Long-term stability study of fiber Bragg grating sensors integrated into a lithium-ion pouch cell

Monitoring the temperature of lithium-ion batteries (LiB) is crucial for safe operation. The degradation of a LiB is highly dependent on temperature. It is desirable to keep the LiB within a temperature window between 15 °C and 35 °C. Otherwise, enhanced degradation will take place. So far, only the surface temperature of a LiB at a single point is measured, but the internal temperature can be significantly higher during operation. In this work, we integrated fiber Bragg grating (FBG) temperature sensors into lithium-ion pouch cells and evaluated the stability of the sensitivity of the FBG sensors over four months. Two prototype pouch cells were manufactured each with two fibers. Each fiber has three FBGs inscribed, hence there are six temperature measurement points per pouch cell. The FBG temperature sensors comprise a polyimide-coated fiber with the three inscribed FBGs and a polyimide-coated fused silica capillary. The capillary mitigates the influence of external strain on the FBGs because the fiber can move freely in the capillary due to the single mounting point on one side of the capillary. The sensitivity was measured once a month for a period of four months. Although, the Bragg wavelength changed over time, and baseline correction is required for reliable temperature calculation. The measurements show comparable stability of sensitivity over time with another publication.