| Power lithium ion battery has the advantages of high specific energy, low pollution and high safety, thus has become one of the main power sources for new energy automobiles at present. The state of health(SOH) of a power battery is the most important performance index in the power battery system of electric vehicles currently. SOH of the power battery is different from other electrical parameters such as voltage, current, internal resistance and temperature in that it cannot be obtained through direct method of measurement by any equipment or instrument, but these electrical parameters will change with changes of SOH of the power battery. For this reason, the online estimation of exact SOH of the lithium battery has become one of the key techniques in the power battery management system of electric vehicles. This paper focuses on factors such as internal resistance, capacity and self discharge of lithium battery, and deeply studies on SOH estimation of the power battery, Extended Kalman Filter(EKF) algorithm, online estimation strategy of Dual Extended Kalman Filter(D-EKF) and comprehensive assessment of Federated Extended Kalman Filter(F-EKF). The specific information is as follows:The result of influences of parameters such as internal resistance, capacity, self-discharge resistance and state of charge(SOC) on service life of the batter is acquired on the basis of the research on equivalent circuit model of batteries with self-discharge resistance and according to the result of battery cycle life test. Combining the features of electrical parameters of the power battery and taking Thevenin’s equivalent circuit model as the prototype, an improved equivalent circuit model with self-discharge factors is built. The conclusion that the difference in selection of initial value of SOC among the batter parameters may result in difference in model parameters is obtained by identifying system parameters of the model.Combining the improved Thevenin’s equivalent circuit model, a state equation and an output equation for SOC of the lithium battery are established in the ampere-hour integral method. Each parameter in SOC system is observed by using EKF algorithm, and therefore the object to exactly observe the SOC of the lithium battery online is achieved. The system simulation observation result indicates that the SOC online estimation of the lithium battery in the EKF algorithm has good precision and accuracy.Based on SOC online estimation of the lithium battery in the EKF algorithm, the estimation researches on SOH of battery capacity, SOH of internal resistance and SOH of self-discharge are carried out. According to diversity and complexity of internal state parameters of the battery, experiments related to internal resistance, capacity and self-discharge are conducted. The experiment result indicates that the SOH online estimation method of the battery based on EKF complexity has good precision and can basically satisfy the requirements for actual service conditions of electric buses.To solve the problem that the initial parameters of the battery model in SOH online estimation of the battery cannot be determined in advance, which results in increased error, the D-EKF algorithm is introduced. Two independent EKFs are established to respectively estimate the state of battery system and the parameters and mutually update their states and parameters. The actual road condition test and data simulation result have verified the validity and precision of D-EKF algorithm.To solve the problem of the standards for SOH of the battery and incomplete description, the concept of "generalized SOH of battery" is put forward from the point of practical application of the battery and metrology and used for evaluating the performance parameters of the battery. A function structure for generalized SOH of the battery with information distribution factors was established, a local filter for SOH of ohmic internal resistance, SOH of self discharge and SOH of capacity was designed, the influence of information distribution factors on distribution of the master filter and the sub-filter was studied, and an optimum allocation algorithm of self-adaption probability distribution factors was given.To solve the problem of shortened service life of high-capacity power battery pack which is composed of single battery packs, a battery sorting method based on cluster analysis method was put forward by conducting fault diagnosis and failure analysis on the lithium battery on the basis of FMEA method. Main factors and electrical performance parameters of the battery related to service life of the battery pack are obtained. The proposed concept of "dynamic consistency sorting of battery" can effectively improve the SOH of the power battery pack.Based on laboratory and road test for the generalized SOH of battery, he research result is verified by combining the actual usage conditions of electric buses. The test result indicates that the online estimation method of lithium power battery based on FKF can satisfy the requirement for precision in SOH estimation of lithium power battery pack for electric vehicles. |
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