When the above electrochemical reaction formula is carried out from left to right, it is a discharge reaction of the battery. When the above electrochemical reaction mode is performed from right to left, it is a charging reaction of the battery.
It can be seen from the electrochemical reaction formula that when the battery is discharged, the positive electrode must have a molecular weight of lead dioxide, the negative electrode must have a gram molecular weight of the sponge lead, and at the same time, there should be two grams of molecular weight sulfuric acid. Participation in this discharge process can proceed smoothly. Using the Faraday constant in Faraday's law, through the above electrochemical reaction equation, it is calculated that the electrochemical equivalent of lead dioxide is 41.46g/s, and the electrochemical equivalent of spongy lead is 33.87s/Ah. That is to say, in order for the VRLA battery to discharge an ampere-hour of electricity, the positive electrode must have 41.46g of lead dioxide active material, and the negative electrode must have 33.87g of sponge-like lead active material in the presence of a sufficient amount of sulfuric acid to achieve the desired. To make the VRLA battery discharge 100Ah of electricity, the positive electrode must have 4146g of lead dioxide, and the negative electrode must have 3387g of sponge lead. This explains in principle how the battery's capacitance is determined by the amount of active material. This is also the reason why users buy a battery, why the weight of the battery is better than the weight of the battery is good. Of course, the electrochemical equivalents listed here are only a theoretical value. In addition to the same working principle as the electrochemical reaction of an open lead-acid battery, the VRLA battery has a different working principle than the open lead-acid battery, which is the cathode absorption principle. The so-called cathode absorption principle refers to the battery. During charging, especially at the end of charging, the positive electrode will generate oxygen. Since the VRLA battery is completely sealed, the generated gas will not be released to the outside of the battery through the opening at any time like the open battery, and the generated gas will be in the battery. Accumulation in the tank. As the amount of gas accumulated inside the battery increases, the pressure inside the battery gradually rises. Because of the internal pressure inside the battery, the oxygen generated by the positive electrode will run to the negative electrode. Since the oxygen atom is formed on the positive electrode, and the oxygen atom has strong oxidizing property, the oxygen atom with strong oxidizing ability runs to the negative electrode, and the negative electrode also has a very active sponge which is formed immediately upon charging. The lead is oxidized to form lead oxide, which in turn reacts with sulfuric acid to form lead sulfate and water. Lead sulfate is the product of the negative discharge. Lead sulfate is formed into a sponge lead when it is charged. The spongy lead absorbs the oxygen produced by the cathode. The formation of lead oxide, so repeated this reaction, the oxygen generated on the positive electrode is absorbed by the negative electrode, no matter how to charge no oxygen will be generated, the internal pressure of the battery will not continue to rise, not to worry about the battery will An explosion occurred. In order to prevent the battery from exploding due to the increase of internal pressure due to the accumulation of gas in the special case, a safety valve is specially provided in the upper cover of the battery when the internal pressure of the battery reaches a certain value. It will automatically open, release a certain amount of gas and reduce the internal pressure, the safety valve will automatically close again. The above is the principle of cathode absorption of VIIIIA batteries. Just because the cathode absorption principle of this battery was discovered and invented, the open lead-acid battery can be made completely sealed, and the VRLA battery can be introduced.
Of course, in order to maintain the cathode absorption principle of the VRLA battery, the first prerequisite is that the battery must be sealed, not sealed, there is no internal pressure inside the battery, and the oxygen generated by the positive electrode cannot be run to the negative electrode. Absorption, oxygen will run out, running away oxygen is equal to the water inside the battery run away, the battery loses water, it should be hydrating, need to replenish water is not called VRLA battery, then become an open battery It is. It can be seen that the sealing performance of VRLA battery is a key technical indicator. Users should pay great attention to this problem when selecting batteries. Even if there is a slight leak or liquid leakage, it will directly affect the battery usage. life. If such a battery appears in the battery pack, it will affect the overall performance of the entire battery pack because the battery first becomes a backward battery, and also causes an imbalance in the voltage of each battery in the battery pack to form a vicious circle. The reason why the YD/T799 standard of the Ministry of Posts and Telecommunications specifies that the gas recombination rate of the battery is above 95% is the reason.
Of course, in order to make the cathode absorption of the VRLA battery well, it is necessary to ensure that its gas recombination rate is high, and the generated gas basically generates water and returns to the battery, except that airtightness is an important problem. It should also consider whether the measures associated with it are effective. For example, in terms of structure, the VRLA battery must be lean liquid, leaving enough space and passage for the oxygen generated by the positive electrode to quickly and smoothly run to the negative electrode and be absorbed by the negative electrode. This is why the VRLA battery does not have excess electrolysis. The reason for the liquid. Another example: the ultra-fine glass fiber separator used should have a large enough porosity to ensure that the oxygen generated by the positive electrode can be absorbed through the small å‘L of the separator to the negative electrode. Therefore, the quality of the separator used in VBLA batteries is also a critical issue.
When the oxygen generated by the positive electrode of the VRLA battery is absorbed by the negative electrode, the opening can be made into a sealed battery. Then, does the hydrogen generated during the charging of the negative electrode still not exist? Is the battery still dangerous to lose water and explode? The problem is that scientists have changed the anode alloy formulation and used new alloy materials (such as lead-calcium alloy) to increase the potential of hydrogen on this material (to generate hydrogen from electrons). Potential) When the charging voltage reaches a certain value, hydrogen ions are discharged on the cathode to generate hydrogen. Due to the use of lead-calcium alloy, hydrogen ions are not discharged when the charging voltage reaches the original value, and no hydrogen gas is generated. However, no matter how to change the alloy formulation, no matter how to increase the overpotential of hydrogen, when the charging voltage reaches the potential of hydrogen ion discharge, hydrogen is always generated. Why do manufacturers give their batteries a floating charge voltage within a certain range? The reason is to control the generation of hydrogen to prevent the battery from losing water.
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