Lead-Acid Batteries: Container and Active Materials | Electricity

In this article we will discuss about:- 1. Container of Lead-Acid Batteries 2. Plates of Lead-Acid Batteries 3. Active Materials.

Container of Lead-Acid Batteries:

The materials of which the battery containers are made should be resistant to sulphuric acid, should not deform or become porous, or contain impurities deterious to the electrolyte; of these iron and manganese are especially intolerable.

The containers of lead-acid cells are made of glass, lead lined wood, ebonite, hard rubber of bituminous compound, ceramic materials or moulded plastics and are sealed at the top (with three holes, one at each end for the posts and one in the middle for the vent plug and through which the electrolyte is poured and the gases escape out) to avoid spilling of electrolyte.

At the bottom of the container there are four ribs (prisms), on two of them rest the positive plates, and on the other two the negative plates. These prisms serve as supports for the plates and at the same time protect them from short circuits that would otherwise occur as a result of fall of the active material from the plates on to the bottom of the container.

Plates of Lead-Acid Batteries:

The plates of lead-acid cells are of diverse designs; however, they all consist of some form of grid made of lead and the active material. The grid serves as a support for the active material and is also necessary for conducting the electric current and as a means for distributing the current evenly over the active material. If the current is not distributed uniformly, an uneven change will take place in the volume of active material during charging and discharging processes, the result being that the active material will loosen and fall out and the plates warp.

Lighter grids are employed for automotive (or SLI) batteries, designed mainly for short discharges. For motive-power (traction type) and stationary batteries, which are designed for long term service, and in which the discharges are followed by idle intervals or are of long duration, heavier grids are employed.

The grids are cast of an alloy of lead and antimony. These are usually made with transverse ribs that cross the plate at right angle or diagonally. Grids for the positive and negative plates are often of the same design, but the grids serving the negative plates are made lighter, in as much they are not as essential to uniform conduction of the current and are less subject to corrosion than the grids of the positive plates.

Plates are of two types viz., Formed plates or Plante plates and pasted or Faure plates.

The positive plates are usually formed by the formation process. Plante plates are used largely for stationary batteries as these are heavier in weight and more costly than pasted plates but are more durable and less liable to lose active material by rapid charging and discharging. Plante plates have low capacity- weight ratio (12 to 21 Ah per kg of plate).

Faure process is much suitable for manufacturing of negative plates rather than positive plates.

The negative active material (sponge lead) is quite tough (not hard and brittle as lead peroxide is) and moreover, it undergoes a comparatively negligible change in volume during charging and discharging processes of a cell. So it has no tendency to disintegrate or shed out of the grid but it tends to lose its porosity and become dense and, therefore, lose capacity. So in the manufacture of the negative plates by faure process, a small percentage of certain substances, such as powdered graphite, magnesium sulphate or barium sulphate is added in order to increase the material porosity.

In order to obtain large capacity from a battery, a large surface must be exposed to the electrolyte, and since the size of a single plate is limited so increased capacity is obtained by connecting a number of plates in parallel to form a group. By doing so, internal resistance of the battery is also reduced. Two sets of plates are then sandwiched together, adjoining plates being separated from one another by perforated plastic, fiberglass, hard rubber or wooden separators.

The – ve group of the plates contains one more plate than that of the + ve group so that when interleaved both the end plates become the – ve. This is done so that all the + ve plates can work equally on both sides. During charging lead sulphate (PbSO₄) on the + ve plate is changed to PbO₂ and the active material of the plate expands. If a positive plate were used at one end, one of its sides would remain inactive, when the other side expands and thus the plate is forced to buckle.

Active Materials of Lead-Acid Batteries:

The materials, in a cell (or battery), taking active participation in chemical reaction (absorption or evolution of electrical energy) during charging or discharging are called the active materials of the cell.

The active materials of a lead-acid battery are:

i. Lead Peroxide:

Lead peroxide (PbO₂) dark chocolate brown in colour. It forms the positive active material.

ii. Sponge Lead:

Sponge lead (Pb) grey in colour. It forms the negative active material.

iii. Dilute Sulphuric Acid:

Dilute sulphuric acid (H₂SO₄) is used as electrolyte. The electrolyte of a fully charged battery contains about 31 % sulphuric acid by weight and 21 % by volume for specific gravity of 1.23 at 27°C. This is the recommended value for the tropical climate. Slightly higher values can be used in winter, if desired. The acid should be made from sulphur and not from pyrites, as the latter is liable to contain injurious substances.

The mixture should be made by pouring the acid slowly into the water, never the reverse.

The lead peroxide and sponge lead, which form the positive and negative active materials respectively of a fully charged cell, as mentioned above, have little mechanical strength and, therefore, cannot be used alone. Supports of grids required for this purpose, in modern form, are made of either pure lead or an alloy of lead and antimony.

iv. Separators:

The separators, which are thin sheets of non-conducting porous material made of chemically treated wood, porous rubber or mats of glass fibres, are placed between the positive and negative plates to insulate them from each other. Separators are grooved vertically on one side and are smooth on the other.

The grooved side is placed facing the positive plates to provide greater acid volume next to the surface for improved efficiency, to facilitate acid circulation within the cell, and to minimise the area of contact with the positive plate which has a highly oxidising effect on wood separators.

v. Battery Terminals:

A battery has two terminals—the positive and the negative. The positive terminal with a diameter of 17.5 mm at the top is slightly larger than the negative terminal which is 16 mm in diameter.