1、 Appearance inspection

Soft pack battery: We need to confirm whether the battery is intact, including whether it is bulging, leaking liquid, and whether there is corrosion on the surface. Based on the surface phenomenon, we can preliminarily determine whether the battery can be used or what kind of experience it has experienced.

Square aluminum shell: It is also used to confirm whether the battery has any leakage, deformation, pits, and other phenomena such as pole asymmetry.

Battery bulging, considering whether it is at high temperature or room temperature, to check the consistency of the battery, whether it is a single battery or a whole batch. If it is a whole batch of batteries, considering whether it is a system issue or an overall process issue, the swelling of the lithium-ion titanate battery system is usually more obvious, and when the assembly process and design are unreasonable, it will also cause overall swelling. If it is a single battery, consider whether there has been severe cracking attenuation, gas production, etc.

Battery leakage: First, confirm the location of the battery leakage, whether it is at the pole ear or the liquid injection port. Finally, determine whether it is a design issue with the battery shell or an operational issue, such as not sealing the aluminum plastic film properly or not welding the liquid injection hole properly.

If surface corrosion of the battery is found, consider whether it is caused by electrolyte spillage during liquid injection, battery leakage, or shell corrosion caused by battery short circuit collision or electrochemical corrosion. When problems are discovered, they should be promptly eliminated.

2、 Basic electrochemical performance testing

1. Measurement of OCV, IR, and volume (thickness) of the battery

To obtain a batch of batteries, the first step is to measure the OCV, IR, volume (thickness) and other parameters of the batteries. Testing the OCV can confirm the abnormal initial state of the batteries, and testing the IR can lock the problematic batteries in a timely manner to observe potential problems that may occur in the future, such as cracking and micro short circuits. If there is not much explanation for the test volume or thickness, the battery may swell during use.

2. Confirmation of battery capacity

Generally, it is necessary to confirm the capacity of the battery 2-3 times, why not do it once? Worried about not being accurate enough, doing too much is meaningless.

Usually, for battery capacity testing, choose a current of 0.5C or 1C. The current national standard is 1C charging and 1C discharging to see how much capacity there is. Of course, the discharge capacity at 0.5C will be more than 1C, according to the national standard~

In addition, after confirming the conventional capacity, the battery needs to be charged and discharged with low current to eliminate the influence of polarization impedance, usually 0.1C or less; At this point, the current is small enough and the polarization is small enough, which can be ignored temporarily. It fully displays the basic information of the battery's capacity, medium voltage, cycle curve, and so on.

Further down, if conditions permit, the external voltage data collector can accurately measure the voltage capacity curve of the battery. After differential processing, the internal state of the battery can be further displayed, and the degree and type of battery degradation can be determined by the changes in peak positions.

3. Three electrodes

Due to the current passing through the battery system, the solution voltage drop and polarization of the electrode occur, so it is difficult to accurately measure the potential of the working electrode, so the reference electrode should be introduced. The reference electrode has a very stable potential, and the current will not cause polarization without passing through the reference electrode, so the potential of the working electrode can be obtained from the reference electrode, and the current is obtained from the working electrode auxiliary electrode circuit. Electrochemical testing usually considers the three electrode method, and the research on lithium-ion batteries is also the same.

Through the disposal of three electrodes, it is possible to clearly distinguish the electrode potentials of the positive and negative electrodes, and determine which side has the problem or potential deviation.

Here, the reference electrode, usually a lithium metal electrode, should be placed between the positive and negative electrodes, not just inside the battery.

The production of three electrodes should ensure that it does not affect the state of the battery itself as much as possible, to prevent short circuits and contact effects.

4. EIS testing

EIS (Electro chemical Dependence Spectroscopy) is an electrochemical impedance spectrum. For electrochemistry, whether the impedance can be well fitted and analyzed represents whether the analysis of the battery is accurate.

According to the law of conservation of energy, everything consumes energy, and impedance accurately measures how this portion of energy is consumed.

A simple battery impedance map, Nyqist map, and bode frequency map can intuitively determine the reaction of the battery, the several interfaces experienced, their respective impedance sizes, and the subsequent diffusion parts.

Generally speaking, impedance diagrams are compared, that is, taking a known and an unknown and comparing them with each other, in order to more intuitively see which one is better or worse, where the advantages lie, and where the disadvantages lie. Where are the future improvement directions or where may the problem points be located.

The combination of three electrodes and EIS impedance can provide more abundant information, and the impedance of the battery can be specifically divided into the impedances of the positive and negative electrodes, providing a clearer description of the distribution of the internal impedance of the battery. Provide specific information about the battery without violent damage.

3、 Performance testing of lithium-ion batteries

After basic electrochemical performance testing, lithium-ion batteries can undergo in-depth performance research and select batteries with good consistency and low impedance for various performance tests.

1. Cycle life

The number of cycles in a lithium-ion battery reflects how many times the battery can be repeatedly charged and discharged. According to the different environments in which lithium-ion batteries are used, the cycle life can be tested to determine how much the battery can achieve at low temperature, room temperature, and high temperature. The standard for battery disposal is usually selected based on the purpose of the battery. If the battery is used for power lithium batteries (electric vehicles, electric forklifts), etc., a discharge capacity maintenance rate of 80% is generally chosen as the standard parameter for disposal. However, if the battery is used for energy storage, energy storage, etc., it can be relaxed to 60%. If the discharge capacity/initial discharge capacity of the battery that we often come into contact with is less than 60%, it is no longer necessary to use it and cannot last for half a day.

2. Multiplication

Nowadays, lithium-ion batteries are not only used for 3C, but also increasingly used in power lithium-ion batteries. Electric vehicles need to change the current when driving under different working conditions. In the current fast-paced life, the shortage of electric vehicle charging stations has increasingly high requirements for fast charging of lithium-ion batteries. Therefore, it is necessary to test the rate performance of lithium-ion batteries. Testing can be carried out according to the national standard for power lithium batteries. Nowadays, both domestic and foreign battery factories produce special high magnification batteries to meet market demands. The design of high magnification batteries can be started from the types of active materials, electrode surface density, compaction density, electrode ear selection, welding process, and assembly process. Interested friends can learn about it themselves.

3. Safety testing

Safety can be the top concern for battery users, whether it's the explosion of mobile phone batteries or the ignition of electric vehicles, it's enough to make people tremble. The safety of lithium-ion batteries must be inspected, including overcharging, over discharge, short circuit, drop, heating, vibration, squeezing, puncture, and so on. However, from the perspective of lithium-ion batteries, these safety tests are passive safety tests, which means holding a battery and letting foreign objects actively damage it to test its safety. When submitting for inspection, it is necessary to design the battery and module accordingly to undergo safety testing. However, in actual use, such as electric vehicles losing control and colliding with other vehicles or objects, it is an irregular collision and may face more complex situations. However, the cost of testing is higher, and only relatively reliable testing content can be selected.