Research On Isolated Equalizer For Lithium-ion Battery String

With the popularization of renewable energy power generation and electric vehicles,the efficient storage of electricity becomes more and more important.Lithium batteries have been widely used in energy storage system because of their high non-memory effect,energy density and long cycle life.Overcharge and over-discharge of batteries may result in shorter lifetime and higher potential of security risk when they are connected in series to meet load voltage requirements.Therefore,equalizers are required for a series-connected battery string to eliminate individual difference among batteries in the battery pack.Compared with the non-isolated equalizer,the isolated equalizer has the advantages of simple structure,short equalization path and high efficiency.Hence,the isolated equalizer is studied in this paper.Firstly,a modularized equalizer based on push-pull converter is proposed in this paper.The proposed equalizer can transfer energy from high voltage batteries to low voltage batteries directly to short the transmission path by utilizing synchronous control.Besides,it eliminates the magnetic coupling effects caused by multi-winding transformer and increases the circuit's expansibility.In this paper,operating principles of n equalization cells are declared by analyzing operating principles of two equalization cells.What's more,characteristics of the proposed circuit are illustrated while components of the equalizer are non-ideal.To improve the equalization speed and efficiency,the improvement plan is also put forward.Besides,six types of modular battery equalizers based on DC-AC circuit are concluded and compared.Finally,the theoretical analysis results are verified by experimental results.Although the modularized equalizer based on push-pull converter has many advantages,a large number of switches and transformers are employed with the increase of the number of batteries in string.Therefore,a voltage equalizer using half bridge inverter and full wave rectifier for series-connected battery cells is proposed.The proposed voltage equalizer not only realizes battery equalization but also features with low cost,small size and simple control method.The operation principle and characteristics of the proposed battery equalizer are analyzed,and its equivalent circuit model is derived to illustrate the principle of equalization.Furthermore,the influence of the parameters on the equalization circuit is studied.Finally,a prototype is built and experimental results are provided to verify the theoretical analysis results.