# Machine Learning-Based Lithium Battery State of Health Prediction Research
## 3 Data Analysis of Lithium Battery Aging and Extraction of Health Factors
### Extraction of Health Indicators  
The selection of HIs is critical to the accuracy of SOH prediction, as appropriate HIs not only enhance model performance, but also reduce the time required for parameter adjustment [34]. Since battery SOH is influenced by various internal and external indicators and is closely related to the aging mechanism, it is difficult to obtain directly through measurement. However, studies have shown that HIs can be extracted from the patterns of external data such as voltage, current, and temperature generated during the aging process to indirectly predict SOH. In this paper, we take the B0005 battery as an example for feature extraction. The graph in Figure 2a shows that the time to reach peak temperature shortens as the number of cycles increases, reflecting capacity decay. Therefore, this value can be used as a health indicator, denoted as HI1. The peak temperature under different cycle counts also varies, with the highest peak temperature during charging denoted as HI2. To fully utilize the existing temperature data, the average charging temperature is used as a health indicator, denoted as HI3. Figure 2b shows the discharge temperature curve, where the time to reach peak temperature shortens, the peak temperature increases, and the slope of the curve becomes steeper, indicating that the average discharge temperature is rising. Therefore, the \begin{table}
\begin{tabular}{c c c c c} \hline
**Battery** & **Charge Current/A** & **Cutoff Current/mA** & **Discharge Current/A** & **Discharge Cut-Off Voltage/V** \\ \hline B0005 & 1.5 & 20 & 2 & 2.7 \\ B0006 & 1.5 & 20 & 2 & 2.5 \\ B0007 & 1.5 & 20 & 2 & 2.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Specific experimental parameters for batteries. peak time, peak temperature, and average discharge temperature are taken as HI4, HI5, and HI6, respectively. From Figure 2c, it can be seen that the initial charging voltage decreases with the number of cycles, the difference between the charging voltage and cutoff voltage gradually decreases over the same time interval, and the area under the voltage curve during constant  
Figure 2: Temperature, voltage, and current versus time for different cycles of the battery. (**a–f**) represent the change curves of the battery charging temperature, discharging temperature, charging voltage, discharging voltage, charging current, and discharging current under different cycles. current charging also decreases. These changes reflect capacity decay, so the initial charging voltage, the isochronous average charging voltage difference, and the area under the voltage curve during constant current charging are denoted as HI7, HI8, and HI9, respectively. Figure 2d shows that over the same time interval, the rate of discharge voltage decrease increases, indicating battery capacity decay. Therefore, the isochronous discharge voltage drop is denoted as HI10, while the average discharge voltage is taken as HI11 due to varying working conditions during actual use. From Figure 2e, it can be observed that the maximum slope of the current decreases with the number of cycles, especially between 0.5 A and 0.2 A, where the decline is more gradual, making it easier to analyze. Thus, the isochronous current drop time from 0.5 A to 0.2 A is denoted as HI12. Additionally, the difference between the charging current at the same moment and the constant current gradually increases, so the isochronous average charging current difference is denoted as HI13. The integral area of the current curve over time gradually decreases, indicating a decrease in charging power; thus, the area under the constant voltage charging current curve is taken as HI14. From Figure 2f, it is evident that the discharge current fluctuates significantly, indicating the complexity of the discharging process. Therefore, the average discharge current is chosen as HI15. By analyzing the variations in the charging voltage and current curves, it can be observed that the charging time varies with the number of cycles.