Autonomous Battery Equalization Module Using Capacitively Coupled Input-Parallel Output-Series Structure

An autonomous battery equalization module, which utilizes an energy circulation technique to equalize the voltage across individual battery cells in a series-connected batteries, is presented. Its structure consists of two power conversion stages. The first stage converts the battery string voltage into a high-frequency ac voltage to form a capacitively coupled ac link. The second one contains multiple diode-based rectifiers with their inputs connected to the ac link via coupling capacitors and their outputs connected in series to individual battery cells. Based on the property that the coupling capacitor voltage equals the battery cell voltage, and the rectifier input voltage are shared across the ac link, the magnitude of the current delivered by the second-stage converter is determined by the difference between the coupling capacitor voltage and the ac-link voltage. As the current taken from the first stage is common to all battery cells, charging or discharging of individual battery cells is autonomously adjusted by the output current of the second stage. Apart from equalizing the voltage of the battery cells within a module, the proposed architecture also allows multiple modules connected via the ac link. Individual battery cell can be charged by or discharged to the rest of battery cells in the same module or other modules. Two four-cell modules for equalizing eight 3.6 V, 18 650 batteries have been built and evaluated.