Study On Model Of LiFePO₄ Batteries Used By Electric Vehicles

In the first half of 2015, the cumulative production of new energy vehicles is 78,500, an increase of 3 times, at the same time, the global market for electric cars has also been quite popular. Lithium-ion batteries with its energy density, long cycle life, high-voltage, high-capacity, etc. are used for most electric vehicles. Li Fe PO4 batteries are used widely in our country.Batteries packs are used in electric vehicles and there will be inconsistencies in long time. That will lead to a battery overcharge or over-discharge problem, which affects the whole battery packs. Battery model is one important way to solve this problem, an efficient battery model can accurately estimate battery capacity and ensure battery management to implement reliable. In most battery models, the equivalent circuit battery model is convenience for electronic engineers to use. However, there are some simulation and estimate problems in these equivalent circuit battery models.A model is proposed in this paper, which is convenient in simulation and has small errors aiming at above problems.First, this paper studies batteries voltage, resistance, capacitance, and other basic features, from the internal structure and working principle of lithium iron phosphate departure, it introduces the definition of different battery voltages and measurement methods, describes the different definition of capacity and use way, and presents the different resistance and the relationship between the resistance and the remaining capacity of the battery. In the study of the basic characteristics of Li Fe PO4 battery for clear conditions, it has joined the ambient temperature, charge and discharge rate, charging frequency and other external factors, and summarizes the law of battery charge and discharge under various operating conditions through a lot of experiments. Secondly a novel equivalent circuit model is presented before it analyzes advantages and disadvantages of now has a variety of equivalent circuit models, the core part of the model is a variable capacitance system, which is composed of a linear capacitance and controlled voltage sources, that can show changes in battery capacity at different temperatures, this paper makes a detailed presentation on the process and the principle of the whole battery circuit equivalent circuit model. Thirdly, a lot of experiments are conducted for the new battery model, including experiments at different temperatures, different charge-discharge rate, charge different frequencies and other conditions. The paper lists the procedure followed each experiment, including the build experimental device, the software program design, and layout of the hardware circuit. Amount of experimental data is recorded through the automatic and perfect experimental platform, then it extracts model parameters from experimental data, and the data were fitted for different parameters to obtain the available data relationships and parameter values. Finally, the battery model is simulated in PSpice and the results show simulation waveforms and experimental waveforms error is very small, can prove a higher accuracy of the model and it is very practical.