| With the continuous development of energy industry,the safety of Li-ion battery in the area of power and energy storage represented by new energy vehicles attracts much attention,especially the thermal safety problem.Any development route improving technologies with high specific energy and long cycle life is a wrong way at the expense of safety.In order to improve the thermal safety performance of battery modules and extending the cycle life of batteries,then promoting the development and application of the area of power and energy storage,in this thesis,several important steps of development process for the 46.8V/8.8Ah battery module was studied in depth.First,every 18650 single cell was tested for electrical performance under different conditions and its heat generation characteristic and thermal regularities of distribution were analyzed by experimental and numerical simulation before battery pack assembling,which was the basic research for the thermal behavior of battery modules.The strategies of pre-preheating at low temperature,semiconductor and heat pipe cooling for battery thermal management system(BTMS)were studied experimentally.And then,the composited phase change material(CPCM)with high cycle characteristic was designed and produced,which passed a series of tests,including heat transfer capability,mechanical strength,high-low temperature impact and material injection molding process.In the test part,the thermal behavior 46.8V/8.8Ah battery module was discussed by experimental and numerical simulation.In addition,for the purpose of improve the second cooling structure of the battery module with CPCM,a kind of liquid cooling plate was simulated and optimized.The main research contents and conclusions are summarized as follows:1.The electrical performance of 18650 single cell were studied and tested under different conditions.And for the discharge platform of high-rate battery under low environment,some pre-heating conditions were studied,with different temperature and discharge rate,in order to provide theoretical support for the battery products.The results showed that:(1)there was no leakage,broke,and deformation under the free-fall and drum tests.And after the low pressure cold/hot impact test,the charge retention kept 96%and return to 99.9%after 24 h.(2)the storage life and capacity were faded sharply under the battery storage and charge/discharge tests under high temperature condition,which would bring risk about to the thermal runaway.(3)the electrochemical decay under low temperature mainly manifest in the increase of internal resistance and the recession of capacity,and the heat generated by battery module could be self-heating energy to warm back the battery module,and the voltage fall-off curve appeared rebound phenomenon.When the temperature decreased more than-15℃,the battery showed bad ability of temperature balance and discharging capacity decreased sharply.(4)after pre-heating under low temperature,The results showed that dimethyl silicone and heat fins played important roles in the pre-heating technology for BTMS,improving the ability of temperature balance and discharging capacity.When the temperature reached-15℃ under 5C discharge condition,the SOC increase rate reached 81%,compared with the battery module with no pre-heating technology.2.The thermal performance of 18650 single cell were studied and tested under different conditions.Several methods of SOC assessment were studied and discussed in detail,and a theoretical and experimental study of battery heat generation assessment was operated and the thermal errors were analyzed by numerical simulation.And then,the strategies of semiconductor and heat pipe cooling for BTMS were designed and studied experimentally.The results showed that:(1)with the discharge rate became larger,the thermal errors of three methods of battery heat generation assessment became larger according to the theoretical and experimental study.And the mean heating power calculated by instant integration was the nearest result to the value of simulation,no more than 4%.(2)battery module could be rapidly cooled by semiconductor under the insulated and sealed condition,and the with the discharge rate larger,the cooling effect better.The temperature dropped 14%under 3C discharge.(3)fins and air-forced devices must be operated with the use of heat pipe cooling for BTMS.And with the discharge rate larger,the air speed on the condenser side greater,which made temperature gradient large so the battery module could be cooled so well.The temperature could keep 50℃ under 5C discharge condition.3.In order to solve the form-stable problem of paraffin(PA),metal foam was proposed as a comparison.And then,a novel CPCM using expanded graphite(EG)and epoxy(EP)was prepared,combined with PA,which had good heat transfer ability,mechanical strength and leakage resistance characteristics.Further experiments about the CPCM were operated and the results showed that:(1)due to its porous structure and excellent thermal conductivity,PA/CF(copper foam)showed great performance of BTMS.However,CF has poor adhesive force to form the leakage of PA while phase changing.And CF has characteristics of high production costs and hard to industrialize.(2)according to the test,the EG packing density and expansion ratio had the most stable condition when extrusion temperature reached 900 ℃,which had the least effects on volatile and ash content.(3)the composited PA/EG/EP materials were study in depth,including the thermal physical properties,microstructure characterization,mechanical strength characterization and high/low temperature impact test.If the percentage of EP was higher,the behavior of mechanical strength and leakage resistance were both better,but the value of conductivity and latent heat could decline.Based on the whole material test,PA/EG/EP#3 material(PA:50%、EG:3%、EP:47%)was adopt to be BTMS.(4)after the test of BTMS,PA/EG/EP#3 material showed great cooling and equalizing temperature ability for many cycles without any leakage,and the surface of material had no change before and after the test.4.According to the characterization of PA/EG/EP#3 material,a novel injection mold was designed and manufactured.Batteries were assembled to 46.8V/8.8Ah modules on account of the specific application,and tested under the insulated and sealed condition,studying the thermal behavior in depth.The results showed that:(1)with the discharge rate increase,the PCM module showed much better thermal behavior under the insulated and sealed condition.(2)there was little cooling effect for the center of Blank module with air-forced strategy,due to the small space among each cell in the module.As the air speed increased,the highest temperature difference became larger,even more than 50 ℃,which influenced the electrical/thermal performance of battery module significantly.On the contrary,although the effectiveness of thermal stability using PCM module was not obvious,it also appeared the positive effectiveness for BTMS.(3)there had enough time for the fans to agitate air,in order to promote the cooling effectiveness of Blank module,which was much more effective than the PCM module.The high temperature of PCM module rose slowly at the later stage of cycle test,but on the balance,the PCM module owned a better positive effectiveness for BTMS.(4)in order to solve the thermal inertia problem of PCM,a novel liquid secondly cooling plate for PCM was operate and analized by numerical simulation.Water and heat transfer could both keep the temperature of battery module under 50 ℃ under different discharge rate.In addition,using heat transfer oil as working medium,the max temperature difference was kept under 3.5 ℃,with the same or opposite direction of flow channel,especially kept 2.5℃ in opposite direction condition,which dropped 50%compared to the pure PCM thermal manegemrnt.The result reflected the great performance of temperature control.In summary,the thermal behavior of development process for the 46.8V/8.8Ah battery module was investigated,and the PA/EG/EP materials were designed,studied and developed according to the heat generation of batteries,on macroscopic and microscopic point of views.The in-depth study for BTMS was operated by experimental and numerical simulation in this thesis.The research methods and related results can provide the theoretical guidance and reference for the application of novel high cycle CPCM,high safety battery module and its industrialization. |
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