In the past few years, fiber optic sensors have demonstrated an amazing ability to detect the state of charge (SOC) and electromotive force (EMF) inside a battery in real-time. However, it remains an enormous challenge to characterize the relationship between the spectral shift of the fiber sensor and the internal EMF change of the battery. Here, we propose a method to monitor the electrolyte during the battery discharge process using the integral spectrum of a tiny tilted fiber Bragg grating (TFBG) sensor. The relationship between the fiber optic transmission spectrum and battery EMF was established by using this method. The results show that a TFBG sensor implanted in a lead-acid battery enables rapid EMF detection with a sensitivity of 1.16 × 105 (nm·dBm) /V. This technology provides a fiber optic precision solution for battery operating conditions and has excellent potential for detecting battery failures using traditional EMF methods.