| The slowdown in economic growth has triggered global industrial changes,the previously solidified automotive industry pattern has been constantly broken and reshaped.Automobile manufacturers seek development in the transformation.Electrification,intelligence,sharing,and interconnection have injected new vitality into the automotive industry.Power battery,as the core driving force of the new energy automobile industry,has become a key chip for major car companies to seize the future market.The battery is a temperature-sensitive element,especially for the cold winter in northeast China and the hot summer in the south.The optimal operating temperature range is narrow.In order to break the limitation of time,area and industrial development,the battery needs to run stably in a suitable temperature range.Battery temperature not only restricts battery performance,but also affects the life safety of drivers and passengers.Stable and effective thermal management scheme is a reliable guarantee of battery health and safety.Therefore,exploring the electrical and thermal behavior of the battery during operation,clarifying the battery charge and discharge principle and heat generation rules has become an important basis for battery heat dissipation control.Developing a reasonable cooling structure and formulating an accurate cooling strategy are the core of the thermal management system.This article analyzes and regularly summarizes the effects of battery internal resistance,discharge capacity,and ambient temperature,discharge rate,and depth of discharge on the 18650 cylindrical lithium batteries at medium to high temperatures.A heat generation model is established,and the temperature characteristics of the battery pack are optimized from the aspects of cooling structure and temperature control.The main research contents of this article are as follows:Firstly,disassembling the 18650 cylindrical lithium battery,studying the structural characteristics of the single battery,building a single battery charge and discharge test system,conducting the battery capacity,internal resistance,and temperature rise tests at different ambient temperatures,discharge depths,and discharge rates.The experimental results show that the discharge efficiency increases with the increase of the discharge temperature.The discharge temperature is greater than 45℃,the discharge efficiency is greater than 100%,and the battery is over-discharged.For high-temperature(30~60℃)discharge,the battery resistance value(ohmic internal resistance plus polarization internal resistance)decreases with the increase of discharge temperature,and the maximum resistance reduction amplitude is 15mW.The battery resistance value is obviously dependent on the discharge of depth(DOD).When DOD is lower than 0.8,the battery internal resistance is basically unchanged.When DOD is greater than 0.8,the polarization reaction is intensified and the battery internal resistance is significantly increased.The temperature rise of the surface of a single battery is high at the middle and low at both ends.As the discharge rate increases,the temperature difference increases.The battery heat generation rate function was fitted,and a uniform parameterelectric-cell coupling model was established.The maximum surface temperature of the battery was used as a criterion,and the maximum simulation error was 3.2%.Secondly,establishing the battery pack layout,design a long flat side-mounted liquidcooled compact cooling system,explore the contact area and contact form of the battery and the cooling pipe,and the influence of the cross-sectional shape of the cooling pipe on the temperature characteristics of the battery pack,subdivided into battery and cooling Six cooling structures,such as pipe contact angle,increasing contact angle amplitude along the coolant flow,cooling pipe height,height increase amplitude along the coolant flow pipe,number of cooling pipe layers,and cooling pipe width.Simulation results show that increasing the contact angle between the battery and the cooling pipe can reduce the maximum temperature of the battery pack and improve the temperature uniformity of the battery pack.The contact angle pipes are different in sensitivity to the cooling flow rate.Increasing the cooling flow rate,the large contact angle cooling system improves the temperature of the battery pack more significantly.Increasing the contact angle between the battery and the cooling pipe along the flow direction of the cooling liquid can make up for the insufficient cooling capacity caused by the temperature rise downstream of the cooling liquid,and has a significant effect on improving the temperature uniformity of the battery pack.The analysis of the cooling pipe height and the variable-height cooling system both optimize the temperature characteristics of the battery pack by improving the heat dissipation in the direction of the coolant flow.Increasing the width of the cooling pipe has a small effect on the temperature characteristics of the battery pack,but it can significantly reduce the pressure drop in the pipeline and reduce Small cooling system pump power consumption.The effect of layered cooling on the maximum temperature of the battery pack,the uniformity of the battery pack temperature,and the pressure drop of the battery pipeline are all small.Three kinds of evaluation indexes are selected: the maximum temperature of the battery pack,the difference in battery resistance temperature,and the pressure drop of the cooling pipe.The range analysis using orthogonal analysis is used to evaluate the primary and secondary effects of four typical cooling structure parameters on the three types of evaluation indexes.Thirdly,selecting the contact angle between the battery and the cooling pipe is 40 degree,the variable contact angle amplitude is 5 degree,the pipe height is 60 mm,the pipe width is 2mm,the variable height type is two,and the structural parameters of the single-layer cooling are combined.Analyze the cooling characteristics of the battery pack at different discharge rates,coolant flow rate,contact thermal resistance,coolant temperature,and coolant type on the battery pack under high temperature discharge.The analysis results show that: under the same cooling conditions,the larger the discharge rate of the battery pack,the longer the battery reaches the thermal equilibrium time,the temperature difference of the battery pack will rise significantly in the early stage of discharge,and the increase is affected by the temperature of the coolant.Increasing the cooling flow rate can significantly reduce the maximum temperature and temperature difference of the battery pack,but when the cooling flow rate is increased to a certain value,the improvement of the battery pack temperature characteristics is no longer obvious.The larger the difference between the battery coolant temperature and the initial temperature of the battery pack,the faster the maximum temperature of the battery pack decreases,but the difference in battery temperature will increase rapidly.Reducing the contact thermal resistance between the battery and the cooling pipe can reduce the maximum temperature of the battery pack,but it will increase the temperature difference of the battery pack.Coolants with different water and glycol mixing ratios have little effect on the battery pack temperature.At this time,the coolant configuration should be considered from the perspective of the boiling point of the coolant.According to the temperature environment interval where the battery is located,it is divided into medium temperature zone and high temperature zone.In the middle temperature zone(30℃),the battery pack temperature can be stabilized in a suitable temperature range with passive cooling at a constant cooling flow rate.The control method is simple and the cooling system consumes little power.High temperature zone(40~60℃),at an ambient temperature of 40℃,battery pack adopts "constant coolant temperature + constant cooling flow rate" cooling scheme,to adjust cooling flow rate according to discharge rate.At an ambient temperature of 50℃,battery pack uses "variable coolant "temperature + constant cooling flow rate" scheme,giving priority to considering the temperature uniformity of the battery pack,using constant temperature difference cooling.Ambient temperature of 60℃.The battery pack adopts "variable coolant temperature + variable cooling flow rate".The maximum temperature of the battery pack is greater than 50℃.The highest temperature of the battery pack is given priority.The highest temperature of the battery pack is lower than 50℃.The cooling flow rate establishes a relationship with the battery temperature difference,speeding up the cooling process.The three cooling schemes can be adjusted according to the battery pack maximum temperature and the importance of battery pack temperature uniformity. |
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