In-situ diagnostics of lithium-ion cell parameters using optical fiber Bragg grating technology

The battery management system (BMS) of lithium-ion cells relies primarily on externally measurable parameters, such as voltage, current, and surface temperature. On the other hand, variations in cell external parameters lag behind changes in the cell’s internal physical state and the complex processes occurring inside the lithium-ion cell. The lack of reliable in-situ diagnostic tools capable of resolving internal thermo-mechanical states of lithium-ion cells remains a significant challenge for battery management systems and safety-oriented design. The purpose of the present research is an experimental investigation of the internal thermal behavior of a lithium-ion cell. Bare optical fibers with inscribed multiple fiber Bragg grating (FBG) sensors were embedded into modified lithium-ion cells. Charge–discharge experiments were conducted under various constant-current rates and ambient temperatures. The experimental assessment of the internal temperature map revealed localized hotspots. Additionally, the battery “breath effect”, which connoted the cell volume change due to internal strain variations during cell operation, was identified, and its association with state of charge (SOC) variation was confirmed. A SOC wavelength shift metric is proposed for SOC and health assessment.