| Limited by the capacity and voltage of single cells,electric vehicles need to connect a large number of single cells in series and parallel to meet the performance requirements of electric vehicles.The difference between the cells in the battery pack is inevitable,which will cause a series of problems such as battery capacity loss,maximum output power drop,security risk and so on.This paper focuses on the inconsistency of battery packs,along with the influence,identification,and control of inconsistency,it studies the influence of parameter differences,the estimation of capacity differences,and the balance of battery packs.An ideal battery pack model was established to analyze the maximum usable capacity of the battery pack which has inconsistent SOC,capacity,and internal resistance among cells.It analyzes the characteristics of the voltage curve difference during the constant current charging of different capacity batteries,and proposes a method to identify the capacity difference between the cells.Based on the remaining capacity difference between the cells and considering the charging and discharging status,a balanced control strategy for the battery pack was proposed.(1)Perform performance tests on lithium-ion batteries and analyze the characteristics of OCV and DC internal resistance of different batteries during the aging process.Identify the battery parameters and establish Thevenin equivalent circuit model.(2)Analyze the influence of the parameter difference on the battery pack.An ideal battery model is established to analyze the effect of capacity,SOC,and internal resistance differences on the battery pack.It is pointed out that the capacity difference and SOC difference between the monomers have a greater impact on the available capacity of the battery pack.According to the characteristics of the voltage curve under the capacity difference and the SOC difference,it is believed that the impact of the capacity inconsistency on the battery capacity is caused by the SOC difference.At a given dispersion,the influence of ohmic internal resistance and polarization internal resistance on the capacity of the battery pack is extremely small and can be almost ignored in the test conditions.(3)Propose a method for estimating the difference in capacity between cells.Analyze the difference in terminal voltage curves of different capacity cells during constant current charging,and point out that the difference in terminal voltage curves between cells during charging mainly comes from the difference in SOC between cells caused by capacity differences.A method to normalize the voltage curves of cells with different capacities to estimate the capacity difference between cells using an equivalent time transformation is proposed.The capacity difference problem is converted into an optimization problem,and an adaptive gradient descent algorithm is used to solve it.Design verification experiments verify the proposed algorithm.When the battery pack starts charging from an initial SOC of under 10%,the proposed method has better accuracy.(4)Study of battery balance.The maximum usable capacity of series-connected battery packs with different equilibrium structures is analyzed.The limitation of using SOC as the equilibrium control variable is analyzed,and it is proposed to consider the battery charging or discharging status and use the charging or discharging residual capacity as the equilibrium control variable in the process of charging and discharging.In the bidirectional fly-back equalization circuit,the proposed equalization control algorithm is simulated and analyzed.The results show that this method can well balance the remaining capacity among the cells and achieve the purpose of maximizing the remaining available capacity of the battery pack. |
