Comparison between state of charge determination methods for lithium-ion batteries based on the measurement of impedance, voltage and coulomb counting

Goal of this work was to compare different methods of determining the state of charge (SoC) of lithium-ion-batteries and to investigate the application of these methods on different types of cells. To evaluate the SoC determination techniques, voltage and impedance data were recorded and referenced to the current integrated over time (coulomb counting) and to cell temperature. For 100 randomly spread test measurements for each cell, the SoC was calculated and the difference towards the value indicated by coulomb counting was noted. Regarding the Li(NiCoMn) 1/3O 2-type cell, determining the SoC by using the open-circuit voltage gave the best results. For the LiFePO 4-type cell the most accurate SoC values were achieved by measuring the voltage under load. Impedance based SoC determination g ave less precise results. This is mainly due to the strong temperature dependency of the internal resistance. Also not for both cells under test a unique correlation between impedance and SoC could befound.