Investigation of the internal resistance in LiFePO4 cells for battery energy storage system

Internal resistance is an important element for lithium-ion batteries in battery management system (BMS) for battery energy storage system (BESS). The internal resistance consists of ohmic resistance and polarization resistance. Neither of them can be measured directly and they are identified by some algorithms with battery charging/discharging experiment data. In this paper, several 10Ah LiFePO₄ cells were used for the investigation of the internal resistance. Based on an electric model for the LiFePO₄ cells, methods on estimation of ohmic resistance and polarization resistance were introduced. The cell characteristics were analyzed by pulse charge and discharge experiment data, especially when the state-of-charge (SOC) was neither too high nor too low. Results show that the ohmic resistance and polarization resistance stay at the same order of magnitude. Further, the ohmic resistance changes relatively little with the current and current variation. Meanwhile, the ohmic resistance keeps consistent in charge at different SOC. However, it increases gradually in discharge with the decreasing SOC. Generally, the polarization resistance increased both in charge and in discharge, while there is a reverse change at the SOC about 70%. This result is useful in developing accurate resistance for certain issues, especially for SOC or state-of health (SOH) estimation.