Publication: Active cell balancing in an electric vehicle
Date
2024-08
Authors
Moses, Kueh Guang Jin
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Abstract
The battery is the main energy source in an EV. The need for cell balancing exists to increase the reliability and the longevity of the EV batteries by preventing over-charging and deep discharging of the cells. The active cell balancing technique redistributes charges across the cells and is preferred over the passive method due to shorter balancing time and higher efficiency. An effective cell balancing and selection algorithm, flexible in the number of cells involved in the balancing process and considering multiple criteria in the cell selection judgement, is also required to ensure a short balancing time and the selection of only true fitting cells for discharging. Voltage control is also essential to ensure uninterrupted optimal performance of the EV during the simultaneous cell balancing
process. This research aims to develop an active cell balancing system that utilises fuzzy logic-based cell selection algorithm, performs voltage control to achieve desired reference voltage and perform comparative analysis on the proposed system using three reference voltages i.e. 45V, 60V and 70V. A converter-based active cell balancing system consisting of 24 Li-ion cells, a switching system, a ZET converter, a load, a fuzzy logic-based cell selection algorithm, PWM signal’s duty cycle computation and PWM signal generation is developed and simulated at the mentioned reference voltages. For all tested reference voltages, the observations of the redistribution of charges, the convergence of all cells’ SoC to an approximate equal level, the maintenance of all cells’ SoC at an approximate equal level after the convergence, constant load voltage close to reference voltage, but varying PWM signal’s duty cycle has validated the proposed system. The system functions optimally at the reference voltage of 60V. A higher reference voltage reduces the duration for convergence of all cells’ SoC and the difference between the highest and lowest SoC after the convergence.