Page - 146 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 1503 2.2.DataAcquisitionandSOCEstimationwithSelf-InitializationCapability During the system runningprocess,manyparameters, such as the cell voltage, temperature, packvoltageandcurrent,mustbecollected inreal-time. Thestatusof thebatterypack, suchasSOC, is requiredforcontinuous faultdiagnosis. Therefore, theaccuracyof theSOCestimation iscritical to thesystemperformance.Ahigh-accuracydata-acquisitionmethodusingtheLTC6804 isselectedas ananalog-to-digital converter tomonitor thebatterypack. It canmeasure thebatterycellswitha total measurement errorof less than1.2mV.Themeasurement range from0 to5Vmakes theLTC6804 suitable for theLIBapplication,as itdoesnotconsumemuchpower. Thespeedof thedataacquisition is fastenoughwiththe12-cellvoltages thataresampledat290μs.However, lowerdata-acquisition ratescanbeusedforhighernoisereduction. Theconfigurationof thishardwaremodulecanbeseen in Figure2. Theprecisionof theLTC6804wasaround0.3%.However, theoverall systemaccuracycan beaffectedby theprint circuitboard (PCB)design. Nevertheless, theprecision targetof6%canbe obtainedbycontinuouscalibrationfor thecaseofa lower-currentapplication. Figure2.Hardwareconfigurationofdata-acquisitionmoduleofbatterycells (RC:resistorcapacitor, SPI:SerialPeripheral Interface). The releasable capacity (Creleasable) of an operatingbattery is the released capacitywhen it is completelydischarged. TheSOCisdefinedas thepercentageof thereleasablecapacityrelative to the battery-ratedcapacity (Crated),givenas follows: SOC=Creleaseable/Crated×100%. (1) Afullychargedbatteryhas themaximal releasablecapacity (Cmax),whichcanbedifferent from the rated capacity. In general, Cmax declines as time increases. Hence, theCmax can be used for evaluatingthestate-of-health (SOH)ofabattery: SOH=Cmax/Crated×100%. (2) Whenabattery isdischarging, thedepthofdischarge (DOD)canbedefinedas thepercentageof thecapacity thathasbeendischarged(Creleased) relative toCrated: DOD=Creleased/Crated×100%. (3) 146