| To cope with environmental and energy crisis,electric vehicles are meant to replace traditional internal combustion engine vehicles.Driven by policies and markets,China’s electric vehicles have shown explosive growth in recent years.As the power source of electric vehicles,power batteries directly affect the performance and life of electric vehicles.Among them,lithium-ion power battery has become the first choice of electric vehicle battery because of its high specific energy,low self-discharge rate,long-cycle life.However,the capacity,charging speed and life of lithium-ion power battery are still the key factors that restrict the development of electric vehicles,and the relevant technical level needs to be improved urgently.This paper analyzes the existing charging methods and conducts experiments for the controversial charging method.Ultimately,the multi segment constant current charging method is selected as the research object and the genetic algorithm is applied to optimize the segmented current to reduce the energy consumption.To realize fast charging without lithium plating,the segment pattern is improved,and then Taguchi method is used to optimize the charging current。This paper is organized as follows:1、The existing lithium ion charging technology is analyzed,the constant current constant voltage charging method,multi-stage constant current charging method and pulse charging method are studied,theoretical analysis and experimental results indicate:1)The constant voltage charging is the fastest charging method when the battery voltage reaches the cut-off voltage and multi-stage constant current charging can only shorten the charging time,while charging speed is not really improved.2)Compared with the average constant current charging method,pulse charging cannot increase the charging speed but can make the battery reach its cut-off voltage earlier,shorten the charging time,reduce the charge power,and raise the temperature of the battery.3)The "minimum impedance frequency" is an optimization of the AC component in the charging current,but the AC component itself has no benefits for charging.2、The second-order RC equivalent circuit model of lithium battery is established,and the energy consumption model is built based on it.The model parameters are identified by HPPC experiment,and the validity of the model is verified in MATLAB platform.Considering that the battery internal resistance is changing with the SOC,this paper takes the battery energy consumption as the cost function.Based on the energy consumption model,genetic algorithm is adopted to optimize charging process.The simulation results show that optimization performance of the multi-segment constant current charging method on the energy consumption will be better when the changes of the battery internal resistance are greater and the divisions of the charging segment are finer.For NCM lithium ion batteries,when taking 5%SOC as one segment,the energy consumption can be reduced by 2.2%.The effectiveness of the multi-segment constant current charging method is verified by experimental results.3、Some theories suggest that fast charging without lithium plating can be realized by maintaining the lithium concentration at saturated concentration on the surface of the battery negative electrode.According to the theory,the segmentation method of multi-segment charging is improved in this paper.Considering the battery used,the maximum current 2C charging method is adopted to make lithium ion tending to a saturated concentration rapidly in the 0-40%SOC phase,and after then each of the 10%phase,the charging current decreases to maintain saturation concentration until 80%SOC is reached.Taguchi method is used to design experiments.The cost function is adopted as the weighting sum of charging time,power consumption and battery temperature.The optimal current values of the last four stages are obtained by analyzing the experimental data.Finally,the experiments are conducted. |
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