Battery Handbook Chapter 4

3.1CHAPTER3FACTORSAFFECTINGBATTERYPERFORMANCEDavidLinden3.1GENERALCHARACTERISTICSThespecificenergyofanumberofbatterysystemswaslistedinTable1.2Thesevaluesarebasedonoptimaldesignsanddischargeconditions.Whilethesevaluescanbehelpfultocharacterizetheenergyoutputofeachbatterysystem,theperformanceofthebatterymaybesignificantlydifferentunderactualconditionsofuse,particularlyifthebatteryisdis-chargedundermorestringentconditionsthanthoseunderwhichitwascharacterized.Theperformanceofthebatteryunderthespecificconditionsofuseshouldbeobtainedbeforeanyfinalcomparisonsorjudgmentsaremade.3.2FACTORSAFFECTINGBATTERYPERFORMANCEManyfactorsinfluencetheoperationalcharacteristics,capacity,energyoutputandperform-anceofabattery.Theeffectofthesefactorsonbatteryperformanceisdiscussedinthissection.Itshouldbenotedthatbecauseofthemanypossibleinteractions,theseeffectscanbepresentedonlyasgeneralizationsandthattheinfluenceofeachfactorisusuallygreaterunderthemorestringentoperatingconditions.Forexample,theeffectofstorageismorepronouncednotonlywithhighstoragetemperaturesandlongstorageperiods,butalsoundermoresevereconditionsofdischargefollowingstorage.Afteragivenstorageperiod,theobservedlossofcapacity(comparedwithafreshbattery)willusuallybegreaterunderheavydischargeloadsthanunderlightdischargeloads.Similarly,theobservedlossofcapacityatlowtemperatures(comparedwithnormaltemperaturedischarges)willbegreateratheavythanatlightormoderatedischargeloads.Specificationsandstandardsforbatteriesusuallylistthespecifictestoroperationalconditionsonwhichthestandardsarebasedbecauseoftheinfluenceoftheseconditionsonbatteryperformance.Furthermoreitshouldbenotedthatevenwithinagivencellorbatterydesign,therewillbeperformancedifferencesfrommanufacturertomanufacturerandbetweendifferentver-sionsofthesamebattery(suchasstandard,heavy-duty,orpremium).Therearealsoper-formancevariableswithinaproductionlot,andfromproductionlottoproductionlot,thatareinherentinanymanufacturingprocess.Theextentofthevariabilitydependsontheprocesscontrolsaswellasontheapplicationanduseofthebattery.Manufacturers’datashouldbeconsultedtoobtainspecificperformancecharacteristics.3.2CHAPTERTHREEFIGURE3.1Characteristicdischargecurves.3.2.1VoltageLevelDifferentreferencesaremadetothevoltageofacellorbattery:1.Thetheoreticalvoltageisafunctionoftheanodeandcathodematerials,thecompositionoftheelectrolyteandthetemperature(usuallystatedat25
C).2.Theopen-circuitvoltageisthevoltageunderano-loadconditionandisusuallyacloseapproximationofthetheoreticalvoltage.3.Theclosed-circuitvoltageisthevoltageunderaloadcondition.4.Thenominalvoltageisonethatisgenerallyacceptedastypicaloftheoperatingvoltageofthebatteryas,forexample,1.5Vforazinc-manganesedioxidebattery.5.Theworkingvoltageismorerepresentativeoftheactualoperatingvoltageofthebatteryunderloadandwillbelowerthantheopen-circuitvoltage.6.Theaveragevoltageisthevoltageaveragedduringthedischarge.7.Themidpointvoltageisthecentralvoltageduringthedischargeofthecellorbattery.8.Theendorcut-offvoltageisdesignatedastheendofthedischarge.Usuallyitisthevoltageabovewhichmostofthecapacityofthecellorbatteryhasbeendelivered.Theendvoltagemayalsobedependentontheapplicationrequirements.Usingthelead-acidbatteryasanexample,thetheoreticalandopen-circuitvoltagesare2.1V,thenominalvoltageis2.0V,theworkingvoltageisbetween1.8and2.0V,andtheendvoltageistypically1.75Vonmoderateandlow-draindischargesand1.5Vforengine-crankingloads.Oncharge,thevoltagemayrangefrom2.3to2.8V.Whenacellorbatteryisdischargeditsvoltageislowerthanthetheoreticalvoltage.ThedifferenceiscausedbyIRlossesduetocell(andbattery)resistanceandpolarizationoftheactivematerialsduringdischarge.ThisisillustratedinFig.3.1.Intheidealizedcase,thedischargeofthebatteryproceedsatthetheoreticalvoltageuntiltheactivematerialsareconsumedandthecapacityisfullyutilized.Thevoltagethendropstozero.Underactualconditions,thedischargecurveissimilartotheothercurvesinFig.3.1.TheinitialvoltageofthecellunderadischargeloadislowerthanthetheoreticalvalueduetotheinternalcellresistanceandtheresultantIRdropaswellaspolarizationeffectsatbothelectrodes.Thevoltagealsodropsduringdischargeasthecellresistanceincreasesduetotheaccumulationofdischargeproducts,activationandconcentration,polarization,andrelatedfactors.Curve2issimilartocurve1,butrepresentsacellwithahigherinternalresistanceorahigherdischargerate,orboth,comparedtothecellrepresentedbycurve1.Asthecellresistanceorthedischargecurrentisincreased,thedischargevoltagedecreasesandthedischargeshowsamoreslopingprofile.FACTORSAFFECTINGBATTERYPERFORMANCE3.3Thespecificenergythatisdeliveredbyabatteryinpracticeis,therefore,lowerthanthetheoreticalspecificenergyofitsactivematerials,dueto:1.Theaveragevoltageduringthedischargeislowerthanthetheoreticalvoltage.2.Thebatteryisnotdischargedtozerovoltsandalloftheavailableampere-hourcapacityisnotutilized.AsspecificenergyequalsWatthours/gram
VoltageAmperehours/gramthedeliveredspecificenergyislowerthanthetheoreticalenergyasbothofthecomponentsoftheequationarelower.Theshapeofthedischargecurvecanvarydependingontheelectroche