In the operation of lithium-ion batteries, particularly at the high discharge rate, the temperature of the battery significantly increases owing to heat generation. The most accurate and reliable sensor data for the battery management system (BMS) cannot be obtained by relying solely on temperature monitoring of the surface of individual cells. In-situ monitoring of lithium-ion batteries is one of the most effective methods to prevent the thermal runaway of lithium-ion batteries. A sensor is implanted in the heat source core of the lithium-ion battery, and the temperature change can be sensed immediately. A fiber optic composite temperature sensor is embedded in the inner center of the 18650 cylindrical lithium-ion battery, and the cross sensitivity of temperature and stress inherent in the Bragg grating sensing mechanism is eliminated using the Fabry-Perot (F-P) air cavity on the same fiber. The experimental results show that the internal temperature change of the battery can be monitored in real time in the charge and discharge stages, and the optical fiber sensor is compatible with the internal cell of the battery. This can facilitate the long-term health monitoring of large-scale lithium-ion battery-integrated components.