Operando monitoring of battery state-of-charge via tilted fiber Bragg grating cascaded with chirp fiber Bragg grating using integrated area method

An in situ optical sensing method, which is based on a cascaded tilted fiber Bragg grating and chirped fiber Bragg grating (TFBG-CFBG) configuration, is proposed and experimentally demonstrated for monitoring the electrolyte status of the lead-acid batteries. In the proposed system, the TFBG is embedded within the battery to monitor variations in the refractive index (RI) of the electrolyte, whereas the CFBG, placed externally, functions as a spectral filter. The reflected optical signals from both gratings are captured and subsequently measured using an optical power meter (OPM). During battery charging and discharging, changes in the electrolyte concentration alter the RI, thereby modifying the degree of spectral overlap between the TFBG and CFBG, which in turn leads to changes in the reflected power. Experimental results demonstrate that the real-time monitoring of electrolyte dynamics can be achieved by adopting an integrated area interrogation technique, while temperature interference within the range of 22 ℃–60 ℃ is significantly reduced. This study provides an optical sensing strategy for investigating battery reaction kinetics and for dynamically enhancing state-of-charge (SOC) monitoring through the use of compact and cost-effective instrumentation.