Short Circuit Test
Standard requirements: battery short circuit test includes single short circuit test and module short circuit test. The external short circuit of single battery is 10 min, and the external short circuit resistance is not greater than 5 m Ω; During the short circuit test of the storage battery, there shall be no explosion or fire.
Analysis and research of test results: during the short circuit test of single power battery, the external short circuit resistance is 2.7 m Ω, the short circuit current is large, and a large amount of gas is ejected from the safety valve with electrolyte. The module short circuit current is several times of the single short circuit current (100 Ah short circuit current is nearly 4000 A). The power battery reacts more violently, releasing a large amount of heat that melts the plastic shell of the power battery, causing serious deformation. Combustion often occurs in the process of module short circuit, mainly due to several reasons: the insulation material of the metal connecting piece between the single batteries is not flame retardant enough, burning occurs at high temperatures, igniting the power battery;
The insulation material of the plastic shell of the battery has insufficient flame retardant grade, which burns at high temperature and ignites the power battery: the safety valve is set too tightly, and a large amount of heat generated by the battery during short circuit cannot be released in time, resulting in thermal runaway, leading to explosion, fire and other phenomena of the battery: the electrolyte is a flammable liquid, which is easy to burn at high temperature, and appropriate flame retardant is added, It can improve the safety of power battery (the performance of power battery with flame retardant will be reduced), and the addition of flame retardant should be appropriate. If the battery still suffers from fire and explosion after the above problems are eliminated in the test, in-depth analysis shall be conducted on the deep-seated causes such as electrolyte, electrode materials and formula.
Existing problems and suggestions: It is recommended to conduct hardware over-current protection (PTC) for each single power battery (mainly cylindrical small capacity battery): in case of short circuit, the current is too large, the circuit will be automatically cut off, and after the battery temperature recovers, the circuit will be automatically connected to protect the battery. For laminated power batteries, it is recommended to add fuses between several batteries.
At present, the power battery safety test items specified in the international mandatory standards are routine test items. Due to the continuous development of power battery technology, more in-depth research and exploration are needed. Therefore, some enterprises put forward higher test requirements for their products in the production process, such as immersion, fire, shooting, vibration and other R&D tests.
Water immersion test
Test method: put the fully charged battery module into salt water for discharge test (the depth of water, concentration of salt water and discharge current shall be proposed by the enterprise). During the test, there shall be no explosion or fire.
Analysis of test results: This test item mainly simulates the situation of electric vehicle wading or power battery flooded due to natural disasters of flood. As the battery is immersed in water, higher requirements are put forward for the protection level of the battery. If the protection of the battery is poor, water entering the battery will cause short circuit of the battery: the safety valve and electrode are weak parts. Once the salt water is immersed in the battery, causing a short circuit, the battery will release a lot of heat and gas, which will make the salt water generate a lot of bubbles or even boil.
Test method: put a fully charged battery module into a fire and burn it (fire intensity and burning time shall be proposed by the enterprise). During the test, there shall be no explosion or fire.
Analysis of test results This test item mainly simulates the combustion of the vehicle due to abnormal conditions, and the power battery is in the flame. If the power battery explodes or catches fire, it will further intensify the combustion of the vehicle, posing a greater threat to the safety of passengers. In the test, due to the high external temperature of the power battery, higher requirements are put forward for the flame retardancy of the power battery shell, and also for the flame retardancy of the solution.
Test method: place the fully charged battery module vertically for discharge test, and shoot with a gun in the direction perpendicular to the electrode plate (the discharge current, firing position and firing times shall be provided by the enterprise). During the test, there shall be no explosion or fire.
Analysis of test results: This test item mainly simulates that the bullet hits the power battery of the electric vehicle in motion under abnormal circumstances such as terrorist attacks. Bullet shooting at the power battery will cause internal short circuit of the battery, releasing a lot of smoke and heat. If the heat is not released in time, it may be accompanied by combustion, explosion and other phenomena.
Test method: fix the fully charged battery module on the vibration table as per the actual vehicle status for discharge (vibration conditions and discharge current shall be provided by the enterprise). During the test, there shall be no explosion or fire.
Analysis of test results: This test item mainly simulates the vibration generated by vehicles traveling on different roads. Under the condition of vibration, the active substances on the battery pole piece may fall off, and the battery electrode, spot welding contact, lug, external wiring, solder joint, etc. may break and fall off, thus causing external or internal short circuit of the battery.