1.The information of the L FP cells used in this experiment is shown in the following table:
Table 1: Battery information
2.Test equipment and process: in-situ expansion tester (I EST-SWE2100) and charge and discharge equipment (CT-8002-5V100A-NTFA).As shown in the figure below.
Figure 1. Expansion force test equipment
Figure 2. Battery test process
Figure 3 shows the voltage curve and expansion force change curve obtained under 1 / 25C ratio. It can be seen that the graph has a voltage platform in 27%~94%SOC, and the voltage change is only 0.07V. However, the expansion force change in this stage is mainly caused by the phase change of the negative graphite from Li C12 to Li C6, indicating that it is promising to estimate S OC by expansion force, but the expansion force change in this interval is non-monotonic, so it will challenge the accuracy of the prediction.
Figure 3. Changes of voltage and expansion force with S OC under quasi-static conditions
To validate the S OC estimation model, expansion force experiments were performed with different pretension forces (15kg and 30kg) and different test temperatures (25℃ and 45℃) under two dynamic conditions (N EDC and D ST).As shown in Figure 4, the results show that there is still an obvious voltage platform in 20%~90%SOC, and the change trend of expansion force and constant current charging mode, shows that the expansion force is not sensitive to the dynamic change of current is very sensitive, and sensitive to the change of S OC, this is mainly because the voltage depends on the change of ion concentration on the electrode surface, and the expansion force is the electrode phase ion concentration.In addition, the expansion force of the battery will increase significantly with the increase of the pretension force, so the focus should be focused on in the battery module design.
Figure 4. Plot of expansion force and current voltage under N E D C and D ST cycle conditions
Next, the author established the L SSVM model and continuously trained and optimized it. Combined with the S OC estimation of A UKF, the S OC estimation of different temperatures, different current dynamic conditions and different pretension forces can be realized.
Figure 5. Flow chart of the estimated S OC based on A UKF and L SSVM
The author introduces a novel method to estimate S OC of L FP cells, based on L SSVM and A UKF algorithms, can achieve estimation error of less than 1% and suitable for different working conditions of temperature, dynamic current and pretension forces.In the future, this method is expected to expand to other battery systems, and can also further establish S OC estimation models for batteries under different S OH and low temperature conditions.
SWE series in situ expansion analysis system (I EST energy technology): Using a highly stable and reliable automation platform, equipped with high precision thickness measurement sensor, to measure the thickness change and change rate of the whole charge and discharge process of the cell, can realize the following functions:
1.Constant pressure condition to test the battery expansion thickness curve;
2.Test the battery expansion force curve under the constant gap conditions;
3.Battery compression performance test: stress and strain curve-compression modulus；
4.Battery expansion force step by step test;
5.Different temperature control: -20~80℃.
The original literature
A syncretic state-of-charge estimator for LiFePO4 batteries leveraging expansion force. Journal of Energy Storage, 50 (2022) 104559.