Research On Traction Battery Pack Modeling And Its Active Banlance Method

Lithium battery is widely used as power supply of electric vehicle(EV),because it has lots of advantage like high energy ratio,well security,high thermal stability,stable voltage,low self-discharging rate,fast dynamic response and long circling life.In order to meet the demand of EVs' power,hundreds of cells are connected in series.Because of the limit of production process,cells' parameters are different from each other in series and differences will be more apparent during charging and discharging cycles,which will lead loss of capacity and life.Therefore,balancing method of battery pack is studied to solve the problem.Lithium battery's characteristic is needed to acquire for better research of balancing method.This paper carries out a series of tests of lithium battery,aiming at studying a 50 Ah LiFeO4 cell's external characteristic.And then,with the result of tests,the PNGV equivalent circuit model is analyzed and parameters of PNGV model are identified.A PNGV model is made to simulate the 50 Ah LiFeO4 cell and its accuracy is verified,the error between simulation and test during the voltage platform is less than 40 mV.Based on cell's characteristic,this paper analyzes how differences effect battery pack.Topology of switching matrix with concentrated inductance is adopted in this paper to realize the balance.Modes of this balancing topology are studied and five strategies are proposed.Two strategies which can realize energy flowing from higher capacity cell to pack and from higher capacity cell to lower capacity cell are concerned.Equivalent buck-boost circuit is researched.Afterwards,main topology and control circuit are designed.With the design of balance system,aimed at accurate control of balancing current,peak current control is proposed and theory of slope compensation is studied.Logic of switching matrix and the software process are designed to meet two balancing strategies abovementioned.Finally,balancing experiments are carried out to verify the two balancing strategies,which prove that the balancing system can balance battery pack and the standard deviation of voltage between the cells is decrease from 16 mV to about 5mV.The strategy of energy from higher capacity cell to lower capacity cell have a faster speed,which is an hour earlier than cell to pack to reach stability;and the average voltage of cells is almost unchange on the cell to cell balance strategy during the balance,but decrease 6mV during the cell to pack balance,so cell to cell strategy has a better effect.