Abstract: This paper concerns the internal resistance as a function of discharge rate of Lithium-ion batteries. The aim of the work is to improve the accuracy and adaptiveness of internal resistance model for battery management system (BMS), which is of significance to the accurate prediction of the status of batteries such as the state of charge (SOC). We used a second order RC equivalent circuit model to analyze the direct current internal resistance (DCIR) and pulse discharge internal resistance (PDIR) with 25 A•h LiFePO4 batteries under constant discharge and pulse discharge at various discharge rates. Data fittings were done on the DCIR, PDIR1, PDIR2 and PDIRtot at various states of charge as a function of discharge rate and a good agreement was obtained with a double-exponential relationship, whereas a linear relationship held for the PDIR0. The analyses also suggested independence of the internal resistance change to the temperature effect. Based on the chemical equilibrium between the formation and decomposition of the solid electrolyte interface (SEI), we concluded that large DCIR, PDIR1, PDIR2 and PDIRtot observed at low discharge rates was likely to be due to low decomposition rate than the formation rate of the SEI with a high resistance, similar to the standby situation. On the other hand, a lower resistance at a higher discharge rate could be attributed to higher decomposition rates than the formation rates of the SEI, leading to a thinner SEI until a new equilibrium status was reached, and hence a reduced internal resistance.

Key words: LiFePO4, lithium-ion power battery, internal resistance, discharge rate, the second order RC equivalent circuit model, battery management system (BMS)