Page - 147 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 1503 Withameasuredbatterycurrent (It), thedifferenceof theDODinanoperatingperiod(T) canbe computedby: ΔDOD= ∫ t0+T t0 Itdt Crated ×100%. (4) ByconsideringtheSOH, theSOCisestimatedas: SOC(t)=SOH(t)−DOD(t). (5) In order to improve the convergenceperformanceof SOCestimation, an accuratemethod to determine the initial status of the batteries will be used. However, unlike the lab environment, it is hard to know the exact initial SOC andDODof the battery. Therefore, a self-initialization methodfor thebatterysystemisproposedandimplementedtoprovidepriorconfigurationduringthe initializationstage. At thechargingstage, thevariationsof thebatteryvoltageandbatterycurrentwhenthebattery is charged by the constant current–constant voltage (CC–CV)mode are usually specified by the manufacturer.With theconstantchargingcurrent, thebatteryvoltage increasesgraduallyandreaches the threshold.Once thebatteryhasbeenchargedbytheconstantvoltagemode, thechargingcurrent reducesrapidlybeforeitmoregraduallydecreases. Eventually, thecurrentwill reachalmostzerowhen it is fullycharged.Thischargingcurvecanbeconverted into therelationshipbetweenthecharging voltageandtheSOC,duringtheconstantcurrentstage. Byusingtherelationshipbetweenthecharging currentandtheSOCduringtheconstant-voltagestage, the initialSOCvaluecanbeobtained. At thedischargingstage, the typicalvoltagecurvesatdifferentdischargingcurrentsaregiven bythemanufacturer.Ahighercurrentwill causea fasterdecline in the terminalvoltage,andleadto ashorteroperationtime. TherelationshipbetweentheSOCandthedischargingvoltageatdifferent currents canbeobtained. At theopen-circuit stage, the relationshipbetweenopen-circuit voltage (OCV)andSOCisneeded. Thebattery isdischargedbydifferentcurrentsbeforedisconnectingfrom the load. TheOCVcanbeusedtoestimateSOCifa longperiodof relaxation isgivenfor the transient tosettle to its steadystate. Figure3 is theflowchart to indicate theself-initializationprocedures for SOCestimationwhenthesystemrestartsaftera longperiodof relaxation. Figure3.FlowchartofSOCself-initializationandupdating(DOD:depthofdischarge,SOH:state-of-charge, OCV:opencircuitvoltage,EEPROM:electricallyerasableprogrammableread-onlymemory). 2.3. SmartFault-DiagnosisStrategies As the LIBs provide a high-energy-density power, the system must be able to detect the abnormality inreal-timetoensure thesafetyof theusersandefficientpowersupplyunderunexpected conditions, suchasanexternal short circuit. ThesafetyofLi-ionbatteriesdependsontheattributesof systemdesign, suchaselectronics, estimationalgorithms,andthermalandmechanical characteristics, 147