Some useful theory about battery

BATTERY
Battery is direct current producing source. It not only used to make function electronics system, but with the latest invent it is possible to replace of fuel energy to the motor vehicle. Battery produces electricity by chemical method. When two dissimilar metals immersed in a dilute acid will produce electric potential difference between the two plates. Such a unit can be called a cell. It has one positive terminal (called Anode) and one negative terminal (called Cathode) immersed in electrolyte (dilute acid) contained in a insulated container. Battery is a combination of cells. There are different types of cell. Cells are mainly divided into two groups; namely primary cells and secondary cells. Primary cells can not be recharged after its use. But secondary cells can be recharged. The chemical action in secondary cells during discharge is reversed when it is charged.

Lead – Acid Battery

A 12 Volt lead acid battery contains six cells; each cell has two volts potential difference. The cells are connected in series so as to have total electromotive force (EMF) of 12 Volt. Each cell has a lead dioxide (peroxide) (PBO2) positive plate, a pure, spongy lead (pb) negative plate and dilute sulphuric acid (H2SO4) electrolyte.

The plates and electrolyte are enclosed in hard-rubber or plastic container. A screw type vent cap in the top cover of each cell allows access to the electrolyte, which must be 1 to 2 cm above the plates at all times. To prevent the positive and negative plates from touching each other, thin wooden, glass, rubber, or plastic separator sheets are inserted between adjacent plates. To provide mechanical strength, both the negative and positive plates consist of an open framework of an alloy of lead-antimony. The electrolyte is dilute sulphuric acid having a specific gravity of 1.28.

When the battery is in use (i.e while discharging), the sulphuric acid is combining chemically with both the lead dioxide and the pure lead plates, changing them to lead sulphate. When both plates have been changed to lead sulphate, they are no longer a different materials, no potential difference is produced between them, and the battery is completely dead. The sulphuric acid, having combined with the plates, leaves only water as the electrolyte.

A lead- acid cell should never be allowed to become completely discharged. During discharge, lead sulphate forms as crystals on the plates, causing them to swell, which will bend, or buckle, the plates, possibly splitting the separators or even the battery case itself.

To recharge a lead-acid cell, a source of dc emf greater than that of the cell must be used. This forces a current backward through the cell, reversing the chemical action of discharging. The sulphate in both plates is chemically broken down, reappearing as sulphuric acid in the electrolyte, leaving the positive plate lead dioxide, the negative plate pure lead, and the cell again charged. Lead-acid cells must be recharged as soon as possible after being discharged. The lead sulphaate first forms small, soft crystals on the plates. If allowed to age, the crystals grow and turn hard. Soft crystals respond to recharging and easily change back into the sulphuric acid, lead, and lead dioxide. Hard crystals require along, slow recharging to complete the chemical reversal. Sulphated batteries that are allowed to remain in a semi-charged or discharged state for long periods may never completely recharge or may become completely useless.

Specific Gravity

A voltage reading of a lead-acid cell under load gives a fair indication of state of charge in the battery; but measurement of the specific gravity of the electrolyte is another good ;method. The specific gravity of the electrolyte solution in a lead-acid cell ranges from 1.210 to 1.300 for new, fully charged batteries. The higher the specific gravity of the electrolyte, causing the less internal resistance of the cell and the higher the possible output current. However, the higher the specific gravity, the more active the chemical action and the faster the battery will discharge itself. The battery should be recharged before its specific gravity drops by 100 points

Hydrometer

Specific gravity of the electrolyte is measured by using Hydrometer. When hydrometer readings are taken, care must be exercised to prevent drops of the electrolyte from spilling on the top of the cells or touching clothing or hands. The sulphuric acid is very corrosive and will eat holes in fabrics and burn human skin. The cap on a storage cell usually has a tiny hole in it to relieve internal gas that develops, but the hole is small enough to prevent dirt and foreign objects from falling into the electrolyte, whenever the battery is charged, the chemical action produces hydrogen gas on one plate surface and oxygen gas on the other. These gases, bubble to the surface and escape through the vent hole in the cap. Thus, the water in the electrolyte is lost in the cell. The water that escapes must be replaced to maintain the proper electrolyte level. To be safe, only distilled water should be added in the cell.

Capacity of a battery

The capacity of a battery is rated in ampere-hours (Ah). The number of ampere hours produced in a 8-h period to bring a lead-acid cell down to 1.75 V is one standard of measurement used. If the battery is forced to discharge faster than this, the number of ampere-hours produced will be somewhat less. If discharged slowed, the number of ampere-hours obtainable will be greater because of lessened internal heat. The capacity of a storage battery determines how long it will operate at a given discharge rate. An 80 Ah battery must be recharged after 8 h of an average 10 –A discharge.

Some useful hints

• Keep flames and sparks away from a charging or recently charged battery. The mixture of hydrogen and oxygen gases given off during charging is higly exlosive.
• Be carefull while using hydrometer. Avoid spilling acid.
• Keep the top of the cells clean and free from moisture to prevent leakage across the surface of the cell top and to prevent dust and dirt from failing into the electrolyte.
• Keep the terminals of cells coated with petroleum jelly to prevent corrosion.
• Keep cell tops on, or slightly loose, while charging to prevent eleftrolyte droplets from spraying out of the cell.
• Maintain the proper electrolyte level.
• Use only distilled water to replace lost water.
• Take a hydrometer reading weekly.
• Test operate the battery at least once a month.
• Trickle charge unused batteries at least one full day a month.
• Always bring batteries up to full charge after a discharge.
• Use onl7 chemicall7 pure sulpuric acid (diluted) if necessary to add new electrolyte.
• Provide adequate ventilation while charging.
• If a battery box is used, clean the inside surfaces once a year to remove sulphuric acid droplets that may form during charging.
• Do not overcharge a battery, as this causes the grid to deteriorate.
• Batteries may be stored several months if first fully charged and then kept refrigerated(not frozen).
• If necessary to store for a year or more, battery should be fully charged, the electrolyte removed, and the cells flushed with clear water and then filled with distilled water.
• Plates and wooden separator must never be allowed to dry.
• When removing caps, do not turn them over or place them on an unclean surface. This precaution will prevent the transportation of foreign materials into the cells when the caps are replaced.