Page - 169 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 5 However, for the long-termRCnetwork, the voltage varies continuously due to a relatively large timeconstant,as illustrated inFigure7. Thevoltageacross the long-termRCnetworkhasnot reachedtheequilibriumstateat theendof thepulse-dischargingperiod; thus, therealwaysexistsa significantproportionof the loadcurrent I(1−e−D/τlong)flowingthroughthecorrespondingcapacitor. Consequently,VRC,long(0)at thebeginningof therestperiodshouldbewrittenas: VRC,long(0)= IRlong(1−e − Dτlong) (17) where I is the value of the pulse-discharging current. Since the SoC variation in each test cycle is set as 2% in this paper, it can be assumed that themodel parameters keep constant during the pulse-dischargingperiod. 7LPH V 95& VKRUW 95& VKRUW VWDEOH 95& ORQJ 95& ORQJ VWDEOH 95& ORQJ 95& VKRUW &XUUHQW Figure7.ThevoltagecurveofRCnetworksduringonecycleof thedischargingpulse-rest test. 4. ExperimentalResultsandDiscussions 4.1. RCNetworkParameterEstimationResults Basedon theaforementionedanalysis inSection3.1, for the caseof theCCchargingscenario, the chargingpulse-rest test is implementedfirstly. Theparameters are estimated fromthevoltage responseof thepulse-chargingperiod, and theestimationresults are showninFigure8. Figure8a plots two estimated time constants; it can be seen that the general order of themagnitude of the short-termtimeconstant is10s; itfluctuatesgreatlywhentheSoCchanges,especially in themiddle SoC region,while theorderof themagnitudeof the long-termtimeconstant is100s; it is relativelyflat duringthewholeSoC region. Figure8bplots twoestimatedresistances; it canbeobservedthat in the middleSoC range, theshort-termresistancehasa largervalue,whichmeansthat thevoltageacross the short-termRCnetworkaccounts formoreweightduringthisperiod.Hence, it canbeobservedfrom Figures3and8bthat thevariation tendenciesof thepolarizationvoltageandtheshort-termresistance aresimilarduring themiddleSoC range.At theendof thechargingprocess, theshort-termresistance decreasesandstabilizesaroundaverysmallvalue,while the long-termresistance increasesalmost linearlyafter60%SoC, leading toasimilarvariation tendencyof thepolarizationvoltage, comparedto thecorrespondingpart inFigure3.Hence, it canbeconcludedthat the long-termdiffusionprocess playsamajor role in this stage. 169