To overcome the limitation of the battery life for portable electronics, the triboelectric nanogenerator (TENG), a new energy harvesting technology in the spotlight, has been demonstrated as a self-charging solution. However, it is difficult to combine TENGs with existing portable devices because of shape differences between the TENG-based charging system and electronic devices. In this study, we successfully developed an elastic spiral triboelectric nanogenerator (ES-TENG) with cylindrical shape connected to a plate. With the flat plate substrate, devices can be attached easily to the plate and ES-TENG can work as a self-powered case accessory. The elastic sheet inside ES-TENG can generate multiple alternating current outputs during both extraction and self-retraction processes. As the elastic sheet is the key to ES-TENG, it is analyzed through dynamic motion of the elastic sheet. Furthermore, we propose fabrication guidelines for ES-TENG, depending on the size of portable electronics and based on the properties of the sheet. We demonstrate the possibility of ES-TENG as a self-charging case accessory that can light 120 white LEDs and power an engineering calculator and a rechargeable Ni-MH battery. Thus, our study is a potential energy harvesting package system for conventional portable electronics.