**4. Conclusions**

This chapter fully describes all main optical fiber sensing techniques used and developed for tracking critical key parameters in LIBs since the first study in 2013. According to the operating principles, FBGs, FPIs, optical fiber evanescent waves, and optical fiber photoluminescent sensors are being used so far. Regarding all the studies selected to perform this overview, the principal parameters presented in the literature were temperature (heat flow), strain, pressure, electrochemical events (such as electrode lithiation and gassing production), RI, and SoX battery indicators (such as SoC, SoD, and SoH). In a general overview, the FBGs, FPIs, and photoluminescent sensors are mostly used to track the physical parameters instead of the evanescent wave sensors are most used to detect the electrochemical events in LIBs due to the necessity of measuring RI values from the surrounding materials that interact with the optical fiber surfaces, in this case.

Between all OFS used in the battery sensing applications and with an easier correlation with BMS, the FBGs coupled with other types of sensors (interferometers and/or evanescent wave sensors), seem to be the most advantageous in the future battery applications, due to their intrinsic characteristics, of the possibility of multipoint and multiparameter monitor, and easy interrogation, operating in a reflection system. Those factors detected have a good alliance with the battery SoX, thus can greatly reflect the battery failure condition. However, the development of sensors for battery tracking is still not consistent with the goal of massive processing, low cost, and daily applications. Some problems, such as excessive data treatments, and the high fragility of some optical fibers (reduced thickness) still exist. Generally, the optimal result of *in situ*/operando sensing is to real-time monitor and interpret each shift of wavelength into a concrete chemical reaction, so that the precise battery SoX estimation of BMS can be achieved and corresponding actions can be taken place from the external to sustain the continuing operation of batteries.

*Tracking Li-Ion Batteries Using Fiber Optic Sensors DOI: http://dx.doi.org/10.5772/intechopen.105548*

Comparatively, with other sensing tools or instruments that were also used to monitor critical parameters in LIBs, such as TCs, RTDs, thermography, and, strain gauges, the OFS presents several advantages. They can be embedded in the electrochemical environment of the cells, detecting with elevated accuracy and simultaneously, in multipoint and multiparameter, which are until now, completely unknown, such as the internal pressure and RI variations, which are directly correlated with electrochemical cells events (SEI layer formation).

This advancement in sensing internal and operational batteries using OFS will allow for the improvement of their performance and safety and will help in understanding and improving the lifetime and behavior of the next generation of LIBs to be developed.
