Modeling of Ageing of Lithium-Ion Battery at Low Temperaturas
Examensarbete för masterexamen
Electric power engineering (MPEPO), MSc
For mass production of HEVs, performance characteristics and prediction of ageing behaviour of Lithium-ion batteries is essential. This thesis considers the performance and durability of the battery system to be used in commercial Hybrid Electric Vehicles. The purpose of the work presented in this thesis is to link capacity fading to battery usage and to investigate how low temperature conditions and charging rates affect the cycle life and ageing processes of batteries developed for use in HEVs. Ten LiFePO4//Graphite cells optimized for power applications were systematically aged using a simple charge-discharge cycle with constant current rate spanning multiple C-rates (1C, 2C and 4C) and temperatures (0C, -10C and -15C). Extensive impedance spectroscopy tests were conducted in order to build equivalent circuit model of the cell. The experimental impedance spectra of the positive electrode and the fitted one show good agreement for all the cases up to a SoC range of 10%-80% Lithium loss was found to be the dominant ageing mechanism for most of the test cases. A CC/CV charge and a CC discharge even with active air cooling was not found to be a suitable approach at a low temperature of -15C due to dramatic self heating of the cells. A wide SoC range was found to have a profound effect on ageing.
Elkraftteknik , Electric power engineering