The thesis adresses a design of a mathematical model for a Li-ion battery cell, based on an equivalent electrical circuit that reflects the performance of a real battery cell as accurate as possible. Research into Li-ion battery type is of interest because it is currently the most technologically sophisticated in terms of the achieved energy densities. A good battery model is necessary because it allows us to control if the battery is operating within voltage, current and temperature constraints. We want the model to capture the static as well as dynamic properties of the battery. In general, the battery cell equivalent circuit consists of a combination of an ideal voltage source, resistor-capacitor circuits (RC circuits) and a resistor representing the internal resistance of the battery. Accuracy of the model improves by adding more RC circuits but that has negative impact on analyzing and parametrization procedure. Due to quite non-linear battery characteristics, we choose a model that has some reasonable trade-off between complexity and accuracy. In order to determine the values of the model's parameters and to verify the model, a series of measurements are performed. We also derive the mathematical background for model's parameters determination which includes the measurements results. The output of the model is first compared to the results of three measurements, where the cell is discharged with current steps of different amplitudes, which also serve to parameterise the model. We then look at model performance where the electrical current input changes randomly. Finally, we generalise the model to a larger battery pack that we build ourselves. Despite certain problems and limitations, the model provides satisfactory results.
|
