Batteries belong to the specialty of electrochemistry. Because of the comprehensive professional knowledge, technology diffusion is difficult. This chapter introduces the charge-discharge reaction process of lead-acid batteries. The user of the battery needs to master the necessary knowledge in order to provide the battery with reasonable conditions of use. According to the actual situation in this aspect, this chapter introduces some relevant physics and chemistry knowledge. These basic knowledge provide some basis for understanding the battery maintenance process.
What is the charge-discharge reaction process of lead-acid batteries?
When we use electricity, the battery can convert chemical energy into electrical energy and release it, and then when we charge the battery, it can convert electrical energy into chemical energy for storage. The reversibility of this energy conversion can be carried out many times, so we call lead-acid batteries as secondary batteries.
When we decompose a lead-acid battery, we can see that they are all composed of a positive electrode, a negative electrode, a separator, an electrolyte and an outer shell, and the most important ones are the positive electrode, the negative electrode, and the electrolyte.
The negative electrode of a lead-acid battery is composed of pure lead (Pb) powder. When the battery is fully charged, it is in a sponge state and is silver-gray. After being exposed to oxygen, it will quickly turn to blue-gray. Its positive electrode is reddish brown when fully charged. Its chemical composition is lead dioxide (PbO2). The surface of the electrode plate is an irregular porous electrode. The electron micrograph of the surface is shown in Figure 1-1. The electrolyte is an aqueous solution of sulfuric acid (H2SO4).
During charging and discharging, chemical reactions occur on the positive and negative plates. Because this chemical reaction is accompanied by the action of electric current, we call this reaction an electrochemical reaction.
In the charge-discharge cycle, the charge-discharge process of a lead-acid battery is shown in Figure .
Figure ：Lead-acid battery charge and discharge process
Expressed by chemical reaction equation:
PbO2+2H2SO4+Pb=PbSO4+2H2O+PbSO4
The corresponding state of the plate is:
Positive lead dioxide + sulfuric acid + negative lead = positive lead sulfate + water ten negative lead sulfate
There are many types of lead-acid batteries on the market, such as silicone batteries, silicon energy batteries, lead cloth batteries, lead-plastic batteries, lead-carbon batteries, water batteries, etc., all of which are lead-acid batteries. For lead-acid batteries, the electrochemical reactions follow the above equations. Its basic feature is that the no-load voltage is 2 to 2.2V.
Different metals in the periodic table can form batteries due to different electrode potentials, but there are only a few combinations that have industrial value. Lead-acid batteries are the only battery made of one element. Other batteries require two metals. Their no-load voltages are 1.5V for zinc-manganese batteries, 1.2V for nickel-hydrogen batteries, and 3.6V for lithium-ion batteries.
Independence of charge and discharge response
It can be seen from the reaction equation that during the discharge reaction, the positive and negative plates of the battery must participate in the electrochemical reaction at the same time. When two electrons flow through the external circuit, there must be a Pb molecule and a PbO2 molecule to participate in the reaction and become PbSO4 at the same time. At this time, the battery outputs electric energy to the outside. During charging, since the energy required for the reaction is provided from the external charging circuit, the positive and negative charging reactions are not necessarily performed simultaneously. As can be seen in Figure , the reaction and displacement of the SO42- and O2- ions when the positive plate is charged is not necessarily related to the reaction and displacement of the negative electrode SO42-. As long as the charging current flows through the external circuit, the positive and negative charging reactions proceed separately. In the early stage of charging, the PbSO4 on the positive and negative plates is converted into PbO2 and Pb in proportion. Roughly speaking, this kind of reaction based on Bichuang continues until the report with a greater degree of damage should be completed. . The subsequent continuous charging will only be carried out on the other unipolar. Obviously, the charging efficiency in the previous stage is higher. In this stage, the battery has a small gas output and a low temperature rise. In the latter stage, the charging efficiency gradually decreases, a part of the electric energy is consumed in the decomposition of water, and the battery exhibits an increase in gas volume and an increase in temperature.
Scattered batteries have the same degree of damage to the positive and negative plates. They are all single-pole damage to the positive plate or the negative plate. As a result, they are scrapped because they cannot be charged.