The acid battery refers to the battery whose electrolyte is an acidic aqueous solution. At present, the lead-acid battery is the most common application. A typical lead-acid battery consists of positive and negative plates, electrolyte and electrolytic cell, and its basic structure is shown in Figure 1. The active material of the positive plate of the lead-acid seedling battery is lead dioxide (PbO₂), the active material of the negative plate is gray spongy metal lead (Pb), and the electrolyte is a sulfuric acid solution with a concentration of 27% to 37%. When a metal is inserted into a solution containing a metal salt, a certain interfacial potential difference is generated due to the transfer of charged particles between the two phases, which forms the electromotive force of the lead-acid battery.

After a certain concentration of electrolyte is injected into the lead-acid battery, the atoms on the metal lattice of the active material of the positive plate are polarized and attracted by liquid-phase water molecules, and eventually part of the atoms are separated from the lattice and enter the solution in the state of hydrated ions. At the same time, the metal ions in the electrolyte are also partially adsorbed to the surface of the plate metal. The active material (Pb) on the surface of the negative plate dissolves to generate lead ions, and the reaction is as follows
Pb→Pb²⁺+2e

After lead ions (Pb²⁺) enter the electrolyte, the surface of the negative plate accumulates excess electrons and becomes negatively charged. The Pb²⁺ in the electrolyte will be attracted and distributed on the surface of the negative plate. When the Pb²⁺ leaving the negative plate and the Pb²⁺ deposited on the negative plate are equal, dynamic equilibrium is reached, and the negative electrode no longer dissolves. Thus, an interface layer similar to a capacitor is formed at the interface layer between the negative plate and the electrolyte. Due to the opposite potential of the double layers, there is a certain potential difference between the double layers, the positive ions in the electrolyte are adsorbed by the negative plate, and the potential difference generated by the electric double layer at the interface forms the negative potential of the battery (as shown in Figure 2). Show).

At the same time, lead dioxide (PbO₂) on the positive plate of the lead-acid battery reacts with the water in the electrolyte to generate lead hydroxide [Pb(OH)₄]. Lead hydroxide is unstable and dissociates immediately to generate tetravalent lead ions. The reaction process is as follows:
PbO₂+2H₂O→Pb(OH)₄
Рb(ОH)₄→Pb⁴⁺+ 4OH^–

The positive ion Pb⁴⁺ remains on the positive plate, and the negative ion OH^– enters the solution. Due to the accumulation of excess positive ions on the surface of the positive electrode plate, it is positively charged, and the interface electric double layer formed on the surface of the electrode plate forms the positive electrode potential of the battery. Obviously, when the positive and negative plates and the electrolyte form a battery, the positive and negative electrode potentials are formed on the positive and negative plates respectively. This electrode potential is caused by ion diffusion, which is a thermodynamic irreversible process. Once the concentration of the electrolyte is determined, the electrode potential is determined accordingly. The potentials of all interface electrodes on the positive and negative plates constitute the electromotive force of the entire battery.

Lead-acid batteries are cheap, easy to use, easy to maintain, and rich in raw materials. They can be mass-produced and used in large quantities. The disadvantage is that the volume is large, the efficiency is greatly affected by the ambient temperature, and lead is a toxic substance, which is dangerous to a certain extent. At present, the batteries used in photovoltaic power generation systems are still mainly lead-acid batteries.