Lead-acid battery charging and discharging knowledge


1. Charge and discharge characteristics

The characteristics of Lead-acid battery during charging and discharging, including the change of terminal voltage over time and the influence of potential changes and internal resistance during charging and discharging. When charging, the voltage rises sharply at first, then rises slowly, and finally rises rapidly; while when discharging, the voltage starts to fall rapidly, then falls slowly, and finally falls rapidly.

When charging, the battery terminal voltage can be expressed as U=E+△φ++^-+IR.

During discharge, it can be expressed as U=E-^φ+-^Ф–IR.

Among them, U represents the terminal voltage of the battery, and ^φ+ represents the overpotential of the positive plate.

^Ф- represents the overpotential of the negative plate

I represents charge and discharge current

R represents the internal resistance of the battery

When charging, the voltage rises sharply first, then slowly, and finally rapidly.

When discharging, the voltage starts to drop quickly, then drops slowly, and finally drops sharply.


2. During the discharge process

At the beginning of discharge,the concentration of sulfuric acid on the surface of the active material decreases, causing the battery terminal voltage to drop sharply.

As the sulfuric acid concentration on the surface of the active material decreases, the difference between the concentration of the bulk solution and the surface concentration promotes the diffusion process.Supplements the sulfuric acid on the surface of the active material, and makes the terminal voltage relatively stable. However, the overall sulfuric acid concentration is still slowly decreasing, resulting in voltage decline slowly.

At the same time, the positive and negative active materials gradually transform into lead sulfate, resulting in a reduction in the porosity of the active material, making it difficult to diffuse sulfuric acid, and the formation and poor conductivity of lead sulfate lead to an increase in the resistance of the electrolyte. Finally, near a certain point on the discharge curve (point G), the voltage drops sharply and reaches the termination voltage.

3. Voltage changes during charging

When charging begins

The concentration of sulfuric acid on the surface of the active material increases rapidly, causing the battery terminal voltage to rise sharply.

As charging proceeds

The active material is gradually converted into lead dioxide and lead, the porosity increases, and the terminal voltage rises slowly.

When the charging is near the end

The lead sulfate on the plate is insufficient, the polarization of the electrochemical reaction increases, and the potential change of the positive and negative electrodes causes a large amount of oxygen to precipitate.This causes the voltage on the charging curve to rise sharply, gas to evolve, and the water electrolysis process to proceed.

4. Charge and discharge characteristic curve

During the charging process

The electrode potential of the positive plate changes greatly because the water produced and lost during charging causes the sulfuric acid concentration on the surface of the active material to change greatly.

At the beginning of charging

The electrolyte concentration rises slowly. It is not until the negative electrode potential changes from positive to negative and the battery voltage changes sharply that the electrolyte density rises sharply.

During the discharge process

The density of sulfuric acid decreases and the concentration of the electrolyte decreases. During discharge, the density at the upper end decreases rapidly, resulting in the phenomenon that the sulfuric acid is dilute at the top and concentrated at the bottom.

During charging, more sulfuric acid is generated on the surface of the electrode plate, the local density increases.The electrolyte flows by itself, and the density of the electrolyte rises slowly. When the battery charging voltage increases significantly, a large amount of gas precipitates to stir the electrolyte, and the density of the electrolyte increases significantly.

During the charge and discharge process, the degree of change in electrolyte density depends on the amount of electrolyte loaded. Different types of batteries, such as stationary and automotive types, have different changes in electrolyte density.


5. The influence of charging current on terminal voltage

When charging with high current, the voltage rises quickly and the final voltage is high. When discharging with high current, the voltage drops quickly and the final voltage is low.

When discharging at different temperatures, the characteristic curve will also change. At low temperatures, the viscosity of the electrolyte increases, the fluidity is poor. And the diffusion is slow, resulting in a decrease in the average voltage during discharge and an increase in the charging voltage. On the contrary, at high temperatures, the average discharge voltage increases and the charging voltage decreases.

In Conclusion

When charging, the voltage gradually increases, and when discharging, the voltage gradually decreases. As the charge-discharge characteristic curve includes stages of sharp changes and slow changes, high-current charging causes the voltage to rise quickly, and high-current discharge causes the voltage to drop quickly. The voltage will also be affected when the temperature changes. The voltage decreases at low temperatures and increases at high temperatures.






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