Literature Appreciation: A Method for Estimating the LiFePO4 SOC By Using the Expansion Force

1. Author Information and Article Summary

In 2022, Dr. Peipei Xu of Beijing University of Science and Technology developed a method to estimate the LiFePO4 battery SOC based on the expansion force curve of the LFP battery. Through experimental verification, it was found that under different working conditions of the battery, the expansion force was more sensitive to the change of SOC than the voltage. Therefore, this paper proposed a method to estimate the expansion force of SOC. First, LSSVM was used to build the expansion force model, which could solve the non monotonic change problem between the expansion force and SOC, Combined with the floating window method to improve the applicability and prediction accuracy of the model, the proposed SOC estimation method can achieve the prediction error of 1%~0.54% under different ambient temperatures and different preloads of the battery, which is a novel method for estimating the LiFePO4 battery SOC.

2. Test Scheme

2.1 The LFP battery used in this experiment is shown in the following table 1

Table 1: Battery Information

2.2 Test Equipment and Process: in-situ expansion tester (IEST-SWE2100) and charging and discharging equipment.

(CT-8002-5V100A-NTFA). As shown in the figure below.

Figure 1. Expansion force test equipment

Figure 2. Battery Test Process

3. Result Analysis

Figure 3 shows the voltage curve and expansion force change curve obtained at 1/25C magnification. It can be clearly seen from the figure that there is a voltage plateau in the voltage curve at 27%~94% SOC. At this time, the voltage change is only 0.07V. However, the expansion force change in this range is very obvious. The expansion force change in this stage is mainly caused by the phase transition of cathode graphite from LiC12 to LiC6, indicating that it is very promising to use expansion force to estimate SOC, However, it is also seen that the change of expansion force in this range is non monotonic, so it will also challenge the accuracy of prediction.

Figure 3. Variation of voltage and expansion force with SOC under quasi-static conditions

In order to verify the SOC prediction model, expansion force experiments were carried out under two dynamic conditions (NEDC and DST) with different preloads (15kg and 30kg) and different test temperatures (25 ℃ and 45 ℃). As shown in Figure 4, the results show that there is still an obvious voltage platform at 20%~90% SOC, and the change trend of the expansion force is similar to that under the constant current charging mode, indicating that the expansion force is not sensitive to the dynamic change of the current, but is very sensitive to the change of SOC. This is mainly because the voltage depends on the change of the ion concentration on the electrode surface, and the expansion force is the change of the ion concentration of the electrode body phase. In addition, the expansion force of the battery will increase significantly with the increase of the preload, so we should focus on the size of the preload in the design of the battery module.

Figure 4. Swelling force and current voltage curve under NEDC and DST cycle conditions

Next, the author established the LSSVM model, continuously trained and optimized it, combined with the AUKF to predict the SOC, which can realize the SOC prediction for different temperatures, different current dynamic conditions, and different preloads.

Figure 5. Flow chart of SOC estimation based on AUKF and LSSVM

4. Summary

In this paper, the author introduces a new method to estimate the LiFePO4 battery SOC by using expansion force. Based on LSSVM and AUKF algorithm, the estimation error can be less than 1%, and it is applicable to different operating conditions of temperature, dynamic current and preload. In the future, this method is expected to be extended to other battery systems, and it can also further establish SOC prediction models for batteries under different SOH and low temperature conditions.

5. Original Documents

Peipei Xu, Junqiu Li, Qiao Xue, Fengchun Sun. A syncretic state-of-charge estimator for LiFePO4 batteries leveraging expansion force. Journal of Energy Storage, 50 (2022) 104559.

Recommendation of Test Equipment Related to IEST

SWE series in-situ expansion analysis system (IEST): Using a highly stable and reliable automation platform, equipped with high-precision thickness measurement sensors, it can measure the thickness change and change rate of the entire charge discharge process of the electric core, and can achieve the following functions:

1.Test the battery swelling thickness curve under constant pressure.

2.Test the battery swelling force curve under the condition of constant gap.

3.Battery compression performance test: stress-strain curve compression modulus.

4.Step by step test of battery expansion force.

5.Different temperature control: – 20~80 ℃.