Research On Active Thermal Management And Energy Management System To Slow Down Battery Attenuaion For Hybrid Electric Vehicle

With the prominent role in energy conservation and emission reduction,hybrid power technology is one of the crucial technical routes for Chinese automobile industry to achieve the“carbon peaking and carbon neutrality”targets.Apart from continuous improvement of the fuel saving rate of hybrid electric vehicles,it is also hoped that the power battery has the same life as the vehicles.Based on a power-split hybrid vehicle with multiple operating modes,this study explored the active control strategies of battery thermal management and vehicle energy management by analyzing factors that affect battery cycle life,in order to reduce vehicle fuel consumption and delay battery attenuation.（1）Based on the battery life research,the objectives of battery thermal management and vehicle energy management were put forward.According to the working characteristics of hybrid electric vehicles,the characteristic parameters that affected battery cycle life including battery temperature,state of charge（SOC）,depth of discharge（DOD）and effective current were extracted to formulate an orthogonal matrix cycle life experimental scheme.In addition,the influences of various characteristic parameters on the battery cycle life and the electrochemical mechanism of life attenuation were obtained through the regular battery performance tests during the experiment and the physicochemical analyses afterwards.Furthermore,the thermal management objectives of the battery system and the active control requirements of energy management for SOC,state of power（SOP）,state of health（SOH）were proposed.（2）A kind of battery thermal management system that considered the maximum temperature and temperature difference was designed based on the liquid cooling scheme.For the high-rate cylindrical Ni-MH battery,the heat generation process and heat transfer process of Ni-MH battery were analyzed comprehensively.Besides,a liquid cooling structure with parallel serpentine flat tube branches was designed,and the mathematical relationship between the head loss of the cooling circuit and the coolant flow was established.Furthermore,the temperature rise and temperature difference distribution of the battery at different positions were calculated based on the heat generation model of the battery and the one-dimensional heat transfer model from the battery interface to the coolant.In addition,the effects of cross-section ratio（δ）on flow deviation（β）and temperature deviation(T_diff),and the effects of inner support wall（N）on the maximum temperature(T_max)and pipeline pressure drop（Δp）were studied comprehensively.Then,a liquid cooling thermal management system,which considered cooling performance as well as engineering realization,and greatly improved the cooling capacity compared with the first generation of air-cooled system,was proposed.On this basis,the temperature field distribution of the whole battery pack under different boundary conditions was simulated and studied.As verified by the experiments,the results provided the basic data for the active thermal control strategy of battery management system（BMS）.（3）The control strategy of active battery thermal management system was developed,and a real-time estimation and monitoring scheme for battery status was proposed.The online prediction model of battery internal and surface temperature was designed based on Kirchhoff equivalent circuit thermal model.Then,an active control strategy of thermal management,which considered the maximum temperature and temperature differences between batteries,was formulated.In this way,the battery temperature could always stay within the optimal range.Based on the second-order RC equivalent circuit model,a fused SOC estimation method that combined SOCi and SOCv was designed,which not only avoided the computational complexity of algorithms such as Kalman filter and Neural network,but also ensured the reasonable accuracy of the estimation.Thus,the algorithms could be better utilized in embedded engineering application scenarios.Subsequently,based on the power look-up table method and the real-time calculation method of terminal voltage,the impact of the battery’s historical average power and power limit under special conditions were investigated,and the available power（SOP）algorithm of battery under multiple constraints was designed accordingly.Apart from that,a semi-empirical life model was established based on the accelerated aging test data of battery life under standard cycle conditions.By using cumulative ampere hours,the random working conditions were transformed equivalently into the number of standard cycles.At the same time,the moving average equivalents were applied to temperature,discharge rate and DOD to design a battery SOH estimation scheme for random real-time working conditions.Four kinds of functional safety objectives,including the prevention of insulation leakages,vehicle collisions,high voltage short circuits and high temperature fire risks were realized through redundant strategies in different paths.Meanwhile,the status of SOC,SOP and SOH were also monitored.Thus,the accuracy of the status BMS reported and the automaticity of independent processing could be effectively guaranteed,and the functional safety requirements of ISO26262 were also fulfilled.（4）The adaptive energy management strategy for hybrid vehicles was developed based on the real-time feedback states of the battery.The dynamic torque control of four-axis power split hybrid system was studied,and a rule-based（RB）working mode switching strategy was designed.Based on the precise estimation of battery state,an adaptive energy active management strategy of A-ECMS（Adaptive-Equivalent Consumption Minimization Strategy）was designed for accurate battery power control.While considering both battery life and fuel economy,it realized the objectives of engine operating point optimization and SOC balance control.In this strategy,the system mechanical power and electrical power balance equation were used to accurately calculate the power consumption of high-voltage components,inertia of rotating parts and transmission resistance,and the driving power of transmission.Furthermore,the calculations were adapted to the engine power,and the power distribution and engine operating point were optimized to reach the goal of minimum fuel consumption.It is noteworthy that the ECMS algorithm could accommodate real-time embedded computing without engineering calibrations or adjustments following off-line computing.According to the results,compared with the traditional RB+ECMS strategy,the A-ECMS energy management strategy based on power priority and SOC feedback could reduce the DOD fluctuation range by 44.3%.Moreover,the equivalent factor（S）was close to the optimal solution of the system,achieving the preset optimal control object.As verified by the co-simulation and real vehicle tests,the system control strategy enhanced fuel efficiency and realized accurate control of battery power.In addition,the simulation curves of the main components were basically consistent with the test curves,the distribution of operating points were almost the same,and the deviation of the key indicators was within 5%.Beyond that,the fuel consumption of the hybrid electric vehicle amounted to 5.41L/100km,which was 31%more fuel-efficient than the prototype,and could keep the SOC within 9.8%due to DOD fluctuation range.The hybrid battery passed the 230,000 kilometer durability test without any maintenance,with a cumulative charge and discharge capacity of 86,150Ah.Meanwhile,its service life was more than twice that of 80%DOD standard cycle conditions.In the wake of this endurance test,the maximum deviation of OCV between battery modules was 0.129V,the capacity decreased between 2～15%,and the internal resistance increased by 0.97mΩ.The battery performance was consistent,and relevant index parameters remained in the range of reasonable values.The result verifies that the system control strategy protected the battery well,and no overcharge or overdischarge situation appeared during endurance test.The battery thermal management system enabled the battery to work within the optimum temperature range,which maintained battery health and slowed down battery attenuation.In short,the study is of instructive significance for the life attenuation research,cooling system design,battery system management of other types of batteries,as well as the integration and control of hybrid systems with different structures.