## 3 Results and Analysis
### Deformation of Membrane and Battery  
Through the experiment in Figure 3, the corresponding limit loads in Figure 2a,b were 10.52 and 14.61 kN, respectively. The ratio of actual load to ultimate load is defined as the load level. The PVC membrane material at the thin-film battery B and the thin-film battery in Figure 2 is selected for analysis. Strain clouds at different load levels are shown in Figure 6. The general trend of strain clouds for the remaining batteries is similar and will not be repeated here. Figure 5: Experimental results of PVC membrane under uniaxial tension: (**a**) fiber direction tension, (**b**) tensile force 45 degrees from fiber direction. It can be seen from the figure that the principal tensile strain of the thin-film battery and the principal tensile strain of the PVC film at the same position show non-uniform distribution at different load levels. The principal tensile strain around the PVC membrane is significantly larger than that inside the membrane cell. Through actual observation, it was found that the edge of the film cell and the PVC membrane had slightly peeled off when the PVC membrane was about to be destroyed. This phenomenon explains that thin-film batteries adhere to the PVC membrane material, which results in a joint force between the two. The stiffness of the former was more significant than that of the latter. There was a bond slip between the two at the edge of the thin-film cell. This results in less stress on the membrane cell but more stress on the PVC membrane, which causes a much higher strain on the PVC membrane at the edge. The strain data at each pixel in Figure 6 were extracted, and the mean values of principal tensile strains were calculated, and so on. The relationship between load level and the mean value of principal tensile strain is shown in Figure 7.  
It can be seen from the diagram that the principal tensile strain of all thin-film batteries and PVC membranes increases with increasing load levels. When the load level is less than 0.3, the thin-film battery's principal tensile strain increases rapidly with the load level increase. Meanwhile, when the load level is more significant than 0.3, they increase again slowly. When the load level is less than 0.6, the strain of PVC film increases slowly. While when the load level is higher than 0.6, the strain of the PVC membrane increases rapidly. When the load level is 0.6, the strain of the PVC membrane at positions A and B of the  
Figure 6: Strain cloud maps of thin-film cells and PVC films at different load levels. Figure 7: Relation between principal tensile strain and load level. film cell is about 3%, and at positions C and D of the film cell is about 6%. Simultaneously, only at the edge of the thin film cell, the phenomenon of separating from the PVC film material began to appear. In practical engineering, to ensure that the membrane battery does not disengage from the membrane material during service, the applied load level should be less than 0.6, or the principal tensile strain of the PVC membrane material at the membrane battery should be less than 3%. The saddle-shaped PVC membrane was in a complex stress state in space and usually broke at the four corners first. The strain of the PVC membrane at the membrane cell location was significantly less than that under uniaxial tension. The principal tensile strain of all thin-film batteries shows a similar trend. However, the principal tensile strains of the PVC film at the positions of the thin-film cell C and D are significantly greater than that of the rest. This is because ropes surrounded the saddle-shaped PVC membrane. Compared with the PVC membrane adhering to film batteries A and B, the fibers in the PVC membrane adhering to film batteries C and D were in a more constant tension state and the ultimate strength obtained by experiments was much higher. Therefore, the principal tensile strain of the PVC film at the thin-film batteries C and D was also much more significant at the same load level.