The safety performance of batteries has a significant impact on the smooth and reliable operation of electric vehicles, therefore detailed safety performance testing of batteries is necessary. The safety performance inspection of electric vehicle batteries includes overcharging and discharging tests, short circuit tests, high temperature tests, needle puncture impact tests, mechanical performance tests, corrosion resistance tests, and other items.
1 Test of over charge and discharge
For sealed secondary batteries, overcharging and discharging can cause rapid accumulation of gas in the sealed container, leading to a rapid increase in internal pressure. If the safety valve cannot be opened in time, it may cause the battery to burst. Under normal circumstances, the safety valve will open under a certain pressure to release excess gas. After the gas is released, it will lead to a decrease in the amount of electrolyte, and in severe cases, it will dry up the electrolyte, deteriorate battery performance, and eventually fail. Moreover, during the gas release process, a certain amount of electrolyte is carried out, and most electrolytes are concentrated acids or bases, which have a corrosive effect on electrical appliances.
Therefore, a high-performance electric vehicle battery should have good resistance to overcharging, be less prone to bursting, and be less prone to leakage under a certain degree of overcharging and discharging. The battery shape is also less likely to change.
In the design of batteries, an excess negative electrode is generally used to avoid excessive accumulation of gas inside the battery. To avoid the occurrence of reverse polarity during over discharge, reverse polarity protection is generally implemented by adding reverse polarity material to the positive electrode.
When conducting charging tests, appropriate conditions can be selected based on the specific type and model of battery. Taking MH-Ni batteries as an example, the selection of overcharging current can be determined based on the output power of the constant current source.
For some large capacity batteries (D-type, SC type), the general constant current source cannot output a large current of 1 C, and sufficient safety protection measures should be considered in high current situations. For batteries with relatively small capacity, a larger current multiplier can be chosen. Under different discharge systems, there are corresponding differences in the standards for judging the overcharging capacity of batteries. In practical work, the following two overcharging systems are used.
(1) Charge the battery with a constant current of 0.1 C for 28 days. During the test, the battery must not explode or leak, and its capacity should not be lower than the nominal capacity when discharged at 0.2 C after charging.
(2) Charge at a constant current of 1 C for 5 hours, and there should be no leakage during the first 75 minutes of the test. Leakage is allowed thereafter, but no explosion is allowed. After charging, discharge at 0.2 C, and the capacity should not be lower than its nominal capacity.
During the charging process, leakage detection can be verified by dripping liquid at the sealing point. The redness of the solution or the formation of bubbles are considered as leaks.
When conducting an over discharge test on a battery, the battery should be fully charged first, and then appropriate conditions should be selected for discharge. There are two commonly used testing conditions.
(1) Connect the battery in series with a standard resistor (about 10 Ω, selected according to the battery model) and discharge continuously for 24 hours. The battery should have no explosion or leakage during the discharge process, and its capacity should not be less than 90% of the nominal capacity after over discharge.
(2) First discharge the battery at 1 C to 0 V, then discharge it at 0.2 C to 0 V, and then force over discharge at 1 C for 6 hours. The battery should not explode, but leakage or deformation is allowed. After testing, the battery cannot be used again.
For a battery, a method is generally used to test its ability to withstand leakage.
2 Short circuit test
In the case of a short circuit, electric vehicle batteries can generate a large current, which can instantly raise the battery temperature, and even cause the electrolyte to boil or the sealing ring to melt. Therefore, during short circuit testing, the battery may experience alkali spraying, leakage, and other situations. Usually, good protective measures should be taken.
The common test condition is to fully charge the battery and short circuit the two poles of the battery at room temperature for 1 hour. Leakage is allowed, but the battery must not catch fire or explode.
3 High temperature resistance test
Generally, batteries are prohibited from being put into fire because they can undergo certain changes and may explode at higher temperatures. Therefore, it is necessary to test the safety performance of batteries at appropriate temperatures.
The general testing temperature range is divided into high temperature zone and low temperature zone. The high temperature zone is put into fire for testing, and the low temperature zone is 100-200 ℃. The common low-temperature testing conditions are as follows.
(1) The fully charged battery (100 ℃) should be kept for 2 hours and there should be no explosion or leakage.
(2) The fully charged battery should be kept in a constant temperature box at 150 ℃ for 10 minutes, and there should be no explosion or leakage.
The internal resistance and open circuit voltage of the battery will undergo certain changes after passing the test in the low-temperature zone, but the battery should still be able to continue to be used. The testing of batteries in high temperature areas is destructive, and the tested battery will no longer be able to be used. After the battery is put into fire, the temperature can reach 800 ℃, and the sealing ring and other plastics inside the battery will all melt and catch fire. Gas precipitation is allowed, but explosion must not occur.
4 EV battery nail penetration test
When electric vehicle batteries are impacted by sharp objects from the outside, the shell may be punctured. If the punctured object is conductive, a short circuit may occur between the positive and negative electrodes, posing a certain danger. Therefore, for batteries used in some special occasions, puncture experiments should also be conducted, and the battery should be in a fully charged state before drilling tests. The test conditions are as follows: the needle diameter is φ Pierce the battery in the diameter direction with a diameter of 1.0 mm. After piercing, the battery should not explode, but leakage and heating are allowed.
5 Mechanical properties
The mechanical properties include collision resistance, impact resistance, and vibration resistance tests. The commonly used methods for testing mechanical properties are collision tests.
Collision test
Firstly, charge the battery and discharge it at a constant current of 0.2 C after the test. Method 1 requires that the discharge capacity after the test should not be lower than the nominal capacity; Method 2 requires that there is no significant difference in battery capacity loss after the test and before the test. After the test, the battery does not deform or leak. The sample battery used for collision testing should be fixed on the impact table in half the vertical axis and half the parallel axis for testing. Mechanical performance testing can also be done through simple collision tests, where the battery can be randomly dropped from a height of 1m in different directions onto a 2 cm thick oak board four times. After the experiment, there should be no visual changes or leakage in the appearance of the battery. In addition, the battery voltage and internal resistance should not change.
6 Corrosion resistance testing
The commonly used corrosion testing methods include electrochemical testing, salt spray testing, etc. The experiment was conducted in a salt spray box. Expose the battery to the test chamber, spray atomized test solution into it, and the fine mist evenly settles on the surface of the sample under its own weight. The test solution is a 5% NaCl (mass percentage) solution, with a total solid content not exceeding 20%/(μg/g), pH 6.5-7.2. During the experiment, the temperature inside the salt spray box remained constant at (35 ± 1) ℃. The battery is kept in the salt spray chamber for 48 hours.
After the test, there should be no significant difference in the capacity of the battery. A small amount of rust is allowed on the top (seal) and bottom of the battery, but there should be no perforation or very obvious pitting. The battery must not leak or explode.