The most commonly used equivalent circuit model of a lithium-ion battery is the second-order RC equivalent circuit model. Parameter identification based on this model has the disadvantages of many identification parameters and a large amount of calculation. At the same time, there are complex exponential operations in the state of charge (SOC) estimation, and these make it difficult to estimate the SOC of multiple series batteries on-line. Therefore, a simplified second-order battery model is proposed, one which ignores the internal polarization reaction and only pays attention to its external characteristics, so as to reduce the amount of parameter identification. The simplified model also reduces the number of state variables to be estimated in the state space equation, avoids complex exponential operation, reduces the computational complexity and the overall amount of operation, and is conducive to the real-time online estimation of the SOC of multiple series lithium batteries. Through the parameter identification and SOC estimation test of a single lithium-ion battery and series lithium-ion batteries, it is verified that the proposed simplified model not only ensures the accuracy of parameter identification and SOC estimation, but also greatly increases the operational speed of the system, and then improves the speed of SOC estimation. This work is supported by the National Natural Science Foundation of China (No. 51577155).