Page - 127 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 563 influencedbythedischargerate,but thebatterycapacity isalsodiminishedathighdischargerates. Thisoccursasasharpvoltagedropat theendof thedischargeprocess,as indicatedinFigure1.At that rate thedischarge terminatesatVcut-off [4]. Figure1.Typicaldischargecharacteristic curveofLi-ionbattery [4,5]. The vehicle under test (VUT) is equippedwith battery cells of a cathode type LiFeMgPO4. Lithium iron phosphate cells are characterized by their flat open circuit voltage curve (OCV). Hence, the cell voltage staysalmost constantover thecomplete stateof charge (SOC) range [2,3,6]. Thebatterypackof theVUTconsistsof19modules. Eachmodulecomprisessixcellblocksconnected inseries;asinglecellblock isconstructedoutof50LiFeMgPO4–graphitecells, connected inparallel. In total, thereare300cellswithin thesinglebatterymodule. Eachcellblockhasanominalvoltageof 3.2V,amountingtoatotalvoltageof19.2V.ThebatteryspecificationsaregiveninTable1.AController AreaNetwork (CAN)communicationenvironment isused for thecontrol andmanagementof the batterymodules.More technical informationabout theVUTisavailable in theAppendix. Table1.TechnicaldataofLiFeMgPO4Battery [7]. Parameter (Unit) Value NominalModuleVoltage (V) 19.2 NominalModuleCapacity (Ah) 69 MaxContinuousLoadCurrent (A) 120 PeakCurrent for30s (A) 200 Inthispaper,wewill investigatethedifferentbatterymodelingmethodstodecidewhichapproach willbe themostappropriate formodeling thebatterypackofVUT.Then,wewill selectagroupof the most thoroughmodels forLi-ionbatteries. Thesemodelswill contribute to thedevelopmentof the proposedbatterymodel,whichwillbeconsideredformodelingactualbatterypackvoltageresponse whentheVUTundergoesseveredrivingmaneuvers. Thepaper isorganizedas follows. Section2presents thedifferentmodeling techniquesofLi-ion batteries. Threemodelsareelected fromthereviewedmodelsandarediscussed inmoredetails in Section3. InSection4 the thermalbehaviorof thebattery iselaborated. Theexperimental testsare described in Section 5. Themodels are evaluated and anew, improvedmodel is developed and proposedinSection6. Finally, theconclusionsarestatedat theendof thepaper. 127