In a Lead Acid Battery, several similar plates are properly spaced and welded or lead-burned to a strap to form a plate group. The plate consists of perforated grids into which lead or the lead peroxide has been pressed. The grids are made of an alloy of lead and antimony, which make them resistant to electromechanical corrosion; and gives them strength and rigidity. There are two types of plate groups in each cell-positive plate group and negative plate group. The plate group connected to the positive terminal of the cell consists of grids filled with a paste of lead peroxide (brown in colour). The plate group connected to the negative terminal consists of grids filled with metallic lead. It is spongy and dull grey in colour. Each group of plates is held together by a post strap. to which each individual plate is welded. These staps extended up through the cell cover to provide the cell terminals to connect one cell to the other. The plate groups are arranged in the cell so that the positive and negative plates alternate.
It is to be noted that the positive plates are filled with lead peroxide active material. This is a dark brown crystalline material which consists of very small grains or particles, disposed of so as to provide a high degree of porosity in order to allow the electrolyte to penetrate the plate freely. Negative plates are filled with a porous mass of lead, in spongy form, in which electrolyte can penetrate freely. The active material also contains so-called expanders included to prevent the sponge lead from contracting the reverting to the solid inactive state during the life of the Lead Acid Battery.
The plates of a lead-acid storage battery consist of an electrically conducting grid framework in the meshes of which the active materials are incorporated by the electrochemical process. These grids serve to conduct the current to and from the active materials of the positive and negative plates. An alloy consisting essentially of lead and antimony is used for the grids. The antimony stiffens and strengthens the soft lead. The presence of antimony also facilitates casting the fine detail of the wire structure of the grids and enables the Lead Acid Battery weight to be kept to a minimum.
Separators are placed between the negative and positive plates to keep them separate between each other. The separators are designed to hold the plates apart so that they do not touch and the same time they must be porous enough to permit liquid to circulate between the plates. Separators are usually made o specially treated wood, hard rubber, resin impregnated fibre alone or in combination with rubber or mats of glass fibres. Some batteries have separators made of polyvinyl chloride or polyethene saturated cellulose.
Separators have ribs on the side facing the positive plates to provide greater acid volume next to the positive for reasons of improved efficiency and to facilitate acid circulation within the cell. The ribs also minimize the area of contact with the positive plate which has a highly oxidizing effect on most separators. Glass fibre retainer mats or perforated rubber or plastic sheets are sometimes placed between the positive plate and the separator to retard the loss of active material from the plate and to protect the separator from oxidation.
|Lead Acid Battery
5. Element construction
The positive plates are welded to a post strap forming a plate group, and the negative plates are welded to a similar post strap, forming a negative group. The two groups are put
together. Separators are inserted between the plates with the grooved faces of the separators next to the positive plates. These groups, assembled with separators insulating them, are called an Element. One element is used per cell. The post straps extend up through the cell cover and provide terminals for connecting one cell to another.
There may be any desired number or size of plates used in an element, depending upon how much energy is to be stored but there will usually be one more negative plate than there are positives for reasons of improved performance. The greater the plate surface area used per element, the higher will be the voltage during discharge at high rates and low temperature. However, the open circuit voltage of a fully charged cell, no matter what the size of the cell or the number of plates in the element, is only a little over two volts. The Lead Acid Battery voltage is the sum of the voltages of its cells.
6. Cell cover
Each cell is sealed by a cover of hard rubber through which the positive and negative terminals project. Adjacent negative and negative terminals are connected by connector straps. Each battery cover has an opening through which liquid is added. A filler cap is screwed or this opening. The filler cap has also an air vent for the escape of gas. In many late model batteries, the one-piece cover is provided that covers all the cells.
Vent plugs of special designs co-operate with the cover vent opening to baffle the gases and electrolyte splashed and sprayed against the underside of the cover, to prevent loss of acid from the cell.
The sponge lead and lead peroxide which fill the respective plates are referred to as the
active materials of the Lead Acid Battery. But these materials cannot become active until they are covered by an aqueous solution of sulphuric acid called the electrolyte. The sulphuric acid of the electrolyte supplies the sulphate ions which combines with each of the plate materials and releases the electrical energy. The sulphuric acid electrolyte is also the carrier for the electric current inside the Lead Acid Battery between the positive and negative plates through the separators. The antimony- lead alloy of the grid framework of the plates carries the electric current to and from the active materials to the outside terminals.
The electrolyte of a fully charged Lead Acid Battery contains about 31% sulphuric acid by weight or about 21% by volume in distilled water. This s corresponds to a specific gravity of 1.230 at 27°C.
The electrolyte used in a lead-acid battery is the solution of sulphuric acid. It consists of 40% sulphuric acid and 60% distilled water. The level of the electrolyte in the container is about 10 mm above the tops of the plates. When the electrolyte has been added and the battery is given an initial forming charge, it is ready for operation.
|Lead Acid Battery
To connect the cells of a Lead Acid Battery, in series, the elements are placed in each cell so that
the negative terminal of one cell will be adjacent to the positive terminal of the next cell and so on throughout the Lead Acid Battery. Cell connectors are placed over the protruding terminal parts and welded to them to connect the cells in series. Connectors must be heavy enough to carry the high current required for starting without overheating.
Battery terminals are of special design, being tapered to specified dimensions in accordance with standards agreed upon by the industry so that all positive and negative cable clamp terminals will fit any corresponding battery terminal interchangeably. The positive terminal is slightly larger (22.5 mm diameter.) at the top than the negative terminal(16 mm diameter.) at the top so as to minimize the danger of installing a Lead Acid Battery in reverse.