3.Research on T6 Test Method
For T6 test, due to the diversity of existing lithium battery types, it is necessary to select different test methods for different types of lithium battery cells. During the research of the lithium battery working group of the United Nations Transportation Expert Committee, the test methods such as crush, nail penetration, NASA blunt puncture, and Japanese PSE forced internal short circuit were included in the discussion.
These tests will be briefly introduced below:
1) Crush: crush test is a very common test method for lithium batteries, which is used in abuse tests of various standards. In IEC 60,086-4 Primary Batteries – Part 4: Safety of Lithium Batteries, this test is used as an alternative method for heavy impact test of some types of batteries. Generally, the battery is squeezed in two planes and released after reaching a certain pressure. This test can not cause internal short circuit to all types of lithium battery cells, but it is one of the more mature and widely used methods.
2) Nail penetration: it is also a common test method for lithium battery abuse. But the damaged part during test is only limited to the nail penetration point. The short circuit part is relatively small and the heat generation is relatively slow, so the effect of causing internal short circuit is limited.
3) Japanese PSE forced internal short circuit: this test is a forced internal short circuit test for battery cells proposed by Japan. In 2008, the Japanese government introduced the PSE certification of lithium-ion batteries, which requires that all lithium-ion batteries exported to Japan after November 20, 2008 and conforming to the provisions of Japan’s Electrical Appliance Safety Law must be certified by PSE. This test is an important test item for the PSE certification of lithium-ion batteries.
The test method is as follows: disassemble the fully charged lithium ion battery cell, place a small nickel sheet between the positive active substance and the negative active substance, as well as between the positive aluminum foil and the negative active substance, wrap the battery cell, mark the nickel sheet position, place it in a sealed manner, apply pressure on the part of the battery cell where the nickel sheet is placed at a speed of 0.1 mm/s under certain temperature conditions, and when the observed voltage drop is greater than 50 mV, Or when the pressure is applied to meet the requirements (cylindrical battery 800 N, square battery 400 N), stop lowering the pressurizing tool for 30s, and then remove the pressure. If the battery cell does not catch fire during the test, the test requirements shall be met.
This test has high requirements for battery cells, but there are also some disputes. For example, if the electrochemical environment of the battery is damaged during disassembly, it is uncertain whether the repackaged battery cells can effectively simulate the internal short circuit of normal batteries; In addition, the safety of the test process and the requirements for personnel shall be considered; This test is only for lithium ion battery cells, not for lithium metal battery cells. Therefore, its scope of application is limited.
4) Blunt: This test is a battery internal short circuit simulation method proposed by UL (Underwriters Labo ratings Inc.), which reduces the risk of battery disassembly by Japanese forced internal short circuit method. The method is to use a steel blunt needle to prick the battery at a slow speed of 0.1 mm/s at the center of the battery, and observe the change of battery open circuit voltage and temperature until the voltage drop reaches 100 mV. The advantage of this test is that it does not need to disassemble the battery and does not damage the electrochemical environment inside the battery, so it has strong operability. However, because the blunt sting occurs in the battery shell, the internal short circuit position of the battery cannot be accurately controlled. Whether this test can fully simulate the internal short circuit of the battery is still under study.
4.Revision of T6 test in UN38.3
Based on the discussion of experts in the lithium battery working group, including the research on the above described test methods, the United Nations Transportation Expert Committee
The committee has reached a resolution to revise the T6 tests of UN38.3. The main contents are as follows:
1) Crush test is added, which is applicable to prismatic, bag, button batteries and some cylindrical battery cells. For cylindrical battery cells beyond the applicable scope of crush, the original T6 heavy impact test method is still used.
Crush method: the crush speed of the first contact point is 1.5cm/s. Extrude until one of the following three conditions is met:
- (1) The crush force reaches (13 ± 0.78) kN;
- (2) Voltage drop shall be at least 100 mV;
- (3) The cell deformation shall be at least 50%.
The test and observation time is 6h. The external temperature of the test sample shall not exceed 170 ℃ during the test and observation time, and no disassembly or fire shall occur.
2) The original weight impact test has also been revised, including: The tolerance range of the diameter of the test bar and the weight of the hammer has been increased; The length of test bar is limited; The material of the bar is limited; It is emphasized through text description that the hammer should fall perpendicular to the sample, and the friction and traction should be minimized when falling.
The main contents after revision are as follows:
Place the sample on a smooth horizontal surface, place a 316 type stainless steel rod with a diameter of (15.8 ± 0.1) mm and a length of at least 6 cm or the longest dimension of the battery cell (the larger one) on the center of the sample, and drop a heavy hammer with a weight of (9.1 ± 0.1) kg from the height of (61 ± 2.5) cm to the intersection of the sample and the rod. When dropping, use a vertical slide rail that is close to frictionless and minimize the traction on the hammer. The slide rail used to guide the heavy hammer shall be kept at a 90 ° right angle to the horizontal supporting surface.
3) The test purpose is revised to simulate the mechanical abuse caused by the impact and crush of heavy objects that may cause internal short circuit. This revision adds a selection method for T6, that is, different test methods can be used for different types of battery cells, which is a breakthrough in this standard, can increase the applicability of the standard, and also lays a good foundation for further refining and strengthening this standard in the future.
Although the UN38.3 T6 test method will be revised, how to simulate the internal short circuit test of lithium battery cells is still a subject that needs to be studied. With the continuous development of lithium battery technology itself and the continuous improvement of safety requirements, it is more important to develop this test project. Only by constantly studying and exploring the electrochemical characteristics and use environment characteristics of lithium ion batteries, can we formulate more scientific, more targeted and operable lithium battery test items and test conditions. Thus, the evaluation of lithium battery safety can be further strengthened, and the assurance of transportation safety can be enhanced.