The state of charge (SOC) is affected by the current, open-circuit voltage, ambient temperature and other factors. The grey relation analysis of grey theory is proposed to determine the degrees of grey relation(r) of each factor with SOC. The grey relation analysis model is build for analyzing the discharge experimental data of lithium-ion battery which were under different working conditions. The results of experimental data show that the degrees of grey relation of these factors on the SOC change as follows: discharge current has the most significant influence on the SOC(r = 0.8690), the impact of open-circuit voltage(r = 0.6005) is the second and the impact of ambient temperature(r=0.5831) is the minimal. These results verify the grey relation analysis model can calculate and quantify the impact of each factor which is related to SOC. This research can provide a basis for establishing the model of SOC estimation and improve the estimation accuracy of battery SOC in the future.
