Throughout the integration of thin-film supercapacitors into composite structures, it is not only possible to store electric energy, but also to use them as sensor systems for strain detection. This paper describes the integration of thin-film supercapacitors into a glass-fiber composite structure, as well as their analyzation under mechanical load. The results are showing that there is a strong link between the mechanical load and the impedance spectra of the supercapacitor, enabling it to act as strain sensors. To characterize the samples, a four-point bending experiment is used, which features a well-defined constant load scenario with two interesting load states, a tensile and compression state, all in one sample. On each side one structural supercapacitor is integrated, placed far away from the neutral axis near to the outside layer, to be under the highest strain possible. Additionally, the device is placed in between the middle load entry points. During the four-point bending experiment the load is kept constant to run an electrochemical impedance spectroscopy to characterize the supercapacitor. This gives an inside view of what is happening in the supercapacitor. The experiments show a strong relation between the mechanical load and the electrochemical impedance measurements.