In recent years, research on lithium–ion (Li-ion) battery safety and fault detection has become an important topic, providing a broad range of methods for evaluating the cell state based on voltage and temperature measurements. However, other measurement quantities and close-to-application test setups have only been sparsely considered, and there has been no comparison in between methods. In this work, the feasibility of a multi-sensor setup for the detection of Thermal Runaway failure of automotive-size Li-ion battery modules have been investigated in comparison to a model-based approach. For experimental validation, Thermal Runaway tests were conducted in a close-to-application configuration of module and battery case—triggered by external heating with two different heating rates. By two repetitions of each experiment, a high accordance of characteristics and results has been achieved and the signal feasibility for fault detection has been discussed. The model-based method, that had previously been published, recognised the thermal fault in the fastest way—significantly prior to the required 5 min pre-warning time. This requirement was also achieved with smoke and gas sensors in most test runs. Additional criteria for evaluating detection approaches besides detection time have been discussed to provide a good starting point for choosing a suitable approach that is dependent on application defined requirements, e.g., acceptable complexity.

The increasing adoption of lithium-ion battery cells in contemporary energy storage applications has raised concerns regarding their potential hazards. Ensuring the safety of compact and modern energy storage systems over their operational lifespans necessitates precise and dependable monitoring techniques. This research introduces a novel method for the cell-specific surveillance of prismatic lithium-ion cells, with a focus on detecting pressure increases through the surface application of a fiber Bragg grating (FBG) sensor on a rupture disc. Commercially available prismatic cells, commonly used in the automotive sector, are employed as test specimens and equipped with proven pressure and innovative FBG sensors. Encompassing the analysis capacity, internal resistance, and pressure (under elevated ambient temperatures of up to 120 °C), this investigation explores the thermal degradation effects. The applied FBG sensor on the rupture disc exhibits reversible and irreversible state changes in the cells, offering a highly sensitive and reliable monitoring solution for the early detection of abuse and post-abuse cell condition analysis. This innovative approach represents a practical implementation of fiber optic sensor technology that is designed for strain-based monitoring of prismatic lithium-ion cells, thereby enabling customized solutions through which to address safety challenges in prismatic cell applications. In alignment with the ongoing exploration of lithium-ion batteries, this research offers a customizable addition to battery monitoring and fault detection.

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In the rapidly evolving field of lithium-ion battery technology, ensuring safety and optimizing performance have become paramount concerns, especially in applications that demand high performance and reliability, such as electric vehicles and portable electronics [1]. Established and state-of-the-art monitoring of voltage, current, and a sparse distribution of surface temperature sensors are often insufficient for early fault detection [2,3]. To address this challenge, this study presents an innovative approach to battery monitoring using referenceless fixed Fiber Bragg Grating (FBG) sensors mounted on the surface of pouch cells. This method takes advantage of the dual functionality of FBG sensors to measure both strain and temperature directly on the exterior of the cell, offering a promising avenue for real-time battery health and safety monitoring [4]. Pouch cells, favored for their lightweight and flexible design, present unique challenges for adding robust strain monitoring technologies due to the anisotropic nature of their functionally optimized foil enclosures. Our research addresses these challenges by detailing an innovative and systematic surface application method for FBG sensors that minimizes intrusion while providing stable conditions on the pouch foil. This method's small form factor and cost-effectiveness are significant advantages, providing a non-invasive means of gathering high-density information about the cell's operational state without compromising its structural integrity. A critical aspect of this study is the on-cell calibration technique developed for the FBG sensors, which enables the unambiguous sensor reading across different positions without the usually required additional reference sensors. This breakthrough addresses a common limitation in FBG monitoring systems for batteries by ensuring that sensor outputs are reliably correlated with specific strain and temperature parameters, respective of their placement on the cell. The calibration process, thoroughly documented in our experiments, underpins the sensors' capability to provide accurate, real-time data crucial for predictive maintenance and fault detection. Furthermore, the study explores the limitations imposed by the pouch foil's mechanical properties, presenting a comprehensive analysis of sensor performance concerning these constraints. Through extensive testing, we establish guidelines for optimizing sensor placement and adhesion techniques to ensure reliable measurements without affecting cell performance. Our experimental findings highlight the FBG sensors' dynamic temperature measurement capabilities, demonstrating sufficient sensitivity to detect abnormal strain developments indicative of potential battery faults. The dual-measurement approach not only allows for the early detection of issues such as thermal runaway but also provides valuable insights into the development of mechanical stresses experienced by the cell during operation. Such insights are instrumental in advancing our understanding of battery behavior under various load conditions, contributing to the development of safer and more efficient battery systems. In conclusion, the improved surface application of reference-less fixed FBG sensors on pouch cells represents a significant advancement in battery monitoring technology. The small form factor, cost-efficiency, and high information density offered by this setup make it an attractive solution for manufacturers and researchers alike. By providing a means to accurately monitor strain and temperature in real-time, our approach facilitates improved safety protocols and performance optimization for lithium-ion batteries. Due to the possible omission of an additional sensor, and higher information density compared to just temperature or strain sensors, this study lays the groundwork for industrially used FBG sensor applications and calibration techniques, paving the way for the broader adoption of FBG sensors in battery safety and performance monitoring systems. References: [1] Y. Zhang et al., Towards High-Safety Lithium-Ion Battery Diagnosis Methods. Batteries 2023, 9 (1), 63. DOI: 10.3390/batteries9010063. [2] X. Feng et al., Thermal runaway mechanism of lithium ion battery for electric vehicles: A review. Energy Storage Materials 2018, 10, 246–267. DOI: 10.1016/j.ensm.2017.05.013. [3] J. Klink et al., Comparison of Model-Based and Sensor-Based Detection of Thermal Runaway in Li-Ion Battery Modules for Automotive Application. Batteries 2022, 8 (4), 34. DOI: 10.3390/batteries8040034. [4] A. Hebenbrock et al., Innovative Early Detection of High-Temperature Abuse of Prismatic Cells and Post-Abuse Degradation Analysis Using Pressure and External Fiber Bragg Grating Sensors. Batteries 2024, 10 (3), 92. DOI: 10.3390/batteries10030092. Figure 1