Electrical Engineering ⇒ Topic : Formation of Plates of Lead-acid Cells
Formation of Plates of Lead-acid Cells
There are two methods of forming the plates of a lead-acid cell i.e., making the positive plate Pb02 and negative plate Pb. These are known after the names of their inventors viz
(i) Plante method
(ii) Faure' method.
(i) Plante method. In this method, we start with two plates of pure lead immersed in dilute sulphuric acid (H2SO4). When direct current is passed through this cell, the anode (i.e., plate connected to +ve terminal of d.c. source) is *covered with an exceedingly thin layer of Pb02. The cathode, however, remains unaffected. The cell is now discharged by connecting it to a load e.g.,galvanometer. As the cell delivers current, both the plates are covered with a coating of **lead sulphate (PbSO4). After some time, the cell is discharged and no more cun-ent can be drawn from it. If the cell is charged ***again, the anode is again covered with a layer (this time somewhat thicker than the initial charging) of Pb02 and the cathode gets transformed to lead (Pb). If this charge-discharge cycle is repeated again and again, the chemical reactions make deeper and deeper penetration into the plates. The result is that a sufficient thick layer of Pb02 is formed on the anode while the cathode gets transformed to spongy lead; spongy lead being lead in a somewhat porous condition. Plates produced in this way are called formed plates.
Plante' plates have a longer life and are less liable to disintegration. However, they suffer from two important drawbacks. First, the process of forming such plates is very time-consuming and expensive. Secondly, these plates are very heavy and cannot be used in portable batteries.
(ii) Faure method. In this method, the plates are constructed in the form of open metallic grids made of lead-antimony alloy. The openings in the positive grid are filled with a paste of red lead (Pb304) and that of a negative grid with litharge (Pb0). A single "forming" charge is sufficient to cause the necessary chemical changes. The paste on the positive plate is converted into Pb02 and that on the negative plate converted into spongy lead (Pb). It may be noted that the grid serves to hold the active materials in place and also to distribute the current evenly over the surface of the plates.The plates formed in this way are called pasted plates See Fig. (a).
Many advantages are claimed for pasted plates. First, such plates can be formed quickly. Secondly, pasted plates are much lighter than formed plates. For a given capacity, the weight of a pasted plate is only about one-third of that of a formed plate.
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