Download Battery Stack Management Makes another Leap Forward

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