Test method for failure causes of traction batteries

BATTERY SKY TEST METHOD FOR FAILURE CAUSES OF TRAINING BATTERIES
The cause of the failure of the traction battery can be tested by testing the potential of the positive and negative electrodes. The relative value of the measured stop and negative electrode potentials should be determined by means of the reference electrode. In actual production, cadmium electrodes are often used as reference electrodes for product performance tests, because cadmium electrodes have the advantages of convenient operation and stable voltage. , The quality of the negative plate provides a reliable basis for improving product quality, and at the same time, it can also timely analyze the reasons for battery capacity reduction and failure through measurement. Therefore, it is of great significance.

1. The method of using cadmium electrode to measure the potential of positive and negative electrodes
The cadmium electrode is made of pure cadmium, and its density is 8.65g/cm3. The potential stability of cadmium is related to impurities. Therefore, chemically pure cadmium is applied, and when it is manufactured, it is melted in a clean container, and then poured into a mold of a copper wire that was placed in advance, and cast into a rod shape of φ7~φ10mm and a length of 60~80mm. Wrap it with porous hard rubber or cover it with a plastic sleeve, the main purpose is to avoid contact with the positive and negative electrodes of the battery during use.
Before the new cadmium rod is used, its potential value is unstable. It must be immersed in dilute sulfuric acid of 1.100g/cm3 in advance, and after 2 to 3 days and nights of sufficient corrosion, cadmium sulfate can be generated to have a stable voltage.
When not in use, it must also be immersed in dilute sulfuric acid to prevent the surface from drying. The dry cadmium electrode should also be soaked in dilute sulfuric acid with a density of 1.100g/cm3 for more than 12 hours before use. If it is not used for a long time, water can be used. Wash and save to prevent moisture.
The voltmeter used to measure the cadmium electrode must be of high resistance (above 1000Ω/V), so as to avoid large errors due to polarization.
specific usage
During discharge, the cadmium electrode is inserted into the electrolyte in the battery through the injection hole, that is, the cadmium electrode is connected to the negative electrode of the voltmeter, the positive electrode of the voltmeter is connected to the positive electrode of the battery, and the potential between the positive electrode and the cadmium electrode is measured. The difference is the positive cadmium voltage (represented by V1, the positive electrode of the voltmeter is connected to the negative electrode of the battery, and the potential difference between the negative electrode and the cadmium electrode is measured, that is, the negative electrode cadmium voltage (represented by V2).
2. Report on the measured value of positive and negative electrode potentials
In the cadmium electrode test, take the battery 20h rate discharge as an example. After a period of time after starting to discharge, if the voltage of each single battery is measured to be 2V, it means that the battery is in good condition. At this time, the negative terminal of the voltmeter can be connected to the copper wire of the cadmium rod, and the positive terminal of the voltmeter is the same as that of the single battery. Connect the negative electrode of the main battery, be careful not to touch the cadmium rod to the positive and negative plates of the battery, measure the cadmium voltage between the cadmium electrode and the negative electrode of the single battery is about 0.1~0.15V, and then connect the positive end of the voltmeter It is connected to the positive electrode of each single cell in turn, and the voltage between the cadmium electrode and the positive electrode of the single cell is measured to be about 2.10~2.15V. This is when the battery capacity is measured and the rated discharge capacity of the starting battery is tested. The difference between the measured voltage value of the negative electrode and the voltage value between the cadmium electrode and the positive and negative electrodes measured under normal conditions can be used to judge whether the decrease in battery capacity is caused by the positive plate or the negative plate. . In addition, by drawing a curve on the changes of the positive and negative potentials of the battery and the terminal voltage and changes, it can improve the product quality level, analyze the cause of the failure and provide data.
For example, most of the single-cell voltage of a battery that has been discharged at a rate of 20h drops to 1.75V. If the voltage between the cadmium electrode and the positive plate in the battery is less than 1.95V, it means that the battery has a bad positive plate. If the voltage value between the cadmium electrode and the negative plate is higher than 0.3V, it indicates that the negative plate is defective. If the battery is discharged to a single cell voltage of 1.75V, the measured voltage between the cadmium electrode and the positive plate is 1.90V, and the voltage between the cadmium electrode and the negative plate is 0.15V, then according to the results, it can be seen that the positive plate is defective , and the battery cannot discharge the rated capacity. If the measured results are opposite, the negative voltage is 0.35V, and the positive voltage is 2.10V, it means that there is a problem with the negative plate.
If the battery is in good condition, judging from the value of the terminal voltage of the battery, the terminal voltage should not be lower than the specified value after discharge to the specified time. When discharging, the time specified by the standard cannot be reached, and the terminal voltage value is already lower than the specified value, which indicates that the battery has serious problems, but it cannot be analyzed whether the problem is due to the positive plate, the negative plate or other reasons. caused.
The use of cadmium electrodes can make up for this deficiency. Practice has proved that using cadmium pressure can test the battery capacity reduction caused by the positive plate or the negative plate. When measuring the discharge termination voltage of the battery, the positive cadmium voltage should not be lower than 1.95V. The higher the value, the better. The cadmium voltage value of the negative plate should not be higher than 0.25V. The lower the value, the better.
In addition, for a fully charged battery, when it is open circuit, the cadmium voltage of the positive plate is about 2.1V, and the cadmium voltage of the negative electrode is about 0.1V, and the difference between the cadmium voltage of the positive and negative plates is equal to the electromotive force of the battery.
When using a cadmium rod to measure the positive and negative voltage of the battery, the following problems should be paid attention to;
1) Newly made cadmium rods must be treated as previously mentioned. For dry cadmium rods (not newly made), soak them in dilute sulfuric acid with a density of 1.100g/cm3 for more than 12 hours before use to corrode the oxide layer on the surface of the cadmium rods to stabilize the potential.
2) The internal resistance of the voltmeter used to measure the cadmium pressure must be greater than 1000Ω/V. Because the internal resistance of the voltmeter is small, the cadmium rod will have a large current passing through, which will affect the measurement accuracy invisibly.
3) The cadmium rod for measuring the cadmium pressure should be taken out of the battery immediately after the measurement, so as to avoid the cadmium rod will be decomposed during the charging process, which will cause the performance of the battery to decrease, self-discharge, and affect the life of the battery.

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