Research On Dynamic Equalization For Series Connected LiFePO₄ Batteries

The role of battery in the field of production and life has been enhanced by the development of new energy technology, thus playing an increasing important and irreplaceable function. Lithium-ion battery presented by the LiFePO4 has features of security, light weight, long service life, green etc and has become main power battery of next generation. To meet the voltage and power demand, the case that battery stacks with series-connected cells are used is general. The relationship between single battery and battery stack like buckets effect. Although battery systems are affected by many factors, the most important one is the cells unbalancing. Aiming at high-performance and power LiFePO4 battery, study and design of efficient, modular and rapid battery balancing system has great application value.Firstly, this paper introduced the classification of Lithium-ion battery, made a comparison of different anode materials of Lithium-ion battery and deeply analyzed the structure, working principle, discharge characteristic, energy-storage characteristic and open characteristic of LiFePO4 battery. The necessity of battery equalization technique has been put forward on the basis of analysis of cause of the inconsistent problem of batteries and we can also find the advantages and disadvantages of existing active and passive balancing topology in this paper.Then a rapid non-dissipative battery equalization circuit based on the flyback DC/DC converter is proposed in this paper. The designed circuit draws the charge from the high voltage cell and recovers it to the whole battery pack. With the polycyclic layered structure and the peak current mode control strategy, the proposed circuit has a fast balancing speed. On the one hand, compared with the traditional structure and control mode, the correctness of this thought is verified by the simulation based on the PSIM software. On the other hand, the simulation analysis has been made on the dynamic equalization and efficiency of equalization for the designed balancing circuit.Finally, the software and hardware implementation of design of equalization has also been finished, including voltage and current detecting circuit, snubber and filter circuit and MOSFET drive circuit. The software design of this balancing circuit based on the STM32F103 controller let us clear about working process and complete function of whole system. The actual test waveforms verified the effectiveness of the design.