Cell Voltage Equalization Circuit for an Electric Double-Layer Capacitor and a Lithium-Ion Battery Using an LC Series Circuit

Abstract

Series connections of energy-storage modules such as electric double-layer capacitors (EDLCs) or lithium-ion batteries result in a voltage imbalance because of the nonuniform properties of the individual modules. Although various types of cell voltage equalizers have been proposed, the required numbers of transformer windings or magnetic components in conventional equalizers proportionally increase with the number of series connections of the cells. Therefore, conventional equalizers tend to be expensive and bulky and to have less extendability to increase the number of series-connected cells. This paper describes a novel cell voltage equalization circuit using an LC series circuit. The proposed circuit reduces the number of magnetic components, which gives cost and size advantages. The fundamental operating principles and detailed design procedures of the key components are presented. Experimental cell voltage equalization tests using the proposed equalization circuit were performed using eight series-connected EDLC and lithium-ion-battery modules whose initial voltages were intentionally imbalanced. The experimental results demonstrated that the proposed circuit was able to equalize the voltage of each cell connected in series. In addition, the standard deviation of each cell voltage was reduced to a sufficiently low value below 10mV after a finite time.