Thermal monitoring of a lithium-ion pouch cell is essential for safe use.  Until now, the temperature has only been monitored at a few preselected points in a battery module and passed on to the battery management system (BMS). The BMS controls the operation of all the cells in a battery pack. It prevents over-charging, over-discharging, over-heating. However, single-point measurement cannot fully represent the thermal state of all cells. Additional sensors are needed. 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 (FBGs with different Bragg wavelengths are inscribed in one fiber), and their sensitivity to more than one parameter, such as temperature and strain. In this work, we use a fiber optic sensor with 9 FBGs inscribed to monitor the surface temperature of a lithium-ion pouch cell. The fiber is a single-mode fiber with polyimide coating. The sensor was calibrated before use. 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.  During calibration, a hysteresis during heating and cooling was observed. This hysteresis is due to the influence of the changing relative humidity on the polyimide coating in the climatic chamber.  Keeping the relative humidity constant throughout the calibration eliminates the hysteresis effect. The maximum calibration error for the linear fit is up to 1.53°C when relative humidity is not kept constant and 1.05°C for the polynomial fit. For a calibration with constant relative humidity, the calibration error decreases to 0.11°C for the linear fit and 0.10°C for the polynomial fit. With this sensor, the surface temperature of a pouch cell from LG Chemical was monitored with high accuracy and spatially resolved during cycling.
 L. Raijmakers, D. Danilov, R.-A. Eichel, and P. Notten, “A review on various temperature-indication methods for li-ion batteries,” Appl. Energy 240, 918–945 (2019).
 M. Nascimento, M.S. Ferreira, J.L. Pinto, Real time thermal monitoring of lithium batteries with fiber sensors and thermocouples: a comparative study, Measurement, 111 (2017), pp. 260-263
 Zhang, Jing and Yongqian Li. “Calibration method for fiber bragg grating temperature sensor.” 2009 9th International Conference on Electronic Measurement & Instruments (2009): 2-822-2-825.
 P. Giaccari, H. G. Limberger, and P. Kronenberg, „Influence of humidity and temperature on polyimide-coated fiber Bragg gratings,“ in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, 2001 OSA Technical Digest Series (Optical Society of America, 2001), paper BFB2.
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