An SOC-based Active Equalizer for Fast Charge Balance of Series-Connected Battery Pack

An equalizer constructed by compensated open-circuitvoltage (OCV)-based state-of-charge (SOC) estimation and adaptive balance charge shuttling is proposed in this paper for fast equalization of series-connected lithium-ion battery (LIB) strings. The power stage of the equalizer is composed of a bidirectional buck-boost converter to shuttle the balancing charge between two individual cells bi-directionally. The SOC-inspected equalization strategy can achieve charge transfer more accurately than that of the conventional voltage-measured process, due to the characteristic curve between the OCV versus SOC of the LIB is flatter among the SOC mid-range values. Hence, a compensated OCV-based SOC estimation method is addressed to lower the inaccurate SOC estimation during the equalization process. Furthermore, the switches' duty cycle of the converter is kept constant in the conventional method, thus the balancing current will decrease and slow down the balancing speed correspondingly with the decrease in SOC differences. To settle this problem, two equalization mechanisms, i.e. duty cycle adjustment (DCA) method and curve-fitting method (CFM), are devised. These presented techniques can modulate the duty cycle based on the SOC difference and keeps the shuttling charge nearly constant during the entire equalizing phase. Experimental results are provided to confirm feasibility and effectiveness. Comparing with fixed-duty-cycle (FDC) approach and DCA method, the proposed method owns the best balancing efficiency of 81.4%, and it can improve the balancing time by 27.1 % and 18.6 %, respectively.