With the continuous increase in global car ownership, problems such as environmental pollution and lack of responsibility for oil resources have become increasingly prominent. Due to its advantages of environmental protection, energy diversification, and high energy efficiency, countries all over the world are actively working on the research and development of electric vehicles that can replace traditional vehicles.
At present, the power batteries for electric vehicles include lead-acid batteries, lithium-ion batteries, and zinc-air batteries. Among them, the lithium-ion battery is the latest research and the fastest commercialization of high-performance batteries. With the characteristics of high voltage, high specific energy, and good cycle performance, it is widely used in electric vehicle power batteries; however, at present, there are two forms of power batteries.
One is to connect small-capacity batteries in parallel to form a large-capacity battery. The advantage is that the energy storage of the smallest energy storage unit (single battery) is small. If a combustion explosion occurs, the consequences will be relatively minor. Its explosion energy will not cause a chain reaction, and other energy storage units will not explode. The disadvantage is that there are too many minimum energy storage units, which are difficult to manage. The other is a large-capacity stacked battery, which has the advantages of compact structure, small volume, and high energy density. The disadvantage is that the energy storage of a single unit is relatively large, and the consequences of an accident are relatively serious.
According to its packaging form, it is divided into “soft case” and “hard case”. The advantage of “soft packaging” is that it will expand to the extent that the outer packaging ruptures under abnormal circumstances, and it is not easy to cause accidents such as explosions; the disadvantage is that the outer shell is weak. Weakness is one of the disadvantages of the shell. Sharp objects can easily penetrate the battery. The disadvantage of the “hard case” is that compared with the “soft case”, it is easy to explode in abnormal situations; the advantage is that the case is relatively strong, sharp objects are not easy to pierce, and it can better protect the power battery.
The standard stipulates that safety tests include: over-discharge, over-charge, short-circuit, drop, heat generation, extrusion, and acupuncture. After a large number of test statistics, the safety test pass rate of batteries below 30Ah is high, and the problems above 30Ah are mainly concentrated in module short circuits, module extrusion, module, and other test items such as acupuncture. In addition to the mandatory inspection items listed above.
Enterprises can also conduct R&D immersion tests, fire tests, shooting, vibration discharge, and other tests according to their own product needs. Based on a large number of power battery safety tests, the test process and results are analyzed and studied. The test samples include monomers and modules (5 or more monomers are connected in series, and the module introduced in this article is composed of 5 monomers connected in series).
1. Short circuit test
Standard requirements: battery short-circuit test includes single-unit short-circuit test and module short-circuit test. The external short-circuit of the single battery lasts for 10 minutes, and the external short-circuit resistance is not greater than 5mΩ; during the short-circuit test of the battery, there should be no explosion or fire.
Analysis of test results: During the short-circuit test of the single power battery, the external short-circuit resistance is 2.7mΩ, the short-circuit current is large, and a large amount of gas is ejected from the safety valve along with the electrolyte. The short-circuit current of the module is several times the short-circuit current of the single unit (the short-circuit current of 100Ah is nearly 4000A).
The reaction of the power battery is more violent, releasing a large amount of heat to melt the plastic casing of the power battery, causing serious deformation. Combustion often occurs during the short circuit of the module, mainly due to the following reasons:
The insulation material of the metal connecting sheet between the single cells is not flame-resistant enough, and combustion occurs at high temperatures, which ignites the power battery;
The insulation material of the battery plastic shell is not flame-retardant enough, and high-temperature combustion ignites the power battery:
The safety valve is set too tightly, and the large amount of heat generated by the battery cannot be released in the time when the short circuit is released, resulting in thermal runaway, resulting in battery explosion, fire, etc:
The electrolyte is a flammable liquid, which is easy to Combustion, adding an appropriate flame retardant can improve the safety of the battery power battery (the performance of the power battery with the addition of flame retardant will be reduced), and the amount of flame retardant added should be appropriate.
If the battery still catches fire and explodes after the above problems are eliminated in the test, there are problems and suggestions: It is recommended to carry out hardware overcurrent protection (PTC) for each single power battery (mainly cylindrical small-capacity battery): in case of short circuit, overcurrent Large, the circuit will be automatically cut off, and when the battery temperature recovers, the circuit will be automatically connected to protect the battery.
For stacked power batteries, it is recommended to add a fuse between several batteries. At present, the power battery safety testing items specified in the international mandatory standards are all routine testing items. Due to the continuous development of power battery technology, more in-depth research and exploration are needed. Therefore, some companies put forward higher testing requirements for products in the production process, such as research and development tests such as immersion, fire, shooting, and vibration.
2. Water immersion test
Test method: Put the fully charged battery module into salt water for a discharge test (water depth, concentration of salt water, and discharge current are proposed by the company). There must be no explosion or fire during the test. Analysis of test results: This test project mainly simulates the situation where electric vehicles are wading or power batteries are flooded due to natural disasters such as floods. Since the battery is immersed in water, higher requirements are placed on the protection level of the battery. If the battery is not well protected, water ingress can short the battery: the safety valve and electrodes are the weak parts. Once the salt water soaks into the battery and causes a short circuit, the battery will emit a lot of heat and gas, causing the salt water to generate a lot of bubbles and even boil.
3. Fire test
Test method: Put the fully charged battery module into the fire and burn it (the intensity of the fire and the burning time are proposed by the company). There must be no explosion or fire during the test. Analysis of test results This test project mainly simulates the combustion of the vehicle due to abnormal conditions, and the combustion of the power battery in the flame. If the power battery explodes or catches fire, it will further intensify the combustion of the vehicle and pose 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 casing, and higher requirements are also put forward for the flame retardancy of the solution.
4. Shooting test
Test method: Place the fully charged battery module vertically for the discharge test, and shoot with a gun perpendicular to the direction of the plate (discharge current, ignition position, and ignition times are provided by the company). There must be no explosion or fire during the test.
Analysis of test results: This test project mainly simulates the bullet hitting the driving battery of an electric vehicle under abnormal circumstances such as terrorist attacks. Bullets fired at the power battery will cause a short circuit inside the battery, releasing a lot of smoke and heat. Combustion, Explosion, and other phenomena may occur if the heat is not released in time.
5. Vibration test
Test method: Fix the fully charged battery module on the vibration shaker table according to the actual vehicle state for discharge (vibration conditions and discharge current are provided by the company). There must be no explosion or fire during the test.
Analysis of test results: This test project mainly simulates the vibration generated by vehicles driving on different roads. In the case of vibration, the active material on the battery pole pieces may fall off, and the battery electrodes, spot welding contacts, lugs, external wiring, solder joints, etc. may break and fall off, causing external or internal short-circuiting of the battery.
6. Extrusion test
Extrusion tests include single and module extrusions. A single extrusion is required to apply pressure perpendicular to the battery plate, and the area of the extrusion head is not less than 20cm, until the battery case ruptures or the internal short circuit (battery voltage becomes 0V); during the extrusion test of the battery core, there must be no explosion or fire. Module extrusion requires pressure to be applied perpendicular to the cell arrangement direction. The extrusion head whose size meets the standard requirements is extruded to 85% of the original size of the battery module for 5 minutes and then squeezed to 50% of the original size of the battery module.
During the extrusion test of the battery module, there must be no explosion or fire. Analysis and research of test results: When the battery core is squeezed due to shell rupture or internal short circuit, the test ends. During the test, use monitoring equipment such as cameras to detect whether the battery case is broken, and use voltage detection equipment to monitor whether the battery voltage is 0V. Most single cells will not explode or catch fire when squeezed, and very few cells will burn. In the extrusion process, battery modules are subjected to relatively harsh test conditions.
During the extrusion process, the battery module is compressed in a large area, resulting in a large area of short circuits inside the battery. Thick smoke was emitted from the battery module, and the electrolyte sprayed out from the safety valve, resulting in an explosion and fire. Different battery modules have different stages of explosion and fire. The battery safety valve of some enterprises is set too tight. When the battery module is short-circuited, it cannot release heat in time, resulting in thermal runaway and a rapid rise in temperature. This is one of the reasons why the battery exploded and caught fire.
7. Nail Penetration Test
Here are some details of a nail penetration test
1) Standard requirements
The nail test includes a single-cell needle and a module needle. The single needle is perpendicular to the direction of the battery plate, the diameter of the needle is Φ3~8mm, the speed is 10~40mm/s, and the needle pierces the battery core; the battery must not explode or catch fire during the acupuncture test. The insertion needle of the battery module should be perpendicular to the direction of the battery plate, the diameter of the insertion needle is 3-8mm, the speed is 10-40mm/s, and the insertion degree of the needle penetrates at least three layers of batteries: during the acupuncture test of the battery module, there must be no explosion, Fire phenomenon.
2) Analysis of test results
A 5 mm steel needle with a diameter of Ф and a length of 350 mm is inserted into the battery at a speed of 20 mm/s. When the steel needle is inserted into the battery, a short circuit occurs inside the battery, and a large amount of smoke and electrolyte is released from the safety valve and the needle insertion position. The heat released will cause the battery casing to expand and deform and even melt part of the plastic casing.
Due to the leakage of the electrolyte during the test, the internal heat generated when the battery is short-circuited is extremely high, and even the pierced 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 flame will ignite the electrolyte to further aggravate the combustion. Do not pull out the battery immediately after the injection, but keep it in the battery. If you pull it out immediately, the needle will not be pulled out until the battery returns to normal temperature, because the battery reacts violently, and oxygen enters the needle-punched place, which is easy to cause combustion.
The mechanism’s penetrating device pierces the battery from top to bottom. Since the placement state of the battery in the test is inconsistent with the actual loading state, the standard only stipulates vertical plate acupuncture and does not specify the placement direction. Transverse needling is the actual loading state. Whether the results of horizontal needles and vertical needles are consistent requires a lot of experiments to verify.
It is recommended that the acupuncture direction of the battery be consistent with the actual loading state. The above-mentioned short circuit, extrusion, acupuncture, and other safety tests are mainly to simulate the situation that may occur under abnormal conditions such as a car accident when the power battery is driving at high speed. A power battery in an electric vehicle is currently divided into several small boxes and installed in different locations.
Each small box is connected by wires. To protect against short circuits, fuses can be connected in series between several application pools. The battery management system (BMS) collects abnormal voltage and reminds the driver of the sound and light on the electric vehicle instrument. To prevent the battery from being crushed during vehicle crashes and rollovers, the battery case needs to be strong and cool well.