Future energy storage applications emerging from technological innovations, such as drones and smart devices, require batteries to be small and slender while maintaining high energy density and power capability, fast charging, long cycling/calendar life, and safety. In this study, coaxial-fibers bundled batteries (CFBBs) are proposed, wherein the center is a negative electrode made of carbon fibers, the inner shell is a separator made of Al₂O₃ and polyvinylidene fluoride (PVDF), and the outer shell is a positive electrode. These batteries have flexibility in size and shape by changing the number of bundled electrodes. A 225 mA h CFBB consisting of 288 fiber-electrodes exhibited a high-rate capability of 180 mA h at a 7.6C-rate and a capacity retention of 92% after 100 cycles without marked degradation. A 3.3 mA h CFBB consisting of four fiber-electrodes exhibited 90% capacity retention after 100 cycles. Electrochemical–thermal coupled simulations of CFBBs predicted high safety in the event of an internal short-circuit, and the nail penetration test on the 225 mA h CFBB at 4.2 V demonstrated that the temperature rise was limited to 48 °C by the characteristic safety mechanism. Manufacturing processes to enable mass production are described and a flight test of a drone with four 225 mA h CFBBs is conducted to demonstrate their performance. A flexure test of the CFBBs consisting of 288 fiber-electrodes demonstrated that CFBBs have sufficiently high strength to be applied as the frameworks of drones and wearable devices and as their power sources.