The estimation methods of state-of-charge (SOC) for lithium-ion batteries are reviewed. SOC is used to characterize the remaining capacity of the battery in the current cycle, which is also an important indicator of battery management in electric vehicles. When an accurate estimation of SOC is obtained, batteries would avoid bad working conditions, such as a run with low capacity, ensure that the battery always runs in a safe-state. Thus, the battery efficiency was improved and the life-time was prolonged. The common estimation methods of SOC are introduced and compared. The ampere-hour integration method is the simplest. However, it is an open-loop estimation system; thus, the estimation error is unable to correct itself. The open-circuit voltage method is used to estimate SOC based on the corresponding relationship between the open-circuit voltage and SOC. The need for long-standing time to obtain stable voltage values makes this method unsuitable for on-line estimation. The Kalman filter family method is a combination of ampere-hour integration and open-circuit voltage, which is suitable for on-line estimation. The system observation value error is used to correct the state estimation. When an appropriate battery model is established, high estimation accuracy can be obtained. The data-driven method needs long-term historical data to build a database. The advantages and disadvantages of these methods and the improvement scheme are summarized. Based on the above analyses, combined with the limitations and challenges of the SOC estimation algorithm in practice, the future research direction of on-line SOC estimation for lithium-ion batteries is presented.

Keywords： lithium-ion battery ; SOC ; on-line estimation