Design And Implementation Of SOC Estimation And Mixed Balance Strategy For Lithium Batteries

People are increasingly aware of the importance of clean and renewable energy,because of the continuous consumption of traditional fossil fuels and the increasingly serious environmental pollution.Lithium batteries have gotten more and more attention due to their advantages,for example,they have high energy density,long cycle life and they are friendly to environment.The working voltage of single lithium battery is only 3.6V,but when applied in electric vehicle,energy storage power station and military equipment,dozens or even hundreds of batteries are needed to supply power in series.However,the cells exist inconsistency,Frequent charging and discharging of batteries under complex operating conditions makes the inconsistency more obvious,which directly affects the performance and service life of batteries.Therefore,in the process of using lithium batteries,it is necessary to monitor and manage their working status,among them,the estimation of the state of Charge(SOC)and balancing strategy are the most important of the Battery Management System(BMS).Firstly,this paper elaborates the characteristics of all kinds of mathematical models of lithium batteries,after studying various SOC estimation algorithms of lithium batteries,the second-order RC equivalent circuit model is established as the SOC estimation model.The model has the characteristics of accurate accuracy and moderate amount of calculation.This paper deduces the operation process of Extended Kalman Filter(EKF),and the singular value decomposition(SVD-KF)algorithm is proposed based on the unscented Kalman filter(UKF).This algorithm can overcome the instability of UKF algorithm in matrix decomposition,and build a simulation model of lithium battery SOC estimation based on this algorithm in MATLAB combined with second-order RC model.The results show that the accuracy of the algorithm for SOC estimation is less than 5%.Secondly,this paper analyses the advantages and disadvantages of common equalization topologies,and establishes a hybrid equalization strategy by analyzing the principle of bi-directional flyback converter equalization and resistive passive equalization.The hybrid equalization strategy can make full use of the characteristics of the two equalization modes,besides it can reduce the inconsistency between single batteries quickly and improve the equalization efficiency.Finally,this paper designs a lithium battery management system which can effectively manage 12 batteries.This system can collect the voltage,current and temperature of battery pack.In addition,it can effectively calculate the SOC of battery,and has the function of mixed equalization and perfect system protection.At last,By collecting and analyzing data from the host computer based on LabVIEW,the data meet the design requirements of the battery management system in this paper.