S65,EN
WirelessBatteryManagementSystemsHighlightIndustry’sDriveforHigherReliability
ByGregZimmerSr.ProductMarketingEngineer,SignalConditioningProducts
LinearTechnologyCorporation
Overview
Lithium-Ionbatteriesrequireconsiderablecareiftheyareexpectedtooperatereliablyoveralongperiod.Theycannotbeoperatedtotheextremeendoftheirstateofcharge(SOC).Thecapacityoflithiumioncellsdiminishesanddivergesovertimeandusage,soeverycellinasystemmustbemanagedtokeepitwithinaconstrainedSOC.
Toprovidesufficientpowerforavehicle,tensorhundredsofbatterycellsarerequired,configuredinalongseriesgeneratingasmuchas1000Vorhigher.Thebatteryelectronicsmustoperateatthisveryhighvoltageandrejectcommonmodevoltageeffects,whiledifferentiallymeasuringandcontrollingeachcellinthesestrings.Theelectronicsmustbeabletocommunicateinformationfromeachcellinabatterystacktoacentralpointforprocessing.
Inaddition,operatingahighvoltagebatterystackinavehicleorotherhigh-powerapplicationsimposestoughconditions,suchasoperationwithsignificantelectricalnoiseandwideoperatingtemperatures.Thebatterymanagementelectronicsareexpectedtomaximizeoperatingrange,lifetime,safetyandreliability,whileminimizingcost,sizeandweight.
SteadyadvancesinLinear’sbatterycellmonitoringICshaveenabledhighperformance,increasedlifeandreliabilityofbatterypacksinautomobilestoday.WirelessBMSpromisestofurtherimprovesafetyandreliabilityofthefullbatterysystem.
BatteryMonitoring
In2008,LinearTechnologyannouncedthefirsthighperformancemulticellbatterystackmonitor,theLTC6802.Amongitskeyfeatures,theLTC6802measuresupto12Li-Ioncellswith0.25%maximumtotalmeasurementerrorwithin13ms,andmanyLTC6802ICscanbeconnectedinseriestoenablethesimultaneouscellmonitoringofeverycellofalong,highvoltagebatterystrings.LinearTechnologyhasimprovedupontheLTC6802manytimesovertheyears.AllofthedevicesinLinear’sLTC68XXfamilyareintendedforprecisionbatterymanagementwithinhybrid/electricvehicle(HEVs),electricvehicles(EVs)andotherhighvoltage,highpowerbatterystacks.
TheLTC6811isLinearTechnology’slatestmulticellbatterystackmonitor,incorporatinganultrastablevoltagereference,highvoltagemultiplexers,anddual16-bitdelta-sigmaADCs.AnLTC6811canmeasureupto12series-connectedbatterycellsatvoltageswithbetterthan0.04%accuracy.InthefastestADCmode,allcellscanbemeasuredwithin290μsec.Witheightprogrammable3rdorderlowpassfiltersettings,theLTC6811canprovideoutstandingnoisereduction.Theresultisoutstandingcellmeasurementaccuracy,enablingprecisebatterymanagementforincreasedbatterypackcapacity,safetyandlife.
EachLTC6811includestwobuilt-in1MHzserialinterfaces,anSPIinterfaceforconnectingtoalocalmicroprocessor,andtheproprietary2-wireisoSPIinterface.TheisoSPIinterfaceprovidestwocommunicationoptions:multipledevicescanbeconnectedinadaisychaintotheBMSmaster(hostprocessor)ormultipledevicescanbeconnectedandaddressedinparalleltotheBMSmaster.
ModularBatteryPacksToaccommodatethelargequantityofcellsrequiredforhighpoweredautomotivesystems,batteriesareoftendividedintopacks,anddistributedthroughoutavailablespacesinthevehicle.With10to24cellsinatypicalmodule,modulescanbeassembledindifferentconfigurationstosuitmultiplevehicleplatforms.Amodulardesignsimplifiesmaintenanceandwarrantyissues,andcanbeusedasthebasisforverylargebatterystacks.Itallowsbatterypackstobedistributedoverlargerareas,formoreeffectiveuseofspace.
Tosupportadistributed,modulartopologywithinthehighelectromagneticinterference(EMI)environmentofanEV/HEV,arobustcommunicationsystemisrequired.BothisolatedCANBus,andLinear’sisoSPIofferroad-provensolutionsforinterconnectingmodulesinthisenvironment.
GiventhesuccessofCANBusinautomotiveapplications,itprovidesawell-establishednetworkforinterconnectingbatterymodules,butrequiresanumberofadditionalcomponents.Forexample,implementinganisolatedCANBusviatheLTC6811’sSPIinterfacerequirestheadditionofaCANtransceiver,amicroprocessorandanisolator.TheprimarydownsideofaCANBusistheaddedcostandboardspacerequiredfortheseadditionalelements.
Figure 1. Modular BMS Electronics Using CAN Bus
AnalternativetoaCANBusinterfaceisLinearTechnology’sinnovative2-wireisoSPIinterface.IntegratedintoeveryLTC6811,theisoSPIinterfaceusesasimpletransformerandasingletwistedpair,asopposedtothefourwiresrequiredbyCANbus.TheisoSPIinterfaceprovidesahighRFnoiseimmuneinterfaceinwhichmodulescanbeconnectedinadaisy-chainoverlongcablelengthsandoperatedatdataratesupto1Mbps.
Figure 2. Modular BMS Electronics Using isoSPI Connected in a Daisy Chain
Wireless BMS InawirelessBMS,eachmoduleisinterconnectedviaawirelessconnectioninsteadofaCANBuscableoranisoSPItwistedpair.Today,LinearTechnologyisdemonstratingtheindustry’sfirstwirelessautomotivebatterymanagementsystem(BMS)conceptcar.ThiswirelessBMSconceptcarcombinestheLTC6811batterystackmonitorwithLinear’sSmartMesh®wirelessmeshnetworkingproductsinaBMWi3,replacingthetraditionalwiredconnectionsbetweenthebatterypacksandthebatterymanagementsystem.ThisdemonstrationofafullywirelessBMScarrepresentsasignificantbreakthroughthatoffersthepotentialforimprovedreliability,lower costandreducedwiringcomplexityforlargemulticellbatterystacksforelectricandhybrid/electricvehicles.
Automakersarechallengedtoensurethedrivingpublicthatelectricandhybrid/electricvehiclesarebothsafeandreliable.LinearTechnologyisnowlookingbeyondthesafetyandreliabilityofthebatterymonitoringICtoaddressthepotentialmechanicalfailureofconnectors,cablesandwiringharnessesinhigh-vibrationautomotiveenvironments.Todate,themetalandhigh-EMIsurroundingsinvehicleswerethoughttobetooharshforawireless
systemtobereliable.However,SmartMeshnetworkingoffersatrulyredundantinterconnectsystemthroughitsuseofbothpathandfrequencydiversitytoroutewirelessmessagesaroundobstaclesandtomitigateinterference.Field-proveninindustrialInternetofThingsapplications,SmartMeshembeddedwirelessnetworksdeliver>99.999%reliabledatatransmissioninharshenvironments,suchasrailcarmonitoring,mining,andindustrialprocessplants.Bydeliveringthereliabilityofwiresyeteliminatingmechanicalconnectorfailures,thewirelessBMSconceptcar,showsthepromiseofwirelesstechnologytosignificantlyimproveoverallsystemreliabilityandsimplifythedesignofautomotivebatterymanagementsystems.
Figure 3. Modular BMS Electronics Using SmartMesh Network
AdditionalBenefits
ABMSwithaSmartMeshnetworkhasthepotentialfornewfunctionality,currentlyunavailableinawiredsystem.Thewirelessmeshnetworkenablestheflexibleplacementofbatterymodules,andmakespossibletheinstallationofsensorsinlocationspreviouslyunsuitablefora
wiringharness.Additionaldatagermanetotheaccuracyofbatterystateofcharge(SOC)calculations,suchascurrentandtemperature,maybecollectedbytheBMSMasterbysimplyaddingSmartMeshenabledsensors.SmartMeshautomaticallytimesynchronizeseachnodetowithinafewmicroseconds,andaccuratelytimestampsmeasurementsateachnode.Theabilitytotime-correlatemeasurementstakenatdifferentlocationsinavehicleisapowerfulfeatureforcalculatingmoreaccuratelythebatterystateofcharge(SOC)andstateofhealth(SOH).ASmartMeshnodewithlocalprocessingateachmoduleimprovesnormalBMSoperation,andalsopresentsthepotentialforsmartbatterymodules,wheremodulediagnosticsandcommunicationmaybeavailabletoenhanceassemblyandservice.
About SmartMesh NetworksSmartMeshwirelesssensornetworkingproductsarechipsandpre-certifiedPCBmodulescompletewithmeshnetworkingsoftware,enablingsensorstocommunicateintoughindustrialInternetofThings(IoT)environments.
SmartMeshproductsarefieldproven,withover50,000customernetworksdeployedin120countries.Bydelivering>99.999%datareliabilityintoughRFenvironments,SmartMeshwirelesssensornetworksareentrustedbyindustrialIoTproviderstodelivercriticalsensorandcontroldatareliablyformanyyearswithoutrequiringintervention.
Figure 4. SmartMesh IP Wireless Sensor Network
SmartMeshFeatures
• PROVEN:Fieldproven>99.999%reliabilityinIndustrie4.0applications• ROBUST:Timesynchronized,channelhoppingmeshtechnologyautomaticallymitigates
faultsbasedonself-diagnostics• SECURE:RobustsecurityincludesNISTcertifiedAES128encryption
ComparisonofInterconnectsThefollowingdiagramshighlightthevariouselectricalconnectionsbetweenCANBus,isoSPIandSmartMeshnetworks:
Figure 5. Battery Monitoring Interconnections with CAN Bus
Figure 6. Battery Monitoring Interconnections with isoSPI
Figure 7. Battery Monitoring Interconnections with SmartMesh WSN
Figure 8. Battery Monitoring & Additional Sensor Interconnections with SmartMesh WSN
Summary
Withleadingtechnologyinbatterymonitorsandreliablewirelessnetworking,LinearTechnologyiswellpositionedtoassesshow,when,andwherewirelessBMScanbedeployed.ASmartMeshwirelessBMSpromisestoeliminatemaintenance-proneconnectors,cablesandharnesses.WirelessBMSenablesexpandedBMSfunctionalityviaSmartMeshscalabilityandtime-stampeddataacquisitioncapability.LinearTechnologycontinuestocollaboratewithcustomers,providinghighperformanceanalogandpowersolutions.
BenefitsofWirelessBMS
• Eliminatesmaintenance-proneconnectors,cablesandharness.• Permitstheadditionofnewsensorstofurtherimprovereliability.• Simplifiesautoassemblyandbatterymaintenanceandprovidesdesignerswith
additionalmechanicaldesignflexibility.• Allowsforprecisetime-stampeddatacollection,whichmayalsofurtherimprove
SOC/SOHcalculation.