LEE- Elektrik Mühendisliği Lisansüstü Programı
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ÖgeOnline impedance measurement of batteries using cross-correlation technique(İTÜ Graduate School, 2022) Gücin, Taha Nurettin ; Ovacık, Levent ; 723130 ; Elektrik MühendisliğiAlthough the foundation of battery technologies had been laid out quite a long time ago, the recent increase of the interest for technologies such as renewable energies, portable devices, electric vehicles urged the battery technology to emerge as a major research topic. Moreover, for such applications, the batteries generally considered to be one of, if not the most crucial component of the system, as the energy provided within them ensures the continuity of system operations. Additionally, the lifetime of such systems is generally directly correlated with the lifetime of batteries utilized thereof. Consequently, the assessment of battery condition is a crucial aspect for the durability and stability of a very wide range of applications. However, as most of the parameters regarding the status of the batteries are not directly measurable, the battery status generally has to be estimated. This task is generally undertaken by a battery management system (BMS), which often runs some tests on the battery to predict certain parameters such as state of health (SOH) and state of charge (SOC). Electrochemical Impedance Spectroscopy (EIS) principally presents the complex impedance values of a battery over a frequency range of interest and it is widely accepted as the most advanced testing technique for batteries as it allows a foundation for the estimation of various battery parameters such as battery temperature, SOC and SOH. However, very often, these measurements have to be accompanied with advanced estimation algorithms for ensuring reliable and accurate estimation of parameters. This thesis aims to provide a practical implementation of EIS measurements. For this purpose, the study presented in this thesis implements the suggested method into the DC-DC converters, which are almost always present in the systems deploying batteries. The implemented method is based on cross-correlation calculations. In the first part of the thesis the motivation and the objectives of the study is elaborated and a literature overview of the state of the art is provided for representing the current approaches and for emphasizing the need for improvements. The first part is followed by a theoretical background section, where the principles of the cross-correlation calculations and it's adaptation to battery EIS measurements are explained in detail. Several improvements for the method, especially aimed for battery applications, are also provided. In the subsequent part, the theoretical findings are supported by simulations, which are created in MATLAB by using the Simulink graphical programming environment. In this part, the theory is preliminarily tested by simulations regarding impedance measurements of a passive RLC circuit. In the latter part, a test bench is designed for performing experiments to serve as proof of concept for the suggested approach. The test bench comprises a digitally controlled boost converter that is configured for charging a 12-V, 7-Ah sealed lead-acid (SLA) battery. The boost converter is controlled by an FPGA based platform, namely the Nexsys 4 DDR of Xilinx. The digital controller also comprises subprogram for injecting the necessary signals to the battery. The waveforms that occur during the tests are then recorded by a data acquisition system based on NI cDAQ platform so that the saved data can be processed in MATLAB environment for calculating the EIS diagrams. During the experiments, firstly, the battery is tested via the proposed method at 50% SOC. It is shown that the results of the present approach coincide with those obtained by a commercially-available, laboratory-type, high-precision instrument. Finally, the tests were also repeated for 25% and 75% SOC values. Additionally, further results are also presented to prove the validity of the approach even when the DC-DC converter is configured to provide a constant current under closed feedback loop. In conclusion, it is shown that the proposed approach can be reliably used to analyse the impedance of batteries over a wide frequency range during battery charging process.