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bq27500
bq27501
System-SideImpedanceTrack™FuelGauge
SLUS785–SEPTEMBER2007
DsgCurrentThreshold
Thisregisterisusedasathresholdbymanyfunctionsinthebq27350todetermineifactualdischargecurrentisflowingintoor
outofthecell.Thedefaultforthisregisteris100mAwhichshouldbesufficientformostapplications.Thisthresholdshouldbe
setlowenoughtobebelowanynormalapplicationloadcurrentbuthighenoughtopreventnoiseordriftfromaffectingthe
measurement.
ChgCurrentThreshold
Thisregisterisusedasathresholdbymanyfunctionsinthebq27500/1todetermineifactualchargecurrentisflowingintoor
outofthecell.Thedefaultforthisregisteris50mAwhichshouldbesufficientformostapplications.Thisthresholdshouldbeset
lowenoughtobebelowanynormalchargecurrentbuthighenoughtopreventnoiseordriftfromaffectingthemeasurement.
QuitCurrent,DsgRelaxTime,ChgRelaxTime,andQuitRelaxTime
TheQuitCurrentisusedaspartoftheImpedanceTrack™algorithmtodeterminewhenthebq27500entersrelaxationmode
fromacurrentflowingmodeineitherthechargedirectionorthedischargedirection.ThevalueofQuitCurrentissettoadefault
valueof10mAandshouldbeabovethestandbycurrentofthesystem.
Eitherofthefollowingcriteriamustbemettoenterrelaxationmode:
1.|AverageCurrent()|<|QuitCurrent|forDsgRelaxTime.
2.|AverageCurrent()|<|QuitCurrent|forChgRelaxTime.
Afterabout30minutesinrelaxationmode,thebq27500attemptstotakeaccurateOCVreadings.Anadditionalrequirementof
dV/dt<4μV/secisrequiredforthebq27500/1toperformQmaxupdates.TheseupdatesareusedintheImpedanceTrack™
algorithms.ItiscriticalthatthebatteryvoltageberelaxedduringOCVreadingstoandthatthecurrentisnotbehigherthanC/20
whenattemptingtogointorelaxationmode.
QuitRelaxTimespecifiestheminimumtimerequiredforAverageCurrent()toremainabovetheQuitCurrentthresholdbefore
exitingrelaxationmode.
Qmax0andQmax1
GenericallycalledQmax,thesedynamicvariablescontaintherespectivemaximumchemicalcapacityoftheactivecellprofiles,
andaredeterminedbycomparingstatesofchargebeforeandafterapplyingtheloadwiththeamountofchargepassed.They
alsocorrespondtocapacityatverylowrateofdischarge,suchasC/20rate.Forhighaccuracy,thisvalueisperiodically
updatedbythebq27500/1duringoperation.Basedonthebatterycellcapacityinformation,theinitialvalueofchemicalcapacity
shouldbeenteredintheQmaxnfieldforeachdefaultcellprofile.TheImpedanceTrack™algorithmwillupdatethesevalues
andmaintainthemtheassociatedactualcellprofiles.
UpdateStatus0andUpdateStatus1
Bit1(0x02)oftheUpdateStatusnregistersindicatesthatthebq27500/1haslearnednewQmaxparametersandisaccurate.
Theremainingbitsarereserved.Bits1isuser-configurable;however,itisalsoastatusflagthatcanbesetbythebq27500/1.
Bit1shouldneverbemodifiedexceptwhencreatingagoldenimagefileasexplainedintheapplicationnote“Preparing
OptimizedDefaultFlashConstantsforspecificBatteryTypes”(seeSLUA334.pdf).Bit1isupdatedasneededbythebq27500/1.
AvgILastRun
Thebq27500logsthecurrentaveragedfromthebeginningtotheendofeachdischargecycle.Itstoresthisaveragecurrent
fromthepreviousdischargecycleinthisregister.Thisregistershouldneverneedtobemodified.Itisonlyupdatedbythe
bq27500/1whenrequired.
AvgPLastRun
Thebq27500/1logsthepoweraveragedfromthebeginningtotheendofeachdischargecycle.Itstoresthisaveragepower
fromthepreviousdischargecycleinthisregister.Togetacorrectaveragepowerreadingthebq27500/1continuouslymultiplies
instantaneouscurrenttimesVoltage()togetpower.Itthenlogsthisdatatoderivetheaveragepower.Thisregistershould
neverneedtobemodified.Itisonlyupdatedbythebq27500/1whenrequired.
DeltaVoltage
Thebq27500/1storesthemaximumdifferenceofVoltage()duringshortloadspikesandnormalload,sotheImpedance
Track™algorithmcancalculateremainingcapacityforpulsedloads.Itisnotrecommendedtochangethisvalue.
OCV,DefaultRa,Rb,andRaTables
Thesetablescontainencodeddataand,withtheexceptionoftheDefaultRaTables,areautomaticallyupdatedduringdevice
operation.Nouserchangesshouldbemadeexceptforreading/writingthevaluesfromapre-learnedpack(partoftheprocess
forcreatinggoldenimagefiles).
FUNCTIONALDESCRIPTION 22SubmitDocumentationFeedback