Study On The Methods For Determining The Remaining Capacity Of Lithium-Thionyl Chloride Primary Batteries

Determining the remaining capacity of lithium thionyl chloridebatteries is receiving increasing attention as the higher and higherdemand for battery quality and performance by battery applicationapparatus and institutions. In order to find methods for determining theremaining capacity of Li/SOCl₂ batteries, electrochemical characteristicsof the cell are analysed. The multiple factors affecting the batteryremaining capacity and the major difficulty to predict the remainingcapacity are discussed. And there is a review of the methods forforecasting SOC (state-of-charge) of various secondary or primarybatteries. In this paper, the methods for estimating the remaining capacityfrom the cell s internal resistance, pulse voltage and ElectrochemicalImpedance Spectroscopy are tested and verified.Firstly, the dependence of Ohm resistance at the frequency 1000Hzon the remaining capacity of ER14250 Li/SOCl₂ cell is measured at roomtemperature. It is found that the internal resistance is almost stable beforethe discharge capacity reached 0.8Ah and increased clearly whendischarge capacity is more than 0.8Ah. So the remaining capacity, 30% ofbattery total capacity (0.4Ah), would be likely to be predicted from cell sOhm resistance at the frequency 1000Hz.Secondly, the Electrochemical Impedance Spectra of ER14250Li/SOCl₂ cells are analysed in the frequency range 20kHz～5mHz. Theimpedance is measured for different states of charge at open circuit andduring continuous discharge at 330Ωload. It was found that the EISaltered with strong regularity during the discharge, which externalizedthe two semicircles in EIS becoming bigger gradually, as well as the secondary semicircle drifting upwards gradually. It is possible to use theimpedance spectra alter characteristics for estimating the cell'sstate-of-charge. The most informative parameters for the state-of-chargetest in the region 40%～0% are the ImZ*, log (Z*) and phaseθ* on theextreme point of an impedance spectrum.Finally, the pulse voltages of ER14250 Li/SOCl₂ batteries atdifferent pulse currents of 5mA, 10mA and 20mA are tested during thedischarge with microampere current at the different temperatures of -20℃,60℃and room temperature when the SOC is＜40%. It is found that thereis no dependence of the pulse voltage at 10mA pulse current on remainingcapacity of the battery at room temperature. While the pulse voltage ofU_1s and U_5ms at 5mA pulse current decrease notably at -20℃when thebattery remaining capacity is about 0.03Ah, 5% of the battery totalcapacity(0.6Ah) and the pulse voltage of U_1s at 20mA pulse currentdecrease visibly at 60℃and room temperature when the batteryremaining capacity is about 0.26Ah, 21% of the battery totalcapacity(1.2Ah). And the decreasing magnitudeâ–³V becomes bigger andbigger gradually with the battery remaining capacity deceasing.As a result, it is possible to determining about 0.03Ah remainingcapacity from the pulse voltage of U_1s and U_5ms at 5mA pulse current at-20℃. Equally, there is a possibility to estimate about 0.26Ah remainingcapacity, 21% of the battery total capacity, from the pulse voltage of U_1sat 20mA pulse current at 60℃and room temperature.