Research On The Equalization Control For Liquid Metal Batteries

Liquid metal battery(LMB)is an emerging battery energy storage technology.Owing to the merits of low price and long cycle life,LMB has shown high potential in the field of gridlevel energy storage.In order to further promote the practical development of LMB,it is vital to research reliable and efficient equalization system of LMBs according to its energy storage characteristics.Equalization system aims to solve inconsistency problem of in-pack cells resulting from discrepancy in work environment and manufacturing technique,thereby improving the security of system operation.This paper has carried out a series of works around the equalization control of LMBs.The specific research contents can be summarized as follows:1.Based on 23Ah-level experimental Li||Sb-Sn LMB,the characteristics of LMB are analyzed and then a simulation model based on Thevenin equivalent circuit is established for LMB.Experimental results using actual current test data show that simulation model has the capability of simulating actual output characteristic of battery.2.Considering the characteristic of low voltage and large current of LMB,a two-stage hybrid equalization circuit is proposed for LMBs.Hierarchical control is adopted to shorten balancing path,thus quickening balancing speed.In order to decrease power loss of circuits,and improving balancing efficiency,MOS devices whose conduction loss is low replace diodes to follow current.3.The state of charge(So C)serving as equalization variable,equalization strategy based on fuzzy logic controller(FLC)was proposed in this paper.Then the equalization system model based on the proposed equalization circuit and balancing strategy for 12 series-connected LMBs is further built in Matlab/Simulink.The simulation experiments under static and dynamic working conditions are performed to verify the feasibility and availability of proposed balancing system.4.Considering that voltage of LMB in the beginning and end of charging/discharging changes dramatically,the balancing strategy based on hybrid equalization variable is proposed,where So C and voltage are served as equalization variable in the voltage platform and in the beginning/end of charging/discharging,respectively.The performance of proposed strategy and the strategy based on signal equalization variable is compared by experiments under constant current charging profile.The simulation experiments under dynamic working conditions have further verified the reliability of proposed strategy.