Influence Of Plate Porosity And Real Surface Area On Battery Discharge Capacity
The so-called porosity of the electrode plate refers to the percentage of the ratio of the total volume of pores in the active material on the electrode plate to the apparent total volume of the lead paste. The influence of porosity on the discharge capacity of lead-acid batteries is dual.
On the one hand, the larger the porosity, the easier the diffusion and the higher the capacity; If it is too large, it will also shorten the life of the battery. Therefore, the porosity of the electrode plate should be reasonably selected during the production of the electrode plate, and the process guarantee capability should be strictly controlled to produce the electrode plate with the best porosity suitable for different use conditions. The porosity of the active substance is determined by the apparent specific gravity of the lead paste and the pressing operation during coating. Generally, the porosity of the positive plate is controlled at about 55%, and the negative plate is controlled at about 60%.
It is important to pay attention not only to the porosity but also to the size of the pores and their distribution. This is because the volume of the active material will increase after discharge. If the hole is too small, it is easy to be blocked and the active material cannot be fully utilized. When the hole is too large, it will affect the connection between the active materials. In valve-regulated sealed lead-acid batteries , and also affects the recombination efficiency of the gas due to the passage of the gas during recombination.
The discharge capacity of the electrode plate with the same total amount of active materials is not only related to the porosity of the electrode plate, but also to the real surface area of the electrode plate. The so-called real surface area, that is, the surface area of the active material that can be in direct contact with the electrolyte, is much larger than the area calculated according to the geometric size of the electrode plate. the surface area of the pores. Because for the electrode plate containing an active material, the smaller the active material particle, the larger the surface area, and the particles of the active material of the lead-acid battery are very small, so the actual surface area of the electrode plate made by them is larger than the surface area of the electrode plate. The apparent surface area is hundreds to thousands of times larger.
When the real surface area is large, the diffusion cross-sectional area and the reaction area increase, so the concentration polarization and electrochemical polarization decrease during discharge, the battery voltage drops slowly, and the discharge capacity increases. Therefore, for a lead-acid battery with a certain volume and weight, in order to increase its discharge capacity, it can be obtained by increasing the real surface area of the plate. In addition, the real surface area is also related to the size of the pores. Divide a large pore into several small pores. Although the total pore volume remains unchanged, the real surface area increases. However, it should be noted that the small holes are easy to be blocked so the surface area of this part cannot be used.