How lead-acid batteries work
In the mid-19th century, the advent of lead-acid batteries solved the problem of random power consumption of some electrical equipment. However, after more than 100 years of development, its working principle has basically not changed. The chemical reaction formula of its normal charge and discharge is:
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 charging and discharging 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 (Pb O2) to lead sulfate (PbSO4), converts chemical energy into electrical energy to supply power to the negative, and the negative electrode changes from sponge lead (sponge Pb) to lead sulfate (PbSO4) , convert chemical energy into electrical energy to supply power to the load.
Of course, the above-mentioned charging or discharging process can only be achieved by the electrochemical reaction of the positive electrode and the negative electrode in the same equivalent and same state (such as charging state or discharging state). complete the above electrochemical reaction. It can be seen from this that if the positive plate in a lead-acid battery is good and 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 and one is less, the extra part is a waste, and each substance participating in the electrochemical reaction is matched with another substance in a different amount, one substance can be used for one ampere hour The amount of electricity that is converted into chemical energy and stored in the substance is called electrochemical equivalent (that is, 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 electrochemical reaction formula (3-8).
When the electrochemical reaction (3-8) proceeds from left to right, it is the discharge reaction of the lead-acid battery. When the electrochemical reaction (3-8) proceeds from right to left, it is the charging reaction of the lead-acid battery.
It can be seen from the electrochemical reaction formula (3-8) that when the lead-acid battery is discharged, the positive electrode must have 1 gram molecular weight of lead dioxide, the negative electrode must have 1 gram molecular weight of spongy lead, and there should be 2 A gram molecular weight of sulfuric acid participates in this discharge process to proceed smoothly. Using Faraday’s constant in Faraday’s law, through the electrochemical reaction formula (3-8), after calculation, it is known that the electrochemical equivalent of lead dioxide is 41.46 g, and the electrochemical equivalent of sponge lead is 33.87 g. That is to say, in order for the lead-acid battery to discharge 1A·h of electricity, the positive electrode must have 41.46 g of lead dioxide active material, and the negative electrode must have 33.87 g of spongy lead active material in the presence of a sufficient amount of sulfuric acid. To make the lead-acid battery discharge 100 A·h of electricity, the positive electrode must have 4146 g of lead dioxide, and the negative electrode must have 3387 g 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 say that a lead-acid battery with a large weight is better than a lead-acid battery with a small weight when purchasing a lead-acid battery. 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–, 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.