOC),orthepresentbatterycapacityasapercentageofmaximumcapacity.WhiletherearebatterygasgaugesinthemarketthatintegratebatterymodelsandalgorithmsinordertoprovidedirectSOCestimations,peelingbacktheonionrevealsthatthesedevicestendtooversimplifySOCestimationatthetragicexpenseofaccuracy.Moreover,thesedevicesusuallyonlyworkwithparticularbatterychemistriesand

requireadditionalexternalcomponentstointerfacewithhighvoltages.EnterLinearTechnology’sLTC2944depictedinFigure1–asimple,60Vbatterygasgaugethatintentionallyprovidesthebareessentialsforaccurate,singleormulticell,batterygasgauging.

Figure1.LTC294460VBatteryGasGaugeCountonCountingCoulombsCurrentstudieshaveshownthatprecisecoulombcounting,voltage,currentandtemperatureareprerequisitesforaccurateSOCestimation,whichsofarhaveresultedinaminimumof5%error.Theseparametersallowustopinpointwherealongachargeordischargecurveabatteryisat,wherecoulombcountingnotonlyreinforcesvoltagereadingsbutalsohelpsdifferentiateanyflatregionsofacurve–Figure2showstypicaldischargecurvesfordifferentbatterychemistries.Countingcoulombshelpsdodgetheexamplesituationofadevicemisleadinglyreporting75%SOCforalongperiodoftimeandthensuddenlydroppingdownto15%SOC,whichiswhattendstohappenindevicesthatonlymeasurevoltagetoassessSOC.Tocountcoulombs,usersinitializeacoulombcountertoaknownbatterycapacitywhenthebatteryisfullychargedandthencountdownwhendischargingcoulombsorcountupwhenchargingcoulombs(toaccountforpartialcharging).Thebeautyofthisschemeisthatbatterychemistrydoesnothavetobeknown.BecausetheLTC2944integratesacoulombcounter,thisdevicecanbeeasilycopied-and-pastedacrossmultipledesigns,agnosticofbatterychemistry.

Figure2.TypicalDischargeCurvesforDifferentBatteryChemistries

TakealookathowtheLTC2944countscoulombsinFigure3.Remember,chargeisthetimeintegralofcurrent.TheLTC2944measureschargewithupto99%accuracybymonitoringthevoltagedevelopedacrossasenseresistorwitha±50mVsensevoltagerange,wherethedifferentialvoltageisappliedtoanauto-zeroeddifferentialanalogintegratortoinfercharge.Whentheintegratoroutputrampstothehighandlowreferencelevels(REFHIandREFLO),theswitchestoggletoreversetherampdirection.Controlcircuitrythenobservestheconditionoftheswitchesandtherampdirectiontodeterminepolarity.Next,aprogrammableprescalerallowsuserstoincreaseintegrationtimebyafactorof1to4096.Witheachunderfloworoverflowoftheprescaler,theaccumulatedchargeregister(ACR)isfinallyincrementedordecrementedbyonecount.

Figure3.TheLTC2944MeasuresChargewithupto99%AccuracyItisworthnotingthattheanalogintegratorusedintheLTC2944’scoulombcounterintroducesminimaldifferentialoffsetvoltageand,therefore,minimizestheeffectontotalchargeerror.Manycoulombcountingbatterygasgaugesperformananalog-to-digitalconversionofthevoltageacrossthesenseresistorandaccumulatetheconversionresultstoinfercharge.Insuchascheme,thedifferentialoffsetvoltagecanbethemainsourceoferror,especiallyduringsmallsignalreadings.Forexample,considerabatterygasgaugewithanADC-basedcoulombcounterandamaximumspecifieddifferentialvoltageoffsetof20µVthatdigitallyintegratesa1mVinputsignal–thechargeerrorduetooffsetwouldbe2%.Bycomparison,thechargeerrorduetooffsetusingtheLTC2944’sanalogintegratorwouldonlybe0.04%,50timessmaller!BacktoBasics–Voltage,Current&TemperatureIfcoulombcountingisresponsibleforreinforcingvoltagereadingsanddifferentiatingflatregionsofachargeordischargecurve,thencurrentandtemperatureareparametersresponsibleforfetchingthemostrelevantcurvetobeginwith.Thechallengeisthatabattery’sterminalvoltage(voltagewhileconnectedtoaload)issignificantlyaffectedbythebatterycurrentandtemperature.Therefore,voltagereadingsmustbecompensatedwithcorrection

termsproportionaltothebatterycurrentandopen-circuitvoltage(voltagewhiledisconnectedfromload)versustemperature.Becauseitisn’tpracticaltodisconnectabatteryfromaloadduringoperationforthesolepurposeofmeasuringtheopen-circuitvoltage,itisgoodpracticetoatleastadjustterminalvoltagereadingspercurrentandtemperatureprofiles.SincehighSOCaccuracyistheultimatedesigngoal,theLTC2944usesa14-bitNoLatencyΔ∑™ADCtomeasurevoltage,currentandtemperaturewithupto1.3%and±3°Cguaranteedaccuracyrespectively.Inreality,thetypicalperformanceoftheLTC2944ismuchbetter.ThegraphsinFigure4showhowcertainaccuracyfiguresofmeritintheLTC2944varywithtemperatureandvoltage.Figure4ashowsthattheADCtotalunadjustederrorwhenmeasuringvoltageistypicallylessthan±0.5%andfairlyconstantoversensevoltage.Similarly,Figure4bshowsthattheADCgainerrorwhenmeasuringcurrentistypicallylessthan±0.5%overtemperature.Lastly,Figure4cshowsthatthetemperatureerroronlyvariesbyabout±1°Covertemperatureforanygivensensevoltage.AlloftheseaccuracyfiguresaddupandcaneasilycompromiseSOCaccuracy,whichiswhyitisimportanttonotehowaccurateaparticularbatterygasgaugemeasuresvoltage,currentandtemperatureamongstmanyspecifications.

Figure4a.VoltageMeasurementFigure4b.CurrentMeasurementFigure4c.TemperatureErrorvs.ADCGainErrorADCGainErrorTemperature

TheLTC2944providesfourADCmodesofoperationwhenmeasuringvoltage,currentandtemperature.Inautomaticmode,thedevicecontinuouslyperformsADCconversionseverycoupleofmillisecondsversusscanmodewhichconvertsevery10sandthengoestosleep.Inmanualmode,thedeviceperformsasingleconversiononcommandandthengoestosleep.Anytimethedeviceisinsleepmode,quiescentcurrentisminimizedto80µA.TheentireanalogsectionoftheLTC2944canalsobecompletelyshutdowntoreducequiescentcurrentevenfurtherto15µA,sincethelastthinguserswantisabatterygasgaugethatironicallyconsumesalotofbatterypower.

ConvenientInterfacingUserscanreadoutbatterycharge,voltage,currentandtemperaturefromtheLTC2944usingadigitalI2Cinterface.Userscanalsoreadoutstatus,controlon/off,andsetalertablehighandlowthresholdsforeachparameterbyconfiguringafew16-bitregistersviaI2C.ThealertsystemeliminatestheneedforcontinuoussoftwarepollingandfreestheI2Cbusandhosttoperformothertasks.Additionally,anALCCpinservesasbothaSMBusalertoutputorachargecompleteinputthatcanbeconnectedtoachargecompleteoutputofabatterychargingcircuit.Withallofthisdigitalfunctionality,someonemightstillask,“Whyweren’tbatteryprofilesorcapacity/SOCestimationalgorithmsbuiltintotheLTC2944?”Theanswerissimple–itallcomesdownto(perhapsnotsurprisingly)accuracy.Whilebatterygasgaugeswithbuilt-inbatteryprofilesandalgorithmsmaysimplifydesigns,theyareoftentimesinadequateorirrelevantmodelsofrealworldbatterybehaviorandsloppilysacrificeSOCaccuracyintheprocess.Forexample,usersmaybeforcedtoworkwithgenericchargeanddischargeprofilesthatweregeneratedbyunspecifiedsourcesoroverunknowntemperatureranges;exactbatterychemistriesmaynotbesupportedwhichcausesanotherhittoSOCaccuracy.ThepointisaccuratebatterymodelingtypicallyconsidersmanyvariablesandiscomplexenoughthatitmakessenseforuserstomodeltheirownbatteryinsoftwareinordertoobtainthehighestlevelofSOCaccuracy,ratherthanrelyoninaccurategenericbuilt-inmodels.Thesebuilt-inmodelsalsomakebatterygasgaugesinflexibleanddifficulttoreuseindesigns.Putanotherway,itisaloteasiertomakechangesinsoftwarethaninhardware,wherechangingapplication-specificcodeismucheasierthanswappingoutabatterygasgaugethatalsoneedstobeconfigured.IfprovidingalloftheessentialbatterymeasurementparameterswithunrivaledaccuracyviaaveryhandyI2Cinterfacewerenotenough,thenthehighvoltagecapabilitiesiswhattrulysetstheLTC2944apartfromotherbatterygasgaugesinthemarkettoday.TheLTC2944canbedirectlypoweredfromabatteryassmallas3.6Vtoafullblownbatterystackupto60V,addressinganyapplicationfromlow-powerportableelectronicstopower-hungryelectricvehicles.Noneedtocomplicatedesignswithadditionallevelshiftingcircuitryonthesupplyormeasurementpins–directconnectionsbetweenthebattery(orbatterystack)andtheLTC2944ispossibleanddrasticallysimplifieshardwaredesign.Minimizingthenumberofexternalcomponentsalsolowerstheoverallpowerconsumptionandincreasesaccuracysincecomponents,likeresistivedividers,arenotpresent.

ConclusionBatterygasgaugingisanartinitselfbecauseofthemanyinterdependentparametersthatinfluenceSOC.Expertsaroundtheworldagreethataccuratecoulombcounting,coupledwithvoltage,currentandtemperaturereadings,providethemostaccuratemethodtoestimateSOC.TheLTC2944batterygasgaugeprovidesallofthesefundamentalmeasurementsanddeliberatelyexcludesinternalbatterymodeling,allowinguserstoimplementtheirownrelevantprofilesandalgorithmsinapplication-specificsoftware.Moreover,measurementsandconfigurationregistersareeasilyaccessedoverI2C,whileupto60VmulticellconnectionscanbemadedirectlytotheLTC2944.Batterygasgauginganynumberofbatterieswithanybatterychemistryhasneverbeeneasieror,moreimportantly,moreaccurate.