Research On Six-cell Direct Equalizer For Series Power Batteries

When the power lithium-ion battery devices is used in the fields of satellite,electric vehicles (EVs), uninterruptible power supply (UPS) and so on as energystorage device, in order to meet the requirement of voltage levels, they are usuallyused in series to achieve the appropriate voltage level. Because of inconsistency ofthe parameters of each cell, the voltage among the monomers becomes unbalancedwhich decreases the effective capacity and the cycle life of the battery pack.Therefore, the research on equalization technique of lithium-ion cells is essential toimprove the performance of batteries in series.The directed graph and weighted matrix are introduced to analyze the existingequilibrium topological structure, and a variety of typical structure of the directedgraph and weighted matrix are built, and the effect of the structures on energytransfer path is emphasized, on this basic, the graph theory model of the idealequilibrium topology is deduced, the differences between achieved equilibriumtopologies and the ideal equilibrium topology are analyzed, the analysis about thedirected graph and weighted matrix of equilibrium topologies provided theoreticalguidance to seek new topology.Based on the analysis of the existing topologies, a equilibrium topology basedon multi-winding transformer is proposed, this equilibrium topology can realizeenergy transfer directly among any monomers in no more than two steps. In thispaper, the detailed principle of the topology for six monomers is given, and theworking process consists of three modes depengding on difljferent cases ofimbalance, namely flyback mode, inductor-less forward mode and two-step flybackmode; based on the theoretical analysis of the six-cell direct equilibrium topology,the parameters of the main circuit are designed; the effect caused by the leakageinductance on equilibrium efficiency is researched which provides theoretical basisto further improve the efficiency.According to the theoretical analysis, the equalizer control circuit is designed,and a battery voltage acquisition module is built through special detection chipLTC6802-1which can improve sampling precision and simplify the structure of thesampling circuit; a current sampling circuit and a logic judgement circuit aredesigned to solve the problem of current reversal caused by bidirectional currentflow in bi-direction switch. Based on a variety of imbalances, the equilibriumstrategy and the operation rules of all two-side switch in different inequalizationconditions are setted to ensure the equalizer reliable and orderly.A equalizer of six monomers is built for50Ah power lithium-ion batteries, and the equalization effect of three switching modes and the overall equalizer is testedand analyzed in different inequalization conditions. Experimental results show thatsix-cell equalizer can equalize power lithium-ion battery pack accurately andreliably, and realize energy transfer among any monomer.