Analysis and Experimental Evaluation on Switched-Capacitor Equalizer for Nickel–Zinc Battery Energy Storage System

Energy storage systems based on batteries (e.g., Lead-acid and Lithium-ion batteries) are widely explored in cooperation with renewable energies such as photovoltaic (PV) power generation to achieve a carbon-neutral sustainable society. Among various batteries, Nickel-Zinc (NiZn) battery could be a suitable candidate for storing renewable energies due to their non-flammable materials and lower cost as compared to other battery technologies like Lithium-ion. However, the voltage window of a NiZn battery is usually smaller (around 1.3V to 1.9V) which is incompatible with power systems running at 48 Vdc or even 400 Vdc bus. To address this, series-connected cells are usually used to provide a higher bus voltage and thus a larger system capacity, which may however suffer from voltage unbalance as the number of the series-connected cells increases.

This paper proposes a switched-capacitor voltage equalizer for 30 series-connected NiZn battery cells. Due to the low-voltage feature of the NiZn battery, it uses low-voltage semiconductor devices and capacitors to automatically transfer power among NiZn cells. Moreover, the switches can operate at a high switching frequency to reduce the overall size of the reactive components. Analyses of the switched-capacitor equalizer are given, followed by simulation and design guidelines on key components. Comparisons of three types of switched-capacitor circuits are presented to discuss the pros and cons. Control strategies for the power switches are also elaborated. Experimental results of the low-voltage, low-profile switched-capacitor equalizer for a battery string consisting of 30 series-connected NiZn battery cells confirm that the proposed switched-capacitor equalizer is able to maintain similar voltages among different cells while providing a cost-effective solution for battery energy storage systems.