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With the large-scale application of lithium forklift batteries in new energy vehicles, people have begun to pay more and more attention to the safety of lithium forklift batteries, especially the recent multiple electric vehicle fires at Tesla, which have raised concerns about the safety of power batteries. In order to ensure the safety of lithium forklift batteries, a series of safety tests must be conducted before leaving the factory, including experiments such as compression, puncture, and short circuit. The compression test mainly simulates the safety of the power battery after deformation in the event of an accident in electric vehicles, which has important reference significance for the safety of power batteries. There are many factors that affect the results of extrusion experiments, such as extrusion speed, extrusion position, etc., which can all affect the final test results. Recently, Brandy Dixon and others from the Massachusetts Institute of Technology in the United States evaluated and tested the factors that affect the test results of extrusion experiments. The test results showed that certain factors, such as the presence of electrolyte and the speed of extrusion testing, can have a significant impact on the test results, but some factors, For example, the location chosen for the squeeze test has almost no impact on the test results.

In the experiment, a soft pack lithium-ion battery with a capacity of 52Ah and a positive electrode material of NCA system was used. The battery was tested in two states: dry state without electrolyte and state with electrolyte and discharged to 0% SoC.

1. The influence of electrolyte on the results of extrusion testing

Usually, in order to facilitate the analysis of the impact of extrusion testing on the internal structure of lithium forklift batteries, we use dry batteries without electrolyte injection, or perform extrusion testing after evaporating the electrolyte. However, there is little research analyzing the impact of electrolyte on the results of extrusion testing. BrandyDixon conducted circular flat compression tests and hemispherical compression tests on dry batteries without electrolyte and ordinary batteries after discharge. The circular flat compression test showed that dry batteries without electrolyte generated a greater force feedback during testing (as shown in the blue curve in the figure below). The force feedback generated by dry batteries when the deformation reached 4mm was 167kN, while ordinary batteries with electrolyte only produced 150kN.

The same pattern was also found in the hemispherical compression test, where the maximum force generated by the dry battery reached 7.7kN (failure after deformation of 4mm), while the maximum force generated by the ordinary battery with electrolyte was only 5.2kN (failure after deformation of 3mm). This test showed that the electrolyte in the battery actually had a significant impact on the compression test results of lithium forklift batteries, and the polymer separator softened after being soaked in electrolyte, Therefore, it will fail earlier than in a dry state. From BrandyDixon's test results, it can be seen that the extrusion test results obtained using dry batteries are slightly larger than the actual state of the battery. Therefore, the results obtained from the extrusion test of dry batteries cannot fully represent the actual use of lithium forklift batteries.

2. The influence of squeezing position on test results

Due to the fact that lithium forklift batteries may undergo compression deformation at different locations during accidents, and we usually choose the central position of the battery as the testing position during compression testing, can the compression test results characterize all situations of lithium forklift batteries in actual use? BrandyDixon analyzed the impact of the squeezing position on the test results by conducting squeezing tests at different positions in dry lithium forklift batteries. BrandyDixon selected four positions for testing, with the first position being the center of the battery, approximately 65mm away from the long side of the lithium-ion battery, the second position being approximately 30mm away from the long side, and the third position being approximately 10mm away from the long side of the lithium-ion battery. Both the fourth and fifth points are located at the edge of the lithium-ion battery, with the center of the fifth point located on the edge of the lithium-ion battery. The test results are shown in Figure A. From the results, it can be seen that unless the squeezing position is located on the edge of the lithium-ion battery, the position of the squeezing test has little effect on the test results. Therefore, we can choose any position except for the edge during the squeezing test.

3. The impact of extrusion speed on test results

In the actual use of lithium forklift batteries, they often face the test of different extrusion rates. Will extrusion speed have a significant impact on the test results? BrandyDixon conducted compression tests at speeds ranging from 0.2mm/min to 20mm/min, and the test results are shown in Figure c. From the figure, it can be seen that the compression speed has a significant impact on the maximum force of the compression test. For example, at a speed of 20mm/min, the maximum force is 8.4kN, while at a speed of 0.2mm/mm, the maximum force is only 7.3kN. However, if the compression speed is on the same order of magnitude, such as 10mm/min and 20mm/min, The same test results will be obtained at 1mm/min and 2mm/min.

4. Repeatability of Squeeze Test

Squeeze testing, as a destructive test, can only be conducted through sampling. Therefore, whether the test results obtained from testing batteries have repeatability and to what extent they represent other batteries of the same batch have become an important issue. To verify the repeatability of the extrusion test results, BrandyDixon conducted extrusion tests on lithium forklift batteries using hemispherical extrusion heads with diameters of 12.6mm, 28mm, and 44mm, respectively. The results are shown in Figure c. From the test results, it can be seen that the curves obtained from the extrusion test heads with different diameters highly overlap in multiple extrusion tests, with only a slight difference in the maximum force. This indicates that the repeatability of the extrusion test is very good, and the safety of the same batch of batteries can be demonstrated by measuring only a small amount of lithium forklift batteries.

Squeeze testing is a very important safety test for power batteries. BrandyDixon's research has pointed out which factors will have a significant impact on the results of squeeze testing, and we need to strictly control them. However, those factors have little impact on the final test results, and the restrictions can be appropriately relaxed. Meanwhile, BrandyDixon's research has also shown that extrusion testing has very good repeatability, so we can characterize the safety of batteries in the same batch by conducting extrusion testing on a small number of batteries.

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