In the process of using a lead-acid battery, the most important concern is its external electrical characteristics, especially its charge-discharge curve and rated capacity.
The various characteristics of the battery are represented by the characteristic curve of the battery. When studying the charging curve, it is generally in the form of current limiting and constant voltage, the charging voltage remains unchanged, and as the battery voltage increases, the current gradually decreases; the discharge curve is carried out at a certain hour rate, generally taking the nominal hour rate of the battery.
In the early stage of charging, since lead sulfate is converted into lead dioxide and velvety lead, sulfuric acid is generated, so the concentration of sulfuric acid on the surface of the active material increases rapidly, and the terminal voltage of the battery rises sharply. When it reaches a certain level, due to diffusion, the concentration of sulfuric acid on the surface of the active material and in the micropores no longer rises sharply, and the rise of the terminal voltage of the battery is relatively slow. In this way, the active material is gradually converted from lead sulfate to lead dioxide and lead, and the pores of the active material are gradually enlarged, and as the charging progresses, the end point of the electrochemical reaction is gradually approached. When there is not much lead sulfate on the electrode plate, and the Pb2+ required for electrochemical oxidation and reduction through the dissolution of lead sulfate is extremely lacking, the polarization of the reaction increases the side reaction on the positive electrode, that is, the oxygen evolution process occurs preferentially when the input power is about 70%, and the voltage at the upper end of the charging curve increases significantly. When the charge reaches 90%, the side reaction on the negative electrode, that is, the hydrogen evolution process occurs. At this time, the terminal voltage of the battery is close to full scale, and a large amount of gas is precipitated on the two poles, and the electrolysis process of water is carried out, and the terminal voltage reaches a new stable value. Its value depends on the overpotential of oxygen and hydrogen, which is normally constant at about 2.6V. When there are impurities with lower overpotential than hydrogen, such as Fe, Cu, Sb, etc., the charging voltage is reduced, and the water electrolysis process is carried out before reaching 2.6V, and the active material is often not completely converted.
Before discharge, the concentration of sulfuric acid in the micropores of the active material is the same as that of the bulk solution outside the plate, and the open circuit voltage of the battery corresponds to this concentration. As soon as the discharge starts, the sulfuric acid at the surface of the active material (including the inner surface of the pores) is consumed, and the sulfuric acid concentration decreases immediately, while the diffusion of the sulfuric acid main solution to the electrode surface is slow, and the consumed sulfuric acid cannot be immediately compensated, so the sulfuric acid concentration at the surface of the active material continues to decline. What determines the electrode potential is the concentration of sulfuric acid at the surface of the active material, which results in a significant drop in the battery terminal voltage. As the concentration of sulfuric acid at the surface of the active material continues to decrease, the concentration difference between the active material and the host solution increases, which promotes the diffusion of sulfuric acid to the electrode surface, so most of the sulfuric acid on the surface of the active material and in the micropores can be supplemented by the diffused sulfuric acid, so the sulfuric acid concentration on the surface of the active material changes slowly, and the battery terminal voltage is relatively stable. However, due to the decrease of the total sulfuric acid concentration consumed and the consumption of active substances during the discharge process, the active area is continuously reduced, the real current density is continuously increased, and the overpotential is continuously increased, so the discharge voltage is still slowly decreasing with time.
As the discharge continues, the positive and negative active materials are gradually transformed into lead sulfate, and extend deep into the active material. The formation of lead sulfate reduces the porosity of the active material, aggravates the difficulty of diffusing sulfuric acid into the micropores, leads to poor electrical conductivity, and increases the internal resistance of the battery. These reasons finally lead to a sharp drop in the terminal voltage of the battery to reach the specified discharge termination voltage.
When charging and discharging, the principle of the battery reaction of lead-acid batteries can be found in the previous article.
During the charging and discharging process of lead-acid battery, the change of terminal voltage can be expressed as:
While charging: U=E+η++η–+IR
When discharging:U=E-η+-η–-IR
In the formula, U is the terminal voltage of the battery during charging and discharging; η+ is the overpotential of the negative plate; η– is the overpotential of the positive plate; l is the charge and discharge current; R is the internal resistance of the battery (Ω).
It can be seen from the discharge curve that the discharge curve basically consists of three parts; the voltage drops rapidly in a short period of time at the beginning of the discharge, then the voltage drops slowly, and the final voltage drops rapidly in a very short period of time. The longer the part 2 time, the higher the average voltage and the better its voltage characteristics.