Voltage equalizer enhancement of series-connected battery strings using variable duty cycle PWM of a dynamic capacitor circuit

Abstract

Electric storage systems like solar systems and electric vehicles use batteries for storing electricity due to their simplicity, efficiency, considerably small size, and dispatchability. These batteries operate on the principle of charging/discharging and require equalization for voltage balance, especially in series-connected batteries. In this research, a novel technique is presented for enhancing batteries’ voltage equalization, which is based on the variable duty cycle, D, of pulse width modulation (PWM) in the dynamic capacitor technique. This method controls two energy storage elements: an inductor and a dynamic capacitor via a variable D of PWM. The presented technique was implemented on lead-acid batteries connected in series using MATLAB/Simulink. The simulation results showed that increasing D to 80% can reduce the equalizing process time from 500 seconds to just 125 seconds, with voltage differences decreasing from 800mV to just 2.2mV, equalized by 99.98%. For comparison, a well-known fixed switched-capacitor technique was used, and results showed that the variation of D had no effect even after 500 seconds of the equalizing process, and the batteries’ terminal difference voltages still were above 220mV (less than 72% equalizing). Thus, the presented technique demonstrates superior performance, highlighting the significant contribution of variable duty cycle PWM in balancing batteries’ terminal voltages.