Research On Active Equalization Technology Of Electric Vehicle Battery String

The SOH(state of health),SOC(state of charge)and OCV(open circuit voltage)imbalance of batteries connected in series is caused by two major categories.They are the internal sources that consist of manufacturing variance in physical volume,impedance and self discharge and external sources such as thermal difference across the pack.This reduces the usable capacity of battery strings,shortens the lifetime,and even poses a safety hazard(e.g.,an explosion or fire,etc.).To eliminate the influence of the imbalance on the cycle life and the available capacity of battery packs,this paper proposes two different equalization topologies,the first one based on buck-boost converter and the other based on LC resonance circuit.To improve the balancing speed,especially for a large battery string,a novel inductor-based layered bidirectional equalizer(IBLBE)based on buck-boost converter is proposed.The equalizer is composed of two layers of balancing circuits connected in parallel.Each layer contains multiple balancing sub-circuits based on buck-boost converters.These balancing sub-circuits can equalize the corresponding cells simultaneously,and allow the dynamic adjustment of equalization path and equalization threshold.Analysis and simulation results demonstrate the IBLBE has a higher level balancing speed than other equalizers based on switched-capacitor or switched-inductor converters,and reduces the balancing time by 30% compared to existing inductor-based parallel architecture equalizers(PAEs).However,the IBLBE needs a large amount of MOSFETs and inductors and the switching loss decreases the equalization efficiency.To reduces the EMI in equalization system and improve the equalization efficiency,a novel direct cell to cell equalizer based on three pulse resonance circuits in proposed.The three pulse resonance equalizer(TPRE)decouples the efficiency and equalization current from voltage ratio,and it can either work in the three-pulse resonance mode or the traditional LC quasi-resonance mode.A joint equalization strategy based on the LC quasi-resonance and the three-pulse resonance is proposed to achieve higher level efficiency and current.Simulation and experimental results are presented to validate the analysis and effectiveness of the two proposed topologies.