18650 battery 4800mah

　　Research on improving the cycle life of 18650 battery 4800mah

　　In recent years, with the huge cost pressure on established battery manufacturers in Europe and the United States and the continued rise in international lead prices, these internationally renowned companies have built factories in China, or simply purchased batteries in China for OEM sales. While this trend has brought considerable profits to domestic enterprises, it has also brought a devastating blow to itself due to product quality problems of some domestic enterprises. The significant difference between the battery product quality of some domestic companies and well-known foreign companies is mainly that the battery service life, especially the cycle life, does not meet the requirements.

　　The end of life of 18650 battery 4800mah is mostly due to insufficient capacity, and for batteries, their cycle life is a key indicator among many indicators. For valve-regulated 18650 battery 4800mah, the recognized measures to extend battery cycle life are adding long-lasting additives to the lead paste formula, using high-tin and low-calcium alloys, high-temperature solidification of the plates, increasing assembly pressure, etc.

　　But even if all the above measures are taken, the battery life produced may not reach the level of foreign battery life. Especially as cost pressure increases, many domestic small and medium-sized enterprises continue to reduce the plate thickness of the battery and increase the proportion of electrolyte in order to reduce production costs and improve the high-current discharge performance of the battery. This has a negative impact on the overall performance of the battery, especially In terms of cycle performance, it is undoubtedly a method of killing the goose that lays the egg.

　　The research focus of this project is to study the effects of the thickness of the positive and negative plates of the battery, the specific gravity of the electrolyte and different charging conditions on the initial capacity, national standard cycle life and 1h rate of the battery when all the above measures to extend the cycle life of the battery are taken. Effect of 100% DOD cycle life.

　　1Test content

　　In view of the above research content, battery structures with two plate thicknesses and four electrolyte specific gravity were used to produce 12V and 7Ah batteries for various performance tests.

　　1.1 Battery manufacturing

　　The battery is manufactured using two structures: 3 positive and 4 negative (the thickness of the positive plate is 3.6mm) and 4 positive and 5 negative (the thickness of the positive plate is 2.8mm). After the battery is initially charged according to the process, the battery weight and internal resistance are tested. The weight of the two structures of the battery is approximately 2.60kg and 2.45kg respectively, and the internal resistance is approximately 19mΩ and 17mΩ respectively. Afterwards, the initial capacity and two cycle lives of various types of batteries were tested respectively. In order to clearly express the characteristics and test items of various types of orthogonal test batteries, the orthogonal test conditions of various types of batteries are shown in Table 1.

　　1.2 Initial performance test

　　After the various types of batteries in Table 1 are produced, the 20h and 3C capacities of each type of battery are tested respectively for comparison and assessment as the initial battery capacity.

　　1.3 National standard cycle life

　　After the battery passes the initial capacity test, the lifespan of the Category 6 batteries in Table 1 is tested according to the national standard for small valve-regulated sealed 18650 battery 4800mah (standard code: GB/T196391.1-2005) 5.18 life test method.

　　1.4 Constant current limit voltage (LV) life test

　　According to the two test conditions of various types of batteries, different constant current and voltage limiting charging methods were used to test the 1h rate discharge 100% DOD cycle life of the 4 types of batteries in Table 1.

　　1.5 Battery Anatomy Analysis

　　Dissect the battery that has ended its life in the previous test step, use chemical methods to analyze the positive and negative active material content, negative lead sulfate content, acid specific gravity, etc., and determine the reason for the end of the battery's life. [page]

　　2 Analysis and discussion of test results

　　2.1 Initial battery performance test

　　After the battery production is completed, randomly select 3 batteries of each type, test the 20h rate discharge and 3C discharge of the battery according to the national standard method, and average the discharge data of the 3 batteries, as shown in Table 2.

　　It can be seen from the data in the table that all types of battery discharge tests can meet the national standard requirements of 20h rate discharge for 20h and 3C discharge for 7min. However, as the plates become thinner and the electrolyte specific gravity increases, both the 20h rate capacity and the 3C capacity show an increasing trend, especially the 3C discharge time.

　　2.2 National standard cycle life

　　According to the initial capacity testing of various types of batteries, the battery cycle life test method specified in Article 5.18 of the national standard for small valve-regulated 18650 battery 4800mah was used to test batteries with 4 acid specific gravity and 4 positive and 5 negative plate structures with 3 positive and 4 negative plates. Two batteries of plate structure with acid specific gravity of 1.29 and 1.31 were selected for cycle life test. The test data are shown in Table 3.

　　In order to understand the impact of the electrolyte specific gravity and plate thickness on the battery cycle life, the data in the table were classified and made into Figure 1 (the influence of the national standard cycle life of the 3 positive and 4 negative structure battery with different electrolytes) and Figure 2 (different electrolytes). Effect of plate thickness on battery cycle life).

　　Figure 1 Effect of electrolyte density on battery national standard life

　　Figure 2 The impact of different plate thicknesses on battery national standard cycle life

　　It can be seen from Table 3, Figure 1 and Figure 2 that the national standard cycle life of the above types of batteries is greater than the standard requirement of 300 times. However, as the specific gravity of the electrolyte increases and the plate thickness decreases, the battery cycle life shows a significant downward trend.

　　2.3 Constant current limiting voltage (LV) life test

　　According to the conditions of the above tests, two types of batteries, A1B2 and A1B3, with a 3 positive and 4 negative plate structure and acid specific gravity of 1.29 and 1.31 respectively, were used to conduct a 100% DOD life test at a rate of 1 hour. The charging method is constant current and voltage limiting, the constant current value is 0.15C, and the voltage limiting values are 14.2V/unit, 14.5V/unit and 14.8V/unit respectively. For each type of battery, three batteries were tested using various charging methods. After the test, the average number of cycles of the three batteries was calculated and listed in Table 4.

　　It can be seen from the data in Table 4: For a battery with an electrolyte specific gravity of 1.29, as the charging voltage limit value gradually increases, the battery cycle life gradually decreases. When charging with a voltage limit value of 14.2V/battery, the cycle life longest. For batteries with an electrolyte specific gravity of 1.31, the battery charged with 14.5V/only voltage limit has the longest life, and the battery charged with the other two voltage limit values has a significantly shorter life.

　　2.4 Battery anatomy analysis

　　After the battery life of each group of batteries undergoing LV testing has expired, a representative battery will be taken from each group, and the active material content of the positive and negative electrodes, the lead sulfate content of the negative electrode, and the specific gravity of the electrolyte in the separator will be dissected and analyzed, and the cause of battery failure will be preliminarily determined. See Table 5 for details.

　　Analyzing the data in Table 5, combined with the cycle life data in Table 4, it can be concluded that the main reasons for the termination of the cycle life of the battery with an acid specific gravity of 1.29 are the muddying of the positive active material and the corrosion of the positive grid during the charging process. and water loss, etc. During the charging process, the battery loses water and also increases the specific gravity of the electrolyte. For batteries with an acid specific gravity of 1.31, the phenomenon and trend are basically the same, except that charging at 14.2V/cell can easily lead to insufficient charging of the battery and sulfation of the negative electrode.

　　3Conclusion

　　By conducting cycle life tests on batteries with different plate thicknesses and adding electrolytes with different specific gravity, the initial capacity, national standard cycle life and different constant current and voltage limiting charging control conditions, as well as an anatomical analysis of the batteries at the end of cycle life, the following results were obtained Conclusion: The thicker the battery plate, the lower the specific gravity of the electrolyte, and the lower the initial capacity of the battery. Especially the high-current discharge performance will be reduced more obviously, but the cycle life of the battery will be significantly longer.

　　For batteries with a large proportion of electrolyte, a reasonable selection of the voltage limit value for constant current and voltage-limited charging can avoid sulfation of the negative electrode and sludge of the positive electrode, and extend the cycle life of the battery.

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