In the mid-19th century, the advent of lead-acid batteries solved the problem of random power consumption of some electrical equipment. But after more than 100 years of development, its working principle has basically not changed. The chemical equation of its normal charge and discharge is:
PbO₂+2H₂SO₄+Pb↔2PbSO₄+2H₂O

The above normal charge and discharge chemical equations are idealized principle equations. It seems that as long as there is no mechanical damage, a lead-acid battery can be used endlessly to complete the charge and discharge process.

During charging, after the positive electrode is converted from lead sulfate (PbSO₄) to lead dioxide (PbO₂), the electrical energy is converted into chemical energy and stored in the positive plate; the negative electrode is converted from lead sulfate (PbSO₄) to spongy lead (sponge-like Pb) After that, the electrical energy is converted into chemical energy and stored in the negative plate.

During discharge, the positive electrode changes from lead dioxide (PbO₂) to lead sulfate (PbSO₄,), which converts chemical energy into electrical energy to supply power to the load, and the negative electrode changes from spongy lead (sponge Pb) to lead sulfate (PbSO₄). The chemical energy is converted into electrical energy to supply power to the load.

Of course, the above-mentioned charging or discharging process can only be realized by the electrochemical reaction of the positive electrode and the negative electrode in the same state (such as charging or discharging state) with the same equivalent at the same time. The above electrochemical reaction is performed alone. It can be seen from this that if the positive plate in a lead-acid battery is good, but the negative plate is broken, it means that the lead-acid battery has become a scrap lead-acid battery. Similarly, if the negative plate in a lead-acid battery is good and the positive plate is broken, the lead-acid battery is also a scrap lead-acid battery. In addition, the amount of substances that can participate in energy conversion (the amount of active substances) in the positive plate and the amount of substances that can participate in energy conversion (the amount of active substances) in the negative plate must match each other. If there is no match, one is more, one is less, the extra part is a waste, and each substance participating in the electrochemical reaction matches another substance in a different amount, and one substance can convert an ampere The amount of the substance that is converted into chemical energy and stored in an hour is called electrochemical equivalent (ie, the equivalent amount of substance that converts electrical energy and chemical energy into each other). The electrochemical equivalent of each active material is calculated from its electrochemical reaction equation. The whole content of the working principle of the lead-acid battery mentioned above (including the electrochemical equivalent) can be expressed by the following electrochemical reaction equation:
PbO₂+Pb+2H₂SO₄↔2PbSO₄+2H₂O

When the above electrochemical reaction formula proceeds from left to right, it is the discharge reaction of the lead-acid battery. When the above electrochemical reaction formula proceeds from right to left, it is the charging reaction of the lead-acid battery.

It can be seen from the above electrochemical reaction formula that when the lead-acid battery is discharged, the positive electrode must have lead dioxide with a molecular weight of 1g, the negative electrode must have a sponge lead with a molecular weight of 1g, and sulfuric acid with a molecular weight of 2g should be involved. process can go smoothly. Using Faraday’s constant in Faraday’s law, through the above electrochemical reaction equation, it is known after calculation that the electrochemical equivalent of lead dioxide is 41.46g/Ah, and the electrochemical equivalent of spongy lead is 33.87g/Ah. That is to say: to make the lead-acid battery discharge 1Ah of electricity, the positive electrode must have 41.46g of lead dioxide active material, and the negative electrode must have 33.87g of spongy lead active material, and it can be achieved in the presence of a sufficient amount of sulfuric acid. To make the lead-acid battery discharge 100Ah of electricity, the positive electrode must have 4146g of lead dioxide, and the negative electrode must have 3387g of spongy lead. This explains in principle that the electrical capacity of lead-acid batteries is determined by the amount of active substances. This is also the fundamental reason why users who buy lead-acid batteries say that a lead-acid battery with a larger weight is better in quality than a lead-acid battery with a smaller weight. Of course, the electrochemical equivalents listed here are only theoretical values.

In fact, the lead-acid battery will have gas evolution during charging, because when it completes the normal charging and discharging process, along with many other chemical reactions, the electrolyte contains Pb⁺, H⁺, HO^–, SO₄^2-, etc. Charged ions, especially at the end of charging, when the positive and negative electrodes of the lead-acid battery are reduced to PbO₂ and Pb respectively, some H⁺ and HO^– will generate H₂ and O₂ gases in the charged state, and the equations are as follows:
2H⁺+2HO^–=2H₂↑+O₂↑