| With the excellent performance of high specific energy,long cycle life,good safety,high output power,and wide charge-discharge ratio,lithium-ion batteries are widely used in consumer electronics,new energy vehicles and industrial/energy storage power stations.In the process of charging and discharging,lithium-ion batteries will generate a lot of heat,which will lead to changes in the overall temperature,thus affecting the capacity and performance of the batteries.In some extreme cases,it will also lead to thermal runaway,resulting in major safety accidents.Therefore,a batteries thermal management system is needed to ensure the reasonable temperature range and temperature uniformity of the batteries.Based on the electrochemical-thermal coupling model,the thermal behavior of lithium-ion battery and battery pack is analyzed in this paper,which provides a theoretical basis for the design of battery and thermal management system,and provides technical support for the further development of lithium-ion battery in the intelligent process.The thermal behavior of 21700 lithium-ion batteries at different charge-discharge rates was investigated by experiment and simulation methods.The temperature,voltage and current changes during charging and discharging at 1 C,1.5 C and 2 C rates were obtained as validation data of simulation methods.On this basis,based on the temperature distribution of battery charging process,the mechanism of heat generation was explored,and the temperature and heat generation of charging process and discharging process at the same rate were compared.The results show that the electrochemical-thermal coupling model can well simulate the thermal behavior of batteries during charging and discharging.In the process of charging lithium-ion batteries,the average temperature first rises and then decreases,the total heat production during charging process is greatly affected by polarization heat generation,and the total heat production during constant current charging process is greater than that during constant voltage charging process,the ratio is about 9:1.For the constant current charging process,with the increase of charging rate,the proportion of polarization heat to polarization heat increases from 40.2%to 50.5%,and the relative proportion of reaction heat to ohmic heat remains basically unchanged.At a lower rate,the maximum temperature of charging process is greater than that of discharging process:At 1 C rate,the average maximum temperature of charging process is 303.36 K,and the discharge process is 301.78 K;at 1.5 C rate,the average maximum temperature of charging process is 308.10 K,and the discharge process is 307.15 K.At high rate,the maximum temperature of the discharge process is higher than that of the charging process:the average maximum temperature of the charging process is 312.16 K,and the discharge process is 313.83 K.Compared with the discharge process,the charging process has higher average temperature peak value and larger heat production,which is mainly affected by the constant current charging process,it has the characteristics of short temperature rise time,fast rise speed and large heat production.Based on the validation of the single cell battery model,a 3×3 type 21700 battery pack was constructed to investigate the thermal behavior of the battery pack during charging and discharging cycles and the influence of related factors.The results show that the maximum temperature and temperature difference of the battery pack increase further than that of the single battery during charging,and the charging process of the battery pack has a cumulative effect of temperature.During the charging and discharging cycle,the cumulative effect of battery pack temperature is further highlighted,the maximum temperature peak is continuously increased,and the temperature difference of single battery in the pack is also increasing.At 2 C rate,the maximum temperature of the battery pack rises by 60.4 K compared with the ambient temperature,which is close to the thermal runaway temperature of lithium-ion batteries.Five convective heat transfer coefficients of 5,10,20,50 and 100 W/(m2·K)were set to investigate the effect of heat transfer conditions.The results show that increasing the heat transfer coefficient can significantly reduce the maximum temperature of the battery pack and the cell in the cell group,but will further increase the temperature difference of all the cell in the cell group.According to the characteristics of thermal behavior during charging and discharging cycles of batteries,a thermal management method for liquid-cooled lithium-ion batteries is proposed,and the correlative effects of liquid-cooled mode,coolant flow rate and inlet temperature are analyzed.The results show that the maximum temperature drop ratios of liquid-cooled batteries are 3.41%,5.48%and 7.21%respectively at three multiples.The maximum temperature of batteries in each cycle is basically the same,and the temperature difference of each battery can be controlled within 2 K.Under passive liquid cooling mode,the heat transfer capacity of stationary refrigerant decreases with the increase of temperature,and the profitability of heat management is poor.For active liquid cooling mode,the influence of coolant flow rate on the thermal behavior of battery pack is small,while the lowering of coolant inlet temperature can significantly reduce the maximum temperature of battery pack.Coolant temperature decreases by 10 K,the maximum temperature of battery pack decreases by 7 K on average,and the temperature difference decreases by 0.5 K.In summary,the thermal behavior of lithium-ion batteries and batteries in charge-discharge cycles with different rates and cooling strategies is explored,and the influencing factors of thermal behavior are discussed,which can provide some references for the design of single batteries and the design of thermal management methods of batteries. |
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