Gas evolution and electromotive force of lead-acid batteries

Gas evolution and electromotive force of lead-acid batteries

This chapter mainly introduces the gas evolution and electromotive force of lead-acid batteries
1.1 Gas evolution of lead-acid batteries
The chemical reaction equation of lead-acid battery charge and discharge:

It can be seen from the reaction equation that during the charging and discharging process of the battery, there are only two states of solid and liquid, and no gas is produced. The reaction expressed by the charging equation is only an effective reaction. The battery in use always produces gas during charging. This is the first time that O2 is precipitated on the positive electrode when the charging voltage rises to the decomposition voltage of water, while H2 and H2 are precipitated on the negative electrode. , Its volume is twice that of O2. Therefore, the negative root shedding caused by the gas swelling of the active material is much more serious than the positive electrode. During the discharge process, there should be no gas evolution, but we can often see gas evolution on the actual standard. This is due to the gas generated by the self-discharge reaction. Generally, there are two types of gas evolution caused by self-discharge.
The above reaction is extremely slow if there are no impurities in the electrolyte. Under the catalysis of impurities, the above-mentioned reaction rate will increase drastically by a million times. If you see that the gas output of a certain battery is obviously too large, the self-discharge of this battery must be very large. All the electrolyte replacement will reduce the impurities in the solution. However, it is impossible to remove all the impurities by simply changing the electrolyte.
It is precisely because of the outgassing of the battery that there is no completely sealed battery. The popular “sealed” batteries on the market all have a safety valve that releases gas. In the new national standard, the term “sealed battery” has been eliminated and replaced with “valve controlled battery”.

1.2 The electromotive force of lead-acid batteries

It can be seen from the reaction formula that every time the negative plate emits 2 electrons during discharge, the positive plate gets 2 electrons at the same time. The power of this electron transfer depends only on hydrogen ions (H+) and sulfate ions (SO42-) in the electrolyte. . The power value depends only on the concentration of the two ions. Generally speaking, the more sulfuric acid content in the solution, the greater the concentration of the two ions. During the discharge process, the ions formed by the sulfuric acid in the solution continuously react with the positive and negative electrodes, and the solid PbSO4 is formed after the SO42-reaction, so the content of H2SO4 in the solution becomes less and less, and the density becomes smaller and smaller. The decrease in density reduces the concentration of SO42-, so the motive force for the transfer of electrons from the negative electrode to the positive electrode is also reduced. And the lower the density of the electrolyte, the greater the resistance, which is why the voltage at the battery terminal that discharges electricity decreases.

In the commonly used range, the change law of its terminal voltage with density is:
In the formula, U——open circuit voltage of battery, V;
d——The density of electrolyte, g/cm3;
It can be seen from the above analysis that if a high no-load voltage is to be obtained, it can be achieved as long as the density is increased. If the battery is not charged, inject high-density sulfuric acid into the battery, and a high no-load voltage will be directly obtained. Some workers “repair” the pool in this way, and sulfuric acid is often contained in commercially available battery replenishers. Supplementing such electrolyte will undoubtedly accelerate the damage of the battery.

It can be seen from the reaction formula that one PbO2 molecule in the positive electrode, two H2SO4 molecules in the electrolyte, and one Pb molecule in the negative electrode can form a battery. This battery can exist in principle, but it can’t be done in practice. , There is no industrial value. In actual battery products, the positive electrode, negative electrode, and electrolyte of the battery are composed of a large number of materials ranging from grams to kilograms. That is to say, the inside of a battery is actually composed of many tiny batteries in parallel.

The electric quantity of one electron is e=1.6021773310-19C, and the electric quantity of 1A.H is Q=1A1h=1C/S3600s=3600C. 1A.h of electricity requires M electrons: M=Q/e=4.45051015.
By understanding these basic data, you can understand the electrochemical microstructure of the battery used.
The damage of a single cell battery in actual use is often caused by the damage of an internal tiny battery. In the circuit diagram, the symbol of a battery is actually connected by countless tiny batteries in parallel. Understanding this point is very important in battery damage analysis. Under the condition of parallel connection, one of the batteries fails, which will cause all the parallel batteries to fail. Normally, the damage of the battery is basically a small short circuit that occurs in a small part of it. Over time, the entire battery will fail.