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M137, EN Gauging the High Accuracy 60V Gas Gauge By Christopher Gobok Sr. Product Marketing Engineer Linear Technology Corporation Introduction Many of us can relate to battery-powered devices which display how much power or run time a device currently has, especially because we have been surrounded by a multitude of gadgets at the home front. From electric shavers to tablets, we rely on all sorts of battery indicators to help determine if and how these devices will be used. In time, we become somewhat familiar with each device’s level of accuracy and know how much confidence to place in a device reporting, for example, 10% power left. In higher power multicell applications, situations can be more critical if users are found without adequate power, such as the case with ebikes, battery backup systems, power tools or medical instrumentation. A spare battery pack may not always be available or continuous operation for a specific length of time may be required, which allows us to appreciate accurate battery gas gauging (aka fuel gauging) or assessing how much charge a battery or battery pack has at any point in time. Battery gas gauging is just one of many functions typically found in a smart multicell battery system in addition to charging, protection and cell balancing circuitry. Regardless of function, battery systems present a unique set of design challenges simply because batteries are always changing in electrical nature. For example, a battery’s maximum capacity (also known as state- of-health or SOH) and self-discharge rate always decrease over time, while charge and discharge rates vary over temperature. Well-designed battery systems continuously address as many of these parametric shifts as possible in order to provide end users with consistently accurate battery performance criteria, such as charging time, estimated power or expected battery lifetime (or number of charges left). Simply put, accurate battery gas gauging requires an accurate battery gas gauge IC and a relevant battery-specific model to ultimately provide systems with the most coveted parameter in battery gas gauging – state-of-charge (SOC), or the present battery capacity as a percentage of maximum capacity. While there are battery gas gauges in the market that integrate battery models and algorithms in order to provide direct SOC estimations, peeling back the onion reveals that these devices tend to oversimplify SOC estimation at the tragic expense of accuracy. Moreover, these devices usually only work with particular battery chemistries and
Transcript of Gauging the High Accuracy 60V Gas Gauge - analog.com · accurate battery performance criteria, such...
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.
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