A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysis

•Build a method for battery pack SOC estimation.•Analyze the effect of the uneven cells problems to the pack SOC.•The SOC is estimated with consideration of different balance control strategies.•The UPF method is used to estimate the SOC to improve the accuracy. The state-of-charge (SOC) is a critical parameter of a Li-ion battery pack. Differences among in-pack cells are inevitable and can change the total capacity of a pack and the remaining available capacity. Because the traditional methods for the estimation of the SOC of a pack did not consider the difference among the cells and the impact of balance control, we developed a new method that accounts for these problems. To accurately estimate the pack SOC, we establish the relationship between the parameters of the pack and those of in-pack cells under different balance control strategies. This paper also studies the two different types of connections of a battery pack: in series and in parallel. Based on the model of the first over-charged cell and that of the first over-discharged cell, the estimation of the SOC of a battery pack is realized by the Unscented Particle Filter (UPF) algorithm. A simulation experiment verified the method for the estimation of the SOC for a battery pack based on actual data and proved that an accurate estimation value can be obtained by the method.