## 2 Experimental Materials and Test Methods
Figure 2: Diagram of the composite structure of film battery and PVC membrane: (**a**) loading along the fiber direction, (**b**) loading at an angle of 45 degrees to the fiber. Before tensioning the PVC membrane, the wire rope was bypassed by the adjustment wheel at the end of the supporting steel tube, and the steel splint of the PVC membrane was connected with the hydraulic cylinder under tension by the wire rope. Support reactions were provided for steel pipes and hydraulic cylinders using platforms in the structure laboratory. The loading device was a hydraulic loading system consisting of two double-eared cylinders and a hydraulic oil pump. The shunt valve was used to make the two cylinders have the same oil pressure to achieve the synchronous loading and unloading of PVC membranes. The pull sensor was used to calibrate the pull force generated by the  
Figure 3: The biaxial loading device of composite structure: (**a**) three-dimensional schematic diagram of the device, (**b**) overall test process, (**c**) pipe and pulley support, (**d**) connection between steel plate clamp and membrane material, and (**e**) voltage test device. hydraulic cylinder before loading. Then, the PVC membranes were stretched at a loading speed of 5 kN/min until they were destroyed. Two 1000 W standard tungsten halogen lamps were irradiated on the surface of thin-film batteries, as shown in Figure 3. The halogen tungsten lamps were turned on only during loading to reduce the effect of temperature on PVC membranes. The multimeters (measuring range: 2 V; accuracy: 0.5% + 1) were used to measure the potential of thin-film batteries. The camcorder was used to record the change of the multimeter value during loading, and the voltage test is shown in Figure 3e. Digital image correlation technology (DIC) has good accuracy and the ability to monitor real-time full-field displacement, so it is widely used to study surface strain field or crack behavior [28; 29]. DIC was used to obtain the strain cloud image on the surface of the thin-film battery. Because the film battery is much smaller than the entire PVC film, even if the film is a saddle-shaped structure, the surface of the film cell can still be regarded as a plane. The speckles were sprayed on three areas of the membrane cell surface with 13 \(\times\) 13 mm area and on the PVC membrane at the exact location as the cell, as shown in Figure 4.  
To establish the relationship between cell deformation and voltage, the average strain under different tensile loads, i.e., the length variation of the green line in Figure 4b, was obtained along with the longitudinal and transverse directions of the thin-film cell. The thin-film cell area change can be calculated from the length change of the green line. For speckle areas on batteries, the expansion area is defined as (area after deformationarea before deformation)/area before deformation. As a result, the relationship between the thin film battery's voltage and its expansion area can be obtained. Industrial cameras (Model: 14 megapixels, JHSM1400; Manufacturer: Shenzhen Jinghang Technology Co., Ltd., Shenzhen, China) were set on both sides of the PVC film. The first industrial camera was filmed perpendicular to the battery surface. Due to space location limitations, based on the mirror reflection principle, a plane mirror was placed at the 45-degree angle of PVC film. Then, another industrial camera was used to take pictures of the PVC film inside the reflector, as shown in Figure 4c. During the loading process, the  
Figure 4: Speckle position and expansion deformation data: (**a**) thin film battery and the speckles on it (unit: mm), (**b**) data extraction of thin-film battery deformation, (**c**) film battery image and speckle inside the mirror. industrial camera was set to take pictures every second. Strain clouds at speckle locations at different loading stages were calculated by using DIC [30; 31].