Lithium-ion batteries are widely used as power sources in portable devices and in hybrid and electric vehicles. The demand for such batteries will continue to grow due to the ongoing electrification of the mobility sector. To ensure a proper and safe operation of lithium-ion cells in a vehicle a battery management system (BMS) is used. These systems monitor the voltage and the current of the cell and typically the temperature at a single point to estimate the state of charge (SOC), state of health (SOH), and state of safety (SOS). However, these measurements cannot represent a spatially resolved thermal state of the cells in the battery modules or packs. Locally occurring temperature changes are difficult to detect with this system. Additional concepts are needed to gain better insight into the thermal behavior of single cells in each module to detect failures such as thermal runaways early and enhance the safety of the battery system. Fiber Bragg grating (FBG) sensors are promising candidates for such sensors because they are immune to electromagnetic interference, are small and lightweight, and perform well in harsh environments. Other advantages of FBGs include the ability to multiplex, and their sensitivity to more than one parameter, such as temperature and strain. One drawback is that the high-end interrogation units for reading the sensors are quite expensive, but smaller and less expensive alternatives have been developed in recent years. In this work, we examine a sensor array of 9 FBGs for high absolute temperature accuracy. Therefore, a calibration setup is introduced comprising of an aluminum heat spreader in a climatic chamber and a high precision reference temperature sensor with an accuracy of 15 mK. The obtained calibration curves are fitted using linear and polynomial methods. The different methods are compared regarding the fitting error. Hysteresis effects during heating and cooling of the FBGs are also considered and studied in more detail. The characterized FBG sensor array is used to monitor the absolute surface temperature of a pouch cell during cycling. First results are presented.