2.1 Test preparation
Cylindrical and square batteries are commonly used as power cell. The research object of this paper is a square cell, which is mainly composed of nickel cobalt manganese positive electrode and graphite negative electrode chemical material system. Single battery size: X Y Z=W x L x H=27 mm x 148 mm x 94 mm, rated voltage: 3.7 V, rated capacity: 37 Ah. The single battery is arranged in the module in the form of superposition along the width direction (X direction) of the single battery. In this paper, the definition of module crush direction is consistent with that of monomer, and the battery monomer and module are shown in Figure 3. The displacement, load, voltage, temperature data and video data of the crush process were collected by extruding the cell and module with the indenter, and the test results were analyzed.
2.2 Results of cell crush test
Carry out the XYZ crush working condition test on the electric core in these three directions, and obtain the stiffness comparison of the three directions as shown in Figure 4.
The X direction stiffness of the electric core is the largest, and it can withstand greater crush force under the same deformation. The Y and Z directions are relatively weak. Shenxin is generally arranged in series in the X direction in the module. When the vehicle is in side impact, the electric core is squeezed and deformed mainly in the X and Y directions.
In order to find the critical point of shell damage and cracking under the crush condition of the electric core, the crush test is carried out in the Y direction of the electric core. Each group of tests was extruded for three times, and the crush speed was 0.2 mm/s.
In the first group of tests, the monomer is continuously loaded until the battery loses control of heat, and the state of charge (SOC) of the battery is 100%;
The second group of tests is to find out the critical point of battery shell cracking. For the convenience of observation, the battery is discharged and loaded 3 mm each time in sections at the same time, and each section is kept for 200 seconds;
The third group of tests was continuously loaded to the critical value of shell cracking and then left standing to analyze whether there was a risk of thermal runaway. The first group of test cores is continuously loaded until crush failure occurs, as shown in Figure 5.
It can be seen from the comparative analysis of tests that there is uncertainty in the failure mode of electric core crush.
In the crush test, the failure mode of the electric core is that the shell is extruded and damaged, without fire and explosion; The shell is extruded without damage, but smoke or fire explosion occurs; The shell is crushed and damaged, causing fire and explosion.
Figures 6 and 7 show the crush deformation of the electric core shell after the second and third group tests. It can be seen from Figure 6 that the negative side of the electric core cracked and the positive side did not;
When the first crush displacement is 12 mm, the negative side cracks, but the positive side does not. Through the analysis of the second group of tests, it is preliminarily determined that the shell is damaged and cracked under the crush condition of the electric core
The critical value is 12 mm. It can be seen from Figure 7 that when the measured crush displacement was 12 mm, the three test cores did not have shell fracture and no fire or explosion. After the test, the measured cores behaved normally after standing for 24 hours.
This group of tests verified that the critical value of crush failure of this type of electric core can be determined 12 mm. Through the analysis of three groups of tests, it is found that the critical point of the failure mode of damage and cracking of the electric core shell is 12 mm, which is lower than the limit value, so the shell cracking risk is small, and fire and explosion will not occur;
If the limit value is exceeded, there is a certain uncertainty whether the core shell is damaged or cracked, which still has a high safety risk; The uncertainty of the failure of the electric core caused by fire and explosion is large, and the failure modes are inconsistent. Therefore, it can be determined that the damage tolerance of the cell is 12 mm, and the crush working condition is relatively stable in the test. However, the conditions under which the battery is extruded in actual application are different. Considering that a certain safety margin is reserved, the damage tolerance of this type of cell is determined to be 10 mm.
2.3 Results of module crush test
The module shall be subject to crush test in X Y direction respectively. The loading speed is 0.2 mm/s. In the test, the module is in full power state, that is, the module SOC is 100%. The X and Y directions correspond to the vehicle side impact crush performance. Repeat the crush test for three times in two crush directions to ensure the effectiveness of the test.
Figure 8 shows the crush state of the module in different directions.
Figure 9 shows the X and Y direction crush process of the module. By comparison, the process of thermal runaway caused by crush in different directions of the module is: the battery module deforms at the initial stage, and smoke or sparks appear when the internal damage of the battery increases with the crush load, further causing fire and explosion.
Figure 10 shows the temperature and voltage change curves of the module in the X and Y directions during crush. Take M2 (X crush) and M4 (Y crush) modules as examples for analysis.
In Figure 10 x direction , during 0~400s, the module is gradually deformed due to crush, and the module temperature and voltage remain stable; When the load reaches 400s, the voltage starts to drop to 0 V, and the temperature increases from 26 ℃ to 156 ℃, at this time, the battery module starts to smoke; With the further increase of crush force, the temperature rises to 500 ℃, and then the battery module fires and explodes.
In Figure 10 y direction, the voltage and temperature of the module are relatively stable within 0~300s. When the temperature rises from 300s to 550 ℃, the module fires and explodes, and the voltage drops to 0V. Through the comparison and analysis of temperature and voltage curves, it is also found that the thermal runaway is more likely to occur in the Y-direction crush of modules.
According to the displacement load curve, voltage and temperature curve collected from the test, the crush distance when the battery module fails in XY 2 directions is analyzed. In the X direction crush, the crush displacement is 40 mm, 42 mm and 30 mm respectively when the No. M1, M2 and M3 modules fail; Extruding in Y direction. When M4, M5 and M6 modules fail, the crush displacement is 21 mm, 15 mm and 24 mm respectively. Through comparison, it can be found that compared with the X direction, the Y direction of the module is more prone to failure after being extruded.