Explain construction and working of lead storage batteries?

Answer 1

CONSTRUCTION :

PlatesThe lead--acid cell can be demonstrated using sheet lead plates for the two electrodes. However such a construction produces only around one ampere for roughly postcard sized plates, and for only a few minutes.

Gaston Planté found a way to provide a much larger effective surface area. In Planté's design, the positive and negative plates were formed of two spirals of lead foil, separated with a sheet of cloth and coiled up. The cells initially had low capacity, so a slow process of "forming" was required to corrode the lead foils, creating lead dioxide on the plates and roughening them to increase surface area. Initially this process used electricity from primary batteries; when generators became available after 1870, the cost of production of batteries greatly declined.Planté plates are still used in some stationary applications, where the plates are mechanically grooved to increase their surface area.

Faure pasted-plate construction is typical of automotive batteries. Each plate consists of a rectangular lead grid alloyed with antimony or calcium to improve the mechanical characteristics. The holes of the grid are filled with a paste of red lead and 33% dilute sulfuric acid. (Different manufacturers vary the mixture). The paste is pressed into the holes in the grid which are slightly tapered on both sides to better retain the paste. This porous paste allows the acid to react with the lead inside the plate, increasing the surface area many fold. Once dry, the plates are stacked with suitable separators and inserted in the battery container. An odd number of plates is usually used, with one more positive plate than negative. Each alternate plate is connected.

The positive plates are the chocolate brown color of Lead(IV) Oxide, and the negative are the slate gray of "spongy" lead at the time of manufacture. In this charged state the plates are called 'formed'.

One of the problems with the plates is that the plates increase in size as the active material absorbs sulfate from the acid during discharge, and decrease as they give up the sulfate during charging. This causes the plates to gradually shed the paste. It is important that there is room underneath the plates to catch this shed material. If it reaches the plates, the cell short-circuits.

The paste contains carbon black, blanc fixe (barium sulfate) and lignosulfonate. The blanc fixe acts as a seed crystal for the lead--to--lead sulfate reaction. The blanc fixe must be fully dispersed in the paste in order for it to be effective. The lignosulfonate prevents the negative plate from forming a solid mass during the discharge cycle, instead enabling the formation of long needle--like crystals. The long crystals have more surface area and are easily converted back to the original state on charging. Carbon black counteracts the effect of inhibiting formation caused by the lignosulfonates. Sulfonated naphthalene condensate dispersant is a more effective expander than lignosulfonate and speeds up formation. This dispersant improves dispersion of barium sulfate in the paste, reduces hydroset time, produces a more breakage-resistant plate, reduces fine lead particles and thereby improves handling and pasting characteristics. It extends battery life by increasing end--of--charge voltage. Sulfonated naphthalene requires about one-third to one-half the amount of lignosulfonate and is stable to higher temperatures.

Practical cells are usually not made with pure lead but have small amounts of antimony, tin, calcium or selenium alloyed in the plate material to add strength and simplify manufacture. The alloying element has a great effect on the life of the batteries, with calcium-alloyed plates preferred over antimony for longer life and less water consumption on each charge/discharge cycle.

About 60% of the weight of an automotive-type lead--acid battery rated around 60 Ah (8.7 kg of a 14.5 kg battery) is lead or internal parts made of lead; the balance is electrolyte, separators, and the case.

WORKING :

• Inside a lead storage battery is a series of plates. Half of the plates are made of lead dioxide and the other half are made of a spongy form of lead. The plates are bathed in a solution of sulfuric acid which serves as an electrolyte (a chemical solution that conducts electricity). Two posts extend to the outside of the battery through sealed openings in the battery wall. One of the posts---the negative post---is connected to the lead plates, and the other---the positive post---to the lead dioxide plates.

• Sulfuric acid, chemically, is composed of two hydrogen atoms, a sulfur atom and four oxygen atoms. Inside the battery, the sulfuric acid molecules are in solution with water and so are dissociated. This means that the sulfur atom with the four oxygen atoms attached to it are in the water, separated from the hydrogen atoms. The sulfur with the four oxygen atoms is called a sulfate ion and has a double negative charge. The free hydrogen atom is called a hydrogen ion and has a positive charge. (An ion is simply an atom or a molecule with a positive or negative charge.)

• The battery performs its function through a series of chemical reactions involving these ions. In the condition described in the introductory paragraph, the battery is charged and has the capacity to supply an electric current through cables connected to the two posts. As the battery supplies electric current, the sulfuric acid reacts by giving up its sulfate ions to the lead and lead dioxide plates. This forms lead sulfate that deposits on the plates. While this process occurs, the sulfuric acid concentration is decreasing, and the battery is discharging.

• Supplying an electric current to the battery rather than drawing it out---such as what happens in an automobile when the engine is running---reverses the chemical reaction and the battery recharges. When recharging, the lead and lead dioxide plates give up sulfate ions to the electrolyte solution. This restores the amount of sulfuric acid in the electrolyte solution and restores the lead and lead dioxide plates to their charged condition.