毕业设计外文翻译燃料电池.docx

毕业设计外文翻译燃料电池.docx

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毕业设计外文翻译燃料电池

附件2：

外文原文

BatteryFuelGauges:

AccuratelyMeasuringChargeLevel

Abstract:

Batteryfuelgaugesdeterminetheamountofchargeremaininginasecondarybatteryandhowmuchlonger（underspecificoperatingconditions）thebatterycancontinueprovidingpower.Thisapplicationnotediscussesthechallengespresentedinmeasuringthechargeremaininginalithium-ionbatteryandthedifferentmethodsofimplementingafuelgaugetoaddressthesechallenges.

Introduction

Sincetheadventofthemobilephone,chargeablebatteriesandtheirassociatedfuel-gaugeindicatorshavebecomeanintegralpartofourinformationandcommunicationsociety.Theyarejustasimportanttousnowasautomotivefuelgaugeshavebeenforthepast100years.Yet,whiledriversdonottolerateinaccuratefuelgauges,mobile-phoneusersareoftenexpectedtolivewithhighlyinaccurate,low-resolutionindicators.Thisarticlediscussesthevariousimpedimentstoaccuratelymeasuringchargelevelsanddescribeshowdesignerscanimplementaccuratefuelgaugingintheirbattery-poweredapplications.

Lithium-IonBatteries

Lithium-ionbatterieshaveonlybeeninmassproductionsinceabout1997,followingtheresolutionofvarioustechnicalproblemsduringtheirdevelopment.Becausetheyofferthehighestenergydensitywithrespecttovolumeandweight（Figure1）,theyareusedinsystemsrangingfrommobilephonestoelectriccars.

Figure1.Theenergydensitiesofvariousbatterytypes.

Lithiumcellsalsohavespecificcharacteristicsthatareimportantfordeterminingtheirchargelevel.Alithiumbatterypackmustincludevarioussafetymechanismstopreventthebatteryfrombeingovercharged,deeplydischarged,orreverseconnected.Becausethehighlyreactivelithiumcanposeanexplosionhazard,lithiumbatteriesmustnotbeexposedtohightemperatures.

TheanodeofanLi-ionbatteryismadefromagraphitecompound,andthecathodeismadeofmetaloxideswithlithiumaddedinawaythatminimizesdisruptionofthelatticestructure.Thisprocessiscalledintercalation.Becauselithiumreactsstronglywithwater,lithiumbatteriesareconstructedwithnon-liquidelectrolytesoforganiclithiumsalts.Whenchargingalithiumbattery,thelithiumatomsareionizedinthecathodeandtransportedthroughtheelectrolytetotheanode.

BatteryCapacity

Themostimportantcharacteristicofabattery（apartfromitsvoltage）isitscapacity（C）,specifiedinmA-hoursanddefinedasthemaximumamountofchargethebatterycandeliver.Capacityisspecifiedbythemanufacturerforaparticularsetofconditions,butitchangesconstantlyafterthebatteryismanufactured.

Figure2.Theinfluenceoftemperatureonbatterycapacity.

AsFigure2illustrates,capacityisproportionaltobatterytemperature.TheuppercurveshowsanLi-ionbatterychargedwithaconstant-I,constant-Vprocessatdifferenttemperatures.Notethatthebatterycantakeapproximately20%morechargeathightemperaturesthanitcanat-20°C.

AsshownbythelowercurvesinFigure2,temperaturehasanevengreaterinfluenceontheavailablechargewhileabatteryisbeingdischarged.Thegraphshowsafullychargedbatterydischargedwithtwodifferentcurrentsdowntoacut-offpointof2.5V.Bothcurvesshowastrongdependenceontemperatureaswellasdischargecurrent.Atagiventemperatureanddischargerate,thecapacityofalithiumcellisgivenbythedifferencebetweentheupperandlowercurves.Thus,Li-cellcapacityisgreatlyreducedatlowtemperaturesorbyalargedischargecurrentorbyboth.Afterdischargeathighcurrentandlowtemperature,abatterystillhassignificantresidualcharge,whichcanthenbedischargedatalowcurrentatthesametemperature.

Self-Discharge

Batterieslosetheirchargethroughunwantedchemicalreactionsaswellasimpuritiesintheelectrolyte.Typicalself-dischargeratesatroomtemperatureforcommonbatterytypesareshowninTable1.

Table1.TheSelf-DischargeRatesofCommonBatteryTypes

Chemistry

Self-Discharge/Month

Lead-acid

4%to6%

NiCd

15%to30%

NiMH

30%

Lithium

2%to3%

Chemicalreactionsarethermallydriven,soself-dischargeishighlytemperature-dependent（Figure3）.Self-dischargecanbemodelledfordifferentbatterytypesusingaparallelresistanceforleakagecurrents.

Figure3.Self-dischargeofLi-ionbatteries.

Aging

Batterycapacitydeclinesasthenumberofchargeanddischargecyclesincreases（Figure4）.Thisdeclineisquantifiedbythetermservicelife,definedasthenumberofcharge/dischargecyclesabatterycanprovidebeforeitscapacityfallsto80%oftheinitialvalue.Theservicelifeofatypicallithiumbatteryisbetween300and500charge/dischargecycles.

Lithiumbatteriesalsosufferfromtime-relatedaging,whichcausestheircapacitytofallfromthemomentthebatteryleavesthefactory,regardlessofusage.ThiseffectcancauseafullychargedLi-ionbatterytolose20%ofitscapacityperyearat25°C,and35%at40°C.Forpartiallychargedbatteriestheagingprocessismoregradual:

forabatterywitha40%residualcharge,thelossisabout4%ofitscapacityperyearat25°C.

Figure4.Batteryaging.

DischargeCurves

Thecharacteristicdischargecurveforabatteryisspecifiedinitsdatasheetforspecificconditions.Onefactoraffectingbatteryvoltageistheloadcurrent（Figure5）.Loadcurrentcannot,unfortunately,besimulatedinthemodelbyasimplesourceresistance,becausethatresistancedependsonotherparameterssuchasthebattery'sageandchargelevel.

Figure5.Battery-dischargecurve.

Secondarylithiumcellsexhibitrelativelyflatdischargecurvesincomparisonwithprimarycells.Systemdeveloperslikethisbehaviourbecausetheavailablevoltageisrelativelyconstant.However,gradualdischargemakesthebatteryvoltageindependentofthebattery'sresidual-chargelevel.

AccuratelyMeasuringChargeLevel

Todeterminetheavailablechargeinabattery,simplemonitoringmethodsarepreferred.Theyshouldconsumelittleenergyandshould（ideally）allowonetodeducethechargelevelfrombatteryvoltage.Suchavoltage-onlymethodcanproduceunreliableoutcomes,however,becausenoclearcorrelationexistsbetweenvoltageandtheavailablecharge（Figure5）.Batteryvoltagealsodependsontemperature,anddynamicrelaxationeffectscancauseaslowincreaseintheterminalvoltageafterareductioninloadcurrent.Thus,purelyvoltage-basedmonitoringisunlikelytoprovidecharge-levelaccuraciesbetterthan25%.

Therelativechargelevel,oftencalledthestateofcharge（SOC）,isdefinedastheratioofresidualchargetothebattery'schargecapacity.Hencechargeflowmustbemeasuredandmonitoredthroughaprocedurecalled"coulombcounting."Inpractice,coulombcountingisaccomplishedbyintegratingthecurrentsflowingintoandoutofthecell.Tomeasurethesecurrentswithahigh-resolutionADC,onetypicallyconnectsasmallresistorinserieswiththeanode.

Fuel-GaugeLearning

ThefunctionalrelationshipbetweenbatterySOCandtheparametersmentionedabovecannotberelatedanalytically,socellcapacityandchargemustbedeterminedempirically.Noextensiveanalyticalmodelsareavailableforcalculating（withsufficientaccuracy）thecapacityofabatteryunderpracticaloperatingconditionssuchastemperature,numberofchargecycles,current,age,etc.Theoreticalmodelsapplyonlytocertain"local"conditions.Fordeterminingrelativechargelevels,theyareappliedlocallyandcalibratedglobally.

Toachievesufficientaccuracywhileabatteryisinuse,themodelparametersmustbecalibratedconstantlythroughaprocesscalledfuel-gauge"learning."Inconjunctionwithcoulombcounting,thatapproachyieldsfuelgaugesaccuratetowithinafewpercent.

Fuel-GaugeSelection

ModernintegratedcircuitscandeterminetheSOCforalltypesofsecondarycells,cellconfigurations,andapplications.Despitetheirlowsupplycurrent（about60µAinactivemodeand1µAinsleepmode）,theseICsachieveahighdegreeofaccuracy.Fuel-gaugeICsfallintothreecategories（Table2）.Becauselithium-basedbatteriesarepreferredformanyapplications,theexamplesshownarebasedonLi-ionandLi-polymerbatteries.

Table2.Fuel-GaugeCircuits.

Part

TypeofFuel-GaugeIC

FunctioninBatteryPack

FunctioninHostSystem

DS2762

Coulombcounter

Measurement

Algorithm+display

DS2780

Fuelgauge

Measurement+algorithm

Display

MAX1781

Programmablefuelgauge

Measurement+flexiblealgorithm

Display

Coulombcounters,sometimesknownasbatterymonitors,areICsthatmeasure,count,andconvertthebattery'sparametersmentionedabove,includingcharge,temperature,voltage,loadcycles,andtime.Becausecoulombcountersdonotprocessthemeasuredvariables,theyarenotintelligent.Onesuchdevice,theDS2762,alreadyincludesanintegrated,highlyaccurate25m

resistorformeasuringcurrent.Itmonitorstemperature,batteryvoltage,andcurrent,anditfeaturesa1-Wire®busthatallowsallreadingstobereadbyamicrocontrollerresidinginthebatterypackorhostsystem.ItalsoofferstherequisitesafetycircuitessentialforsecondaryLicells.Theresultisaflexible,cost-effectivesystemthatrequiresconsiderableknowledgeanddevelopmenteffort（althoughcostsareoffsetbythesoftware,models,andsupportprovidedbytheICvendor）.

Analternativeapproachtothecoulombcounterisprovidedbyfuelgauges.Theseall-in-onedevicesperformfuel-gaugingroutineswithalearningalgorithm,andtheyperformallnecessarymeasurementsontheirown.Fuelgaugesaretypicallydeployedinintelligent,autonomousbatteriescalledsmartbatteries.Becausede