A Switched-Coupling-Capacitor Equalizer for Series-Connected Battery Strings

Abstract

Due to the low cost, small size, and easy control, the switched-capacitor (SC) equalizer is promising among all types of active cell balancing methods. However, the balancing speed is generally slow and the balancing efficiency is seriously low when the SC equalizer is applied into a long battery string. Therefore, an automatic switched-coupling-capacitor equalizer (SCCE) is proposed, which can realize the any-cells-to-any-cells equalization for a battery string. Only two switches and one capacitor are required for each battery cell. All mosfets are controlled by one pair of complementary pulse width modulation signals, and energy can be automatically and directly delivered from any higher voltage cells to any lower voltage ones without the need of cell monitoring circuits, leading to a high balancing efficiency and speed independent of the cell number and the initial cell voltages. Contrary to the conventional equalizers using additional components for the equalization among modules, the proposed equalizer shares a single converter for the equalization among cells and modules, resulting in smaller size and lower cost. A prototype for four lithium battery cells is implemented, and an experimental comparison between the proposed SCCE and the conventional SC equalizer is presented. Experimental results show the proposed topology exhibits a substantially improved balancing performance, and the measured peak efficiency is 92.7%.