With the increasing number of vehicles in the world, the problems of environmental pollution and lack of responsibility for oil resources are becoming increasingly prominent. Because of its advantages of green environmental protection, diversification of energy sources, and high energy efficiency, all countries in the world are actively working to research and develop electric vehicles that can replace traditional vehicles. At present, power batteries for electric vehicles include lead-acid batteries, lithium-ion batteries, and zinc air batteries. Among them, lithium-ion battery is the latest high-performance battery researched and commercialized the fastest. It has been widely used in electric vehicle power batteries because of its high voltage, high specific energy and good cycle performance; However, due to its frequent safety accidents in the actual use process, its safety has been paid more and more attention.
At present, there are two forms of power batteries. One is that small capacity batteries are connected in parallel to form large capacity batteries. The advantage is that the energy reserves of the smallest energy storage unit (single cell) are small. If combustion and explosion occur, the consequences are relatively light. Its explosion energy will not cause chain reaction and other energy storage units will not explode. The disadvantage is that the number of the smallest energy storage units is too large and it is difficult to manage. The other is a high-capacity laminated battery, which has the advantages of compact structure, small volume and high energy density. The disadvantage is that the single energy storage is large, and the consequences of accidents are relatively heavy. According to its packaging form, it can be divided into “soft package” and “hard shell”. The advantage of “flexible packaging” is that in case of abnormal conditions, it will expand to the extent that the external package is broken, which is not easy to cause accidents such as explosion; the disadvantage is that the shell is weak. Sharp objects can easily penetrate the battery. The disadvantage of “hard shell” is that it is easy to explode when encountering abnormal conditions compared with “soft package”; The advantages are that the shell is relatively strong, sharp objects are not easy to penetrate, and the power battery is better protected.
The standard specifies that the safety tests include: over discharge, over charge, short circuit, drop, heating, extrusion and acupuncture. After a large number of tests and statistics, the safety test qualification rate of batteries with a capacity of less than 30 Ah is high, and the problems above 30 Ah are mainly concentrated in the test items such as module short circuit, module extrusion, module acupuncture, etc. In addition to the above mandatory inspection items. Enterprises can also conduct R & D immersion test, fire test, shooting, discharge in vibration and other tests according to their own product needs.
Based on a large number of power battery safety tests, the test process and results were analyzed and studied. The test samples include monomer and module (five or more monomers are connected in series, and the module introduced in this paper is composed of five monomers in series).
Standard requirements: extrusion test includes single extrusion and module extrusion. Single extrusion requires that pressure be applied perpendicularly to the battery electrode plate, and the area of extrusion head shall not be less than 20 cm until the battery shell is broken or internally short circuited (the battery voltage becomes 0 V); During the extrusion test of battery cells, there shall be no explosion and fire. Module extrusion requires that pressure be applied perpendicularly to the direction of battery cell arrangement. The extrusion head whose size meets the standard requirements shall be extruded to 85% of the original size of the battery module for 5 min and then to 50% of the original size of the battery module. During the extrusion test of the battery module, there shall be no explosion and fire.
Analysis and research of test results: the test is ended when the battery cell is extruded due to shell rupture or internal short circuit. During the test, monitoring equipment such as cameras are used to detect whether the battery shell is broken, and voltage detection equipment is used to monitor whether the battery voltage is 0 V. Most single batteries will not explode or catch fire when squeezed, and a few batteries will burn. The end conditions of the test of the battery module during the extrusion process are relatively harsh. During the extrusion process, the battery module is compressed in a large area, resulting in a large-area short circuit inside the battery. The battery module is accompanied by strong smoke, electrolyte is ejected from the safety valve, explosion and fire occur. Different battery modules have different explosion and fire stages. In some enterprises, the safety valve of the battery is set too tightly. When the battery module is short circuited, it cannot release heat in time, resulting in thermal runaway and rapid temperature rise. This is one of the reasons for the explosion and fire of the battery.
Standard requirements: nail penetration test includes single cell acupuncture and module acupuncture. The single needle shall be perpendicular to the direction of the battery plate, and the diameter of the needle shall be Φ 3 ~ 8 mm speed 10 ~ 40 mm / s, needle penetration through the battery cell; The battery shall not explode or catch fire during the needling test. The needling of the battery module shall be perpendicular to the direction of the battery electrode plate, the diameter of the needle shall be 3 ~ 8 mm, the speed shall be 10 ~ 40 mm / s, and the needling degree shall penetrate at least three battery cells: during the needling test of the battery module, there shall be no explosion and fire.
Analysis and research of test results: diameter Ф A 5 mm steel needle with a length of 350 mm was inserted into the battery at a speed of 20 mm / s. when the steel needle was inserted into the battery, there was a short circuit inside the battery, and a large amount of smoke and electrolyte were released from the safety valve and the needle insertion position. The heat released will cause the battery case to expand and deform, and even melt part of the plastic shell. Due to leakage of electrolyte during the test, the internal heat generated during the short circuit of the battery is extremely high, and even the piercing steel needle will melt. If the flame retardant grade of the material used for the shell is not enough, it may cause combustion, and the open fire will ignite the electrolyte to further aggravate the combustion. The battery cannot be pulled out immediately after the needle is punched, and it should be kept in the battery. If it is pulled out immediately, the needle will be pulled out after the battery recovers to normal temperature because the battery reacts violently and oxygen enters the place where the needle is punched, which will easily cause combustion.
The penetration equipment of the mechanism stabs the battery from top to bottom. Because the placement state of the battery in the test is inconsistent with the actual loading state, the standard only stipulates the vertical pole plate acupuncture, and does not specify the placement direction. The horizontal acupuncture is the actual loading state. Whether the results of the horizontal acupuncture and the vertical acupuncture are consistent needs a lot of tests to verify. It is recommended that the acupuncture direction of the battery is consistent with the actual loading state.
The above safety tests such as short-circuit, extrusion and acupuncture are mainly to simulate the possible situation of the power battery under abnormal circumstances such as car accidents when the electric vehicle is moving at high speed. At present, the power battery in the electric vehicle is divided into several small boxes and installed in different positions of the vehicle. Each small box is connected by wires. In order to prevent short-circuit, fuses can be connected in series between several application pools. The battery management system (BMS) collects the abnormal voltage and alerts the driver with sound and light on the electric vehicle instrument. In order to prevent the battery from being squeezed in the process of vehicle collision and rolling, the battery box needs to be strong and heat dissipation is good. The structural strength of the body where the battery box is installed should be high or special protection should be taken.