Development of Multifunctional Fiber Optic Sensors for Lithium Ion-Battery Monitoring

Abstract

The objective of this thesis is to develop a fiber optic sensor that is capable of simultaneously measuring temperature and the optical properties of lithium ion batteries. The ultimate goal of the project is the integration of this sensor inside of the pouch of a lithium ion battery cell. Estimation of the state-of-charge (SOC) yields a lot of useful information about the properties of the lithium ion battery. However, in moving towards a completed battery management system, the monitoring of temperature is an important aspect. In this thesis, a fluorescence- based fiber optic temperature sensor is developed such that it can be integrated to a previously developed fiber optic sensor for SOC estimation of lithium ion battery cells. The fluorescence-based fiber optic methodology developed for the measurement of temperature and presented in this thesis is compatible with the current fiber optic sensing methodology for SOC estimation developed in our laboratory, which uses Near Infrared (NIR) transmittance through a partially cladded optical fiber. It is also impervious to harsh environments such as the ones existing in an automobile. This new temperature sensor is fabricated in the same fiber optic cable as the SOC sensor by etching away the cladding of a section of the fiber to form the temperature sensing region. A temperature sensitive film is then used to coat this sensing region which was developed using a double dye system of fluorescein and rhodamine B. Poly(methyl methacrylate) (PMMA) was chosen as a suitable matrix in which the fluorophore are dispersed in. The fluorescence signal of these dyes is exhibited around 570 nm when excited by a 470 nm LED. This allows for wavelength based separation of the signal coming from the SOC sensor and the temperature sensitive signal. The coating developed was optimized for high sensitivity to temperature as well as to ensure a uniform high quality coating. During testing, the temperature sensor was mounted outside of the battery cell while the SOC sensor was embedded inside in between the battery electrodes. The sensor thus obtained, allows for real-time monitoring of the optical properties of graphite and simultaneous monitoring of its temperature while the battery is in operation. A resolution of roughly 0.868 °C and a sensitivity of 9.738 lx/°C was obtained for this sensor. Further work to be performed in order to allow for its embedded use in a lithium ion battery is presented. Also presented is the development of a fully integrated photodetector based sensor for dual parameter monitoring in an automotive environment without a spectrometer.