During the driving process of electric vehicles, the on-board power battery continuously charges and discharges, accompanied by a large amount of heat generation. The temperature of the power battery has a significant impact on the electrochemical reaction rate, charging acceptance capacity, safety, cycle life, specific power, and specific energy.
At the same time, due to the different external heat dissipation conditions, thermal management methods, and individual spatial layout positions of the battery, The temperature differences between different parts of the monomers within the battery pack and between different monomers can seriously affect the consistency of battery pack performance.
Therefore, it is necessary to take enhanced heat transfer measures inside the battery pack to improve the temperature uniformity of its monomer surface and overall. High thermal conductivity graphite sheets with high thermal conductivity GTS, with its low density, low coefficient of thermal expansion, and relatively soft surface, can effectively reduce contact thermal resistance, making it an ideal new type of high thermal conductivity carbon material
This article takes square lithium-ion batteries used in electric vehicles as the research object. A control battery pack with and without GTS was made, and a thermal effect test bench was built for the battery pack. By analyzing the changes in temperature difference between the surface of the battery cells and different positions of the battery pack during constant current discharge, the effect of GTS on improving the thermal uniformity of lithium-ion battery packs was studied
1 Test
1.1 battery parameter
Size :180 mm x 100 mm x 32 mm
Weight : 1250 g
Voltage : 3.2 V
Capacity : 40 Ah
Internal resistance : ≤2Ω
1.2 Structural design of battery pack
To verify the effect of GTS on the thermal uniformity of lithium-ion battery monomers and battery packs, two types of battery packs with and without GTS were produced. Battery pack A consisted of six series connected lithium iron phosphate batteries, with temperature sensors attached to the surface of the monomers and PET insulation material on the periphery. Battery pack B served as the control experimental group for battery pack A. GTS was uniformly distributed on the side of each battery cell, totaling 7 pieces. The size of each GTS is 180 mm x 0.27 mm x 100 mm. To reduce the impact of airflow above the incubator on the temperature distribution inside the battery pack, a battery cover was installed above the battery pack for sealing during the experiment.
1.3 Test equipment
A test bench was built to test the thermal efficiency of battery packs during charging and discharging processes. Among them, the electronic load is a battery. The constant temperature chamber is a DGBELL high and low temperature humidity test chamber, with a temperature fluctuation of 0.3 ℃ {-40~100 ℃). The thermocouple temperature acquisition instrument: The temperature sensor is a K-type thermocouple, with an accuracy of ± 0.5 ℃.
1.4 Test process
Build a test bench and write a battery constant current discharge control program. Before the test, the total voltage of the six batteries was 19.62 V, and the temperature of the incubator was set to 18 ℃. When the temperature of the thermocouple measurement point approached 18 ℃ and stabilized, 1 C (I= – 40 A) constant current discharge was started. During this period, the sampling frequency of the thermocouple signal collector was 4 seconds. After the cut-off voltage of
15.6 V, 1 C constant current discharge was terminated, and the data was saved.
2 Test results and analysis
2.1 Thermal uniformity of battery cell surface
During the operation of lithium-ion batteries, the polar ears and center of the battery are representative temperature points. Therefore, two symmetrical measuring points near the positive and negative ear are selected to study the temperature difference in the horizontal direction of the battery cell. Select two measuring points on the vertical centerline of the battery cell surface, located at the center of the battery and near the top of the battery, to study the temperature difference in the vertical direction
When discharging at a rate of 1 C, analyzing the relationship between the absolute value of the temperature difference in the horizontal direction of the single cell surface of the battery and the discharge time, it can be seen that the temperature on the left and right sides of the horizontal direction of the single cell surface of battery pack A is uneven, with a temperature difference of about 0.18 ℃. However, the temperature difference in the horizontal direction of the single cell surface of battery pack B is only about 0.05 ℃, which is 70% less than that of battery pack A, indicating that GTS can play a role in the distribution of horizontal impact heat.
When discharging at a rate of 1 C, analyzing the scatter plot of the absolute temperature difference in the vertical direction on the surface of the battery cell, it can be seen that the temperature change in the vertical direction of the single surface of battery pack A is more severe, and the maximum temperature difference reaches 0.45 ℃. The temperature difference in the vertical direction of the single surface of battery pack B continues to maintain at around 0.05 ℃. After 2700 seconds, the temperature difference begins to increase with the accumulation of heat, with a maximum value of 0.26 ℃, but only 58% of that of battery pack A. This indicates that GTS can also play a role in average heat distribution in the vertical direction.
Through comparison, it can be seen that during 1 C rate discharge, the horizontal temperature difference on the surface of the battery cell is smaller than the vertical temperature difference, and the fluctuation of the vertical temperature difference is more severe, which is related to the structure and physical size of the battery. After arranging GTS on the surface of the battery cell, the temperature difference between the horizontal and vertical directions on the surface of the cell cell is similar and maintained within a small range, indicating that GTS can effectively improve the thermal uniformity of the cell surface.
2.2 Internal thermal uniformity of battery pack
The four sets of temperature difference values are smaller, indicating that when the battery is not working, GTS can improve the uniformity of the battery pack temperature, which is beneficial for extending the battery’s storage life. As the discharge deepens, the internal heat of the battery pack gradually accumulates, and the temperature difference value generally increases. However, compared to battery pack A, the temperature difference trend of battery pack B is more consistent, and the maximum value of the temperature difference is smaller (0.41 ℃), while battery pack A is 0.47 ℃.
The smaller the average temperature difference, the better the temperature uniformity inside the battery pack. From experimental data, it can be seen that before 2600 s, the average temperature difference in battery pack B was smaller and the increase was relatively gentle, indicating that GTS can improve the thermal uniformity of the entire battery pack.
The variance scatter plot shows that the larger the variance value, the greater the dispersion of the temperature measurement values of the five thermocouples at that time, which means the temperature uniformity of the battery pack is worse. The variance value of battery pack B remains around 0.18, with a maximum value of only 0.36. The maximum variance of battery pack A reaches 120, which is 3.33 times that of battery pack B, indicating that GTS can effectively improve the temperature uniformity inside the battery pack
3 Conclusion
By comparing and analyzing the surface temperature difference of battery packs with and without GTS under 1 C constant current discharge conditions, the effect of high thermal conductivity graphite sheets on improving the temperature uniformity of square batteries was studied. The following conclusions were drawn.
(1) During the discharge process, the horizontal temperature difference on the surface of the battery cell is smaller than the vertical temperature difference, which means that high thermal conductivity graphite sheets can effectively reduce this difference, balance the heat between different parts of the battery cell surface, and improve the thermal uniformity of the battery cell surface.
(2) As a heat transfer enhancement measure, high thermal conductivity graphite sheets can effectively improve the thermal uniformity inside the battery pack, and can be used as an auxiliary heat transfer enhancement measure for automotive battery pack thermal management systems such as the use of phase change materials for cooling plates, in order to balance the heat distribution of the battery pack and improve the performance of the thermal management system