Lithium-ion battery cells not only show different behaviors depending on degradation and charging states, but also overcharge and overdischarge of cells shorten battery life and cause safety problems, thus studies aiming to provide an accurate state of a cell are required. Measurements of battery cell impedance are used for cell SoH and SoC estimation techniques, but it generally takes a long time for a cell in each state to be prepared and cell voltage response is measured when charging and discharging under each condition. This study introduces an electrical equivalent circuit model of lithium-ion cells developed in the MATLAB/Simulink environment. Cell SoC, SoH, temperature, and C-rate are considered for more accurate cell impedance prediction, and the simulation results are verified with the measurement results. The developed model is suitable for use in cell SoC and SoH monitoring studies by successfully outputting cell impedance through real-time prediction of cell voltage during discharge.

In this paper, a method for monitoring SoC of a lithium-ion battery cell through continuous impedance measurement during cell operation is introduced. A multi-sine signal is applied to the cell operating current, and the cell SoH and SoC can be simultaneously monitored via impedance at each frequency. Unlike existing studies in which cell impedance measurement is performed ex situ through EIS equipment, cell state estimation is performed in situ. The measured impedance takes into account cell temperature and cell SoH, enabling accurate SoC estimation. The measurement system configured for the experiment and considerations for the selection of measurement parameters are described, and the accuracy of cell SoC estimation is presented.

This paper deals with a method to detect the existence of an over-discharged cell in a lithium-ion battery (LIB) pack by measuring the total harmonic distortion (THD) rate in the voltage response. Over-discharge of the LIB cell reduces the available capacity by irreversible chemical reactions, resulting in serious safety risks such as explosions. Even if only one over-discharged cell exists in the battery pack, it accelerates the decomposition of other cells. In general, the measurement of each cell voltage in a battery pack is required to detect one over-discharged cell. This is because if only the voltage of the battery pack is measured, it cannot be distinguished whether the voltage of each cell is uniformly low or one specific weak cell is over-discharged. The proposed method measures the frequency response through the voltage at only two terminals of the battery pack to detect the presence of one over-discharged cell. When the battery cell is discharged beyond a certain level, the system nonlinearity of the battery pack increases, and it can be detected from the increased THD rate of the battery pack. The proposed method is verified by simulation and measurement.