## 3 Results and Analysis
### Voltage Change Caused by Load  
To analyze the effect of loads on the deformation and output voltage of thin-film batteries, the thin-film cell area ratio can be obtained by dividing the deformed area by the area before loading. The relative initial voltage can be obtained by dividing the thin-film battery's voltage at different load levels by the thin-film battery's initial voltage. The results of the three tests for each load combination were counted, and their mean values were calculated. All the relative errors between the experimental values and the mean values were less than 5%. Figure 8 shows the statistical results of the expansion area ratio and relative voltage of the thin film cell (C and D) under different load levels. The same method was used for the statistics of thin-film cells A and B. Therefore, the average values of the three groups of the test results were used for the following analysis, as shown in Figure 9.  
Figure 9a,c shows that when loaded along the fiber direction, the expansion area ratio of film batteries A and B increases, and their output voltage decreases as the load increases. They change faster first and then slower. Curve fitting shows that their variation law is close to exponential form. The output voltage of thin-film battery A slowly decreases while B decreases faster. Before loading, the initial voltage of battery A was 1.582 V, and when the membrane structure was damaged, the voltage was 1.489 V, which decreased by 5.88%. The initial voltage of battery B before loading was 1.556 V, while the voltage at the breakdown was 1.366 V, which decreases by 12.21%. The o  
Figure 8: Effect of load level on battery deformation and voltage: (**a**) expansion area ratio, and (**b**) relative voltage.  
decays more severely than that of battery A. Battery B is located in the center of the PVC membrane, where the fibers were subjected to symmetrical bi-directional tensile loads. However, the membrane fibers at location A of the battery are subjected to asymmetric loads. The membrane fiber at location A of the battery is more sensitive to load. Figure 9b,d shows that the deformation and voltage variation of thin-film batteries B and D with load levels are similar to those of A and C. However, the voltage of battery C decreases slowly first and then rapidly when the load level is more significant than 0.6. This is consistent with the variation rule of its expansion area. The initial voltage at cell C was 1.568 V, and the output voltage was 1.348 V when the membrane structure is damaged, which decreases by 14.03%. The thin-film battery D's initial voltage was 1.515 V, while the voltage at the breakdown was 1.202 V, which decreased by 20.67%. Initial voltages vary for different batteries because thin-film batteries' voltages were affected by factors such as cell differences and light intensity.  
The parameters corresponding to the critical point of battery voltage failure can be obtained when the batteries' output voltage drops to 90% of the initial value, as shown in Table 2.  
From the table, it can be found that the load level corresponding to the critical point of voltage for film batteries B and D is much lower than that for A and C. This is because thin-film batteries B and D are located at the center of the membrane structure, where the fibers are oriented in the same direction as the forces and are more susceptible to load,  
Figure 9: Relationship between expansion area ratio and relative voltage of thin-film cells and load level: (**a,b**) are expansion area ratio; (**c,d**) are relative voltage.  
i.e., smaller loads can cause more significant deformations. In practical projects, in order to ensure the power generation efficiency of batteries, the principal longitudinal strain of batteries should be less than 2.8%, the principal strain of PVC film adhering to batteries should be less than 3.1%, and the expansion area ratio of batteries should be less than 1.03.