Fiber Optic Sensing Technologies for Battery Management Systems and Energy Storage Applications

The increasing demand for batteries in electric vehicles, hybrid-electric aircraft, and grid-scale energy storage necessitates advanced sensing systems for enhanced battery management. Current BMS rely heavily on voltage, current, and temperature measurements from thermocouples or micro-thermistors, which are susceptible to electromagnetic interference (EMI) and limited in placement. Fiber optic (FO) sensors offer several key advantages: immunity to EMI and RFI, electrical insulation, lightweight and flexible design enabling internal cell deployment, high sensitivity, multiplexing capability, and the potential to measure various parameters (strain, temperature, acoustic emission, chemical species). FO sensors are classified into single-point, quasi-distributed, and fully distributed types, with fiber Bragg grating (FBG) sensors being particularly promising for BMS due to their self-referencing and multiplexing capabilities. The detection of CO2, a major vent gas before thermal runaway, is highlighted as a crucial indicator of battery health. This review comprehensively assesses the application of FO sensing technologies in battery systems, focusing on Li-ion batteries and perspectives for grid-scale batteries, covering various sensor types, monitoring strategies for internal and external cell parameters, and cost considerations.

The review examines recent research (last 5 years) on fiber optic monitoring techniques for Li-ion batteries. It covers point temperature and strain measurements to determine internal cell temperature and SOC, as well as other SOC/SOH-correlated parameters such as pressure, acoustic emissions, transmittance, and refractive index. Monitoring strategies for imminent cell failure, such as thermal runaway, are also discussed at cell and pack levels. The review includes a comparative analysis of different FO sensor types (FBG, evanescent wave, fluorescence-based, Fabry-Perot interferometer) based on their operating principles, spatial distribution topologies, and recent advancements. It further explores quasi-distributed and fully distributed sensing techniques, including wavelength-division multiplexing (WDM), time-division multiplexing (TDM), optical time-domain reflectometry (OTDR), and optical frequency-domain reflectometry (OFDR). The literature review also incorporates studies on CO2 sensing using optical fibers for SOH estimation and early failure prediction.

The review adopts a systematic approach to evaluate the existing literature on fiber optic sensing for battery applications. It begins by identifying potential applications in different scales of energy storage systems: passenger electric vehicles, heavy-duty electric trucks, and utility-scale battery energy storage. A quantitative comparison is provided using a matrix that details the number of cells required for each application and different cell types (cylindrical, prismatic, pouch). The review then delves into the operating principles of various fiber optic sensors, including single-point sensors (FBG, evanescent wave, fluorescence-based, Fabry-Perot interferometer) and distributed sensors (quasi-distributed and fully distributed). It extensively analyzes recent research works on FO monitoring techniques for both internal and external cell parameters, focusing on general Li-ion battery functions and thermal runaway detection. A detailed cost estimation for FBG sensors across different scales is presented, emphasizing the significant contribution of interrogation costs. The review also explores strain-temperature discrimination methods using reference sensors, dual-sensor configurations, and hybrid sensor approaches. Finally, it examines the potential of FO sensors in measuring other parameters relevant to SOC and SOH estimations (strain, pressure, refractive index, acoustic emission) and thermal runaway detection (distributed temperature measurements, vent gas concentration).

Fiber optic sensors offer significant advantages over traditional electrical sensors for battery monitoring, primarily due to their immunity to EMI and RFI. The ability to embed FO sensors within cells allows for accurate measurement of internal parameters, improving SOC and SOH estimations. Research has shown strong correlations between several parameters measured by FO sensors and the battery's health status. Temperature measurements using internal FBG sensors reveal significant temperature differences between the cell's interior and exterior, highlighting the need for internal sensing. Strain measurements correlate strongly with SOC and SOH, enabling improved state estimation. Discrimination techniques between strain and temperature signals are crucial for accurate interpretation. Measurements of pressure and refractive index changes show promise for understanding SEI formation and internal resistance. Acoustic emission measurements offer a direct method to assess internal structural changes related to SOC. Distributed temperature sensing using both quasi-distributed and fully distributed FO sensors effectively identifies temperature hotspots, essential for early thermal runaway detection. Vent gas (CO2) concentration measurements using FO sensors enable early detection of thermal runaway, even before significant temperature rises.

This review highlights the potential of fiber optic sensing technologies to significantly enhance battery management systems. Internal sensing capabilities offer substantial improvements in state-of-charge (SOC) and state-of-health (SOH) estimations, enabling safer and more efficient battery operation. The ability to detect early signs of thermal runaway, through both temperature and gas monitoring, enhances safety and reduces the risk of catastrophic events. While the high cost of interrogation systems presents a challenge, the potential for cost reduction through advancements in photonic integrated circuits and the use of less expensive light sources suggests that FO sensors are a viable technology for future battery systems, particularly for larger-scale energy storage applications where the cost can be distributed across numerous sensing points. The use of less expensive sensors like FOEW and FP sensors will contribute to making FO technology feasible for more applications. The review has identified key areas for future research including cost reduction, enhanced sensor stability and durability, and exploration of different FO sensors for next-generation batteries such as solid-state batteries.

Fiber optic sensors offer substantial advantages for battery monitoring, including EMI immunity, internal sensing capabilities, and multi-parameter monitoring. While cost remains a barrier, advancements in interrogation techniques and the use of lower-cost sensors show potential for wider adoption. Future research should focus on optimizing sensor integration methods, developing lower-cost interrogation systems, and exploring the application of FO sensors to monitor next-generation battery chemistries and designs. The research directions include improved discrimination of multiple parameters influencing Bragg wavelength shifts and extending the long-term durability of FO sensors in battery systems.

The review primarily focuses on Li-ion batteries, and the applicability of these findings to other battery technologies needs further investigation. The cost analysis is based on current technology and pricing, and future advancements may alter the cost-effectiveness of FO sensor systems. While some studies showed comparable capacity retention of fiber-embedded batteries, long-term cycling tests with a larger number of cells are needed for complete validation. The review also primarily focuses on the research aspects of fiber optic sensors and their capabilities. A thorough investigation of the commercialization challenges and the integration of these technologies into commercial BMS is recommended.