Design Of Lithium-ion Battery Equalization System Based On Model Predictive Control

Lithium-ion batteries are among the most promising batteries.However,due to manufacturing variance,state of charge(SOC)imbalance is common in battery string,which can be called battery inconsistency and inevitably results in overcharge or over-discharge,even irreversible damage to cells collected in series.Battery management system with equalizing techniques makes great sense to improve battery uniformity.Energy bus-based approaches have high efficiency and fast equalization speed,all equalizers can work simultaneously to meet fast energy dispatch through energy bus.Meanwhile,the efficiency of equalization can be greatly improved.However,it is hard to be controlled due to its high order,non-linear,multi-parameter coupling and large time delay.By comparing existing equalization techniques and control strategies,model predictive control(MPC)is adopted to provide equalization currents reference of each bidirectional Cuk equalizer,and lag Compensator(PI controller)is implemented to regulate the real-time equalization currents and bus voltage.The main work of this paper is as follows:(1)The operation principle of energy bus-based equalization circuit is analyzed,according to which the state space model with constraints that describe the transfer of energy in the bus-based equalization circuit is derived.(2)Based on the state space model,predicted states can be obtained.Cost function that describes the lost during equalization is set up.Then the prediction of the equalization current references can be solved by non-linear programming.Furthermore,equalization speed is considered into cost function,current references can be obtained by linear programming by approximating equalizer efficiency to a constant.(3)The parallel operation of DC-DC converter is analyzed to come up with the principle of equalization current control and bus voltage control.Small signal model of Cuk converter is built.Equalization current and bus voltage PI controller are derived respectively based on the model to achieve a rapid response without steady-state error.Mutual interference between two control loops is given out.(4)Equalization system is designed to validate proposed control strategy.Host computer is implemented by Matlab GUI and executes the algorithm of MPC.Slave computer is implemented by STM32 and regulate equalization currents and bus voltage in real time.Current control experiment,bus voltage control experiment,energy transferred experiment and 5 cells equalization experiment are carried out and the results show that proposed control strategy can equalize the series connected batteries effectively and rapidly.