Off-line and on-line methods are used to identify model parameters, but the model dynamic characteristic obtained by on-line method is better. Recursive least squares method is simple and it's often used for on-line parameter identification of lithium-ion battery models. However, the RLS has a low identification accuracy. Thus, the forgetting factor recursive least square method was proposed to improve the accuracy for parameter identification. In order to improve the dynamic identification ability, the variable forgetting factor least square (VFFRLS) method and adaptive forgetting factor recursive least square (AFFRLS) method appears. Yet the current adaptive methods tend to ignore the stability of model parameters, and the undetermined coefficient range of this method is large and difficult to confirm. The model parameter changes drastically is easy to cause the divergence of the algorithm. To address these issues, this paper proposes a simpler AFFRLS method without undetermined coefficient. And it takes into account the accuracy and stability of the model.
Firstly, based on dynamic stress testing (DST) and Federal City Operating Conditions (FUDS) data, the FFRLS method with fixed forgetting factor value is simulated and analyzed, and the influence trend of different forgetting factor on the accuracy and stability of model parameters is obtained. Secondly, the proposed AFFRLS method is compared with other AFFRLS and VFFRLS, and the stability and accuracy of the identification parameters are analyzed. Finally, the error tracking ability and convergence speed of the three adaptive methods are analyzed, and the adaptive performance of the proposed AFFRLS to DST and FUDS conditions are analyzed.
The FFRLS simulation results with fixed forgetting factor(λ) value show that when λ value decreases, the algorithm has better tracking ability for time-varying parameters, the convergence speed is accelerated, and the identification accuracy is effectively improved. However, when λ value decreases the parameter changes drastically and the stability decreases. It can be seen that obtaining the appropriate λ value is important for the identification ability of the adaptive methods. The three adaptive methods simulation results show that, the improved AFFRLS in this paper has better tracking ability for time-varying parameters and high model accuracy. And it has better stability of the parameter obtained by FFRLS with fixed λ values of 0.98 and 0.985. It can be seen that the proposed AFFRLS can achieve a better balance between accuracy and stability. The relationship between λ value and error of the adaptive methods show that the improved AFFRLS can track the error variation better. By compared the operation time with three methods, the results show that the proposed AFFRLS has the faster convergence rate. According to the relationship between λ value and time in DST and FUDS conditions, the improved AFFRLS method has the majority of λ value near 0.98 in FUDS condition, and the majority of λ value is 1 in DST condition.
The simulation analysis shows that: 1) The proposed AFFRLS method can improve the accuracy of the algorithm and give the stability of model parameters into consideration, and it has good balance between algorithm accuracy and parameter stability. Applying the proposed AFFRLS method and Kalman filter to predict the state of charge can improve the prediction accuracy better. 2) The proposed AFFRLS method has better tracking ability for error variation and faster convergence speed. 3) The proposed method can improve the algorithm accuracy under both slow and drastic conditions, so it's suitable for different on-line conditions.
范兴明, 封浩, 张鑫. 最小二乘算法优化及其在锂离子电池参数辨识中的应用研究[J]. 电工技术学报, 0, (): 9001-1.
Fan Xingming, Feng Hao, Zhang Xin. Optimization of Least Squares Method and Its Application in Parameter Identification of Lithium-Ion Battery Model. Transactions of China Electrotechnical Society, 0, (): 9001-1.
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