Research On Battery Equalization Using Flying Capacitor With Buck-Boost Converter

Being subjected to the existing battery technology level, Electric Vehicles have to use lots of cells with large capacity connected in series to obtain the required voltage. But, the performance of the monomer battery is unconsistency, which leads to some batteries over-charge and over-discharge. And it consequently affects the service life of the battery, which limits the development and industrialization of Electric Vehicles. At present, the researches on the equilibrium controlling of the series-connected battery string charging and discharging for Electronic Vehicles, which the researchers at home and abroad are making, are focused on the methods of equilibrium of the type of energy transfer, but the circuit structure and control method is relatively complex and the cost is generally higher, which make it difficult to achieve. In addition, most of the equilibrium methods only remain in theoretical research, and without adequate consideration of the battery charge-discharge characteristics. Thus, these methods lack the corresponding balance control strategy, which consequently makes it difficult to popularize and apply.Based on thorough analysis of the research on the traction battery balance technology at home and abroad, this paper puts forward the method of the battery equalization using flying capacitor with buck-boost converter. This method can realize batteries equalization under dynamic and static state, and the balancing current is stable and controllable. It eliminates the pulsing shock, which the balancing current brings about, impacting on the battery string. Based on the method, a battery equilibrium system has been developed, which can realize balanced function for a series-connected battery string including 12 lithium cells. The system consists of a main circuit and equilibrium controller.The equilibrium controller with MCU as the control core, including voltage detection, CAN bus communication module, switch array and the corresponding driving and controlling circuit. Through lots of experiments, the characteristic curve and parameters of the equalizer's equilibrium efficiency, equilibrium speed, equilibrium accuracy etc. Through the analysis and comparis on the above mentioned experimental data, the result proves that the equilibrium solution can make these cells's voltage quickly reaching a consistance under the static state or in the process of charging or discharging, and has a high equilibrium precision and equilibrium efficiency.Finally, in allusion to the shortcomings of the balancing method, this paper also puts forward the improved schemes: battery pack equilibrium method basing on bidirectional Buck-Boost converter and balancing bus structure. It gives the structure and working principle of the equilibrium method, and provides the design principle of the control algorithm. Through designed the simulation comparison experiment, the results prove that this method is feasible.