Fiber Bragg Grating Monitoring of Thermal Dynamics in Immersion-Cooled Lithium-Ion Batteries

Dielectric immersion provides a direct thermal boundary for lithium-ion battery abuse tests, while passive fiber Bragg grating (FBG) sensing enables surface-temperature monitoring within the liquid. We monitored fully charged NCR18650BD cells during external short circuits under natural air and static transformer-oil immersion through two 1 km single-mode fiber leads. A seven-point temperature calibration extended to 132.3 °C yielded a sensitivity of 8.76 pm °C−1 (R2 = 0.99955). In the original tests, peak surface temperatures were 132.3 °C in air and 51.1 °C under immersion, with post-peak half-decay times of 579 and 161 s, respectively. Additional tests reproduced the lower temperature excursion and shorter post-peak half-decay under immersion, although the recorded short-circuit currents differed between cooling conditions. The results characterize these thermal responses and demonstrate the feasibility of electrically passive, long-reach FBG monitoring during immersed battery-abuse experiments.