Charging Systems- Battery

A battery is a name given to a group of two or more electric cells connected together in series. A 12volt vehicle battery has 6 cells, each of 2volts. A battery does not store electricity in the commonly understood sense of the term. The charging current causes chemical changes inside the cells, and most of the electrical energy supplied is converted chemical energy. When the cells are delivering a current, that is, discharging, the chemical energy is converted

Chemical action in a battery is the interaction of different substances to form new substances. For example, when carbon burns, oxygen has become intimately mingled with the carbon, and a substance is produced which is different from either carbon or oxygen it becomes carbon dioxide.

Electrochemistry is when positively charged atoms can be made to accumulate on a metal plate and negatively charged atoms or groups of atoms on another metal plate, the two plates being immersed in a liquid. The liquid used must be a conductor of electricity. If the two plates are connected by a wire, an electric current flows through the wire. This is the basis of the electric cell.

There are two main essentials for an electric cell:

1.       Two plates must be of different substances and both must be conductors of electricity.

2.       The liquid in which the plates are immersed must be such that the chemical action occurs between it and one or both of the plates when the plates are connected by a conductor.

The liquid in the cells is called the electrolyte, and the plates are known as electrodes.

Most vehicles use lead-acid batteries, the cells in a lead-acid battery consist of positive and negative plates, separators and electrolyte. The positive and negative plates correspond to the positive and negative electrodes of a primary cell. These plates, which are thin, are assembled in sets so as to expose as much plate surface to the action of the electrolyte as possible. Both sets of plates are constructed from lattice-like lead-antimony grids filled with the active material which is forces into the plate grids in the form of a paste. The active material of a positive plate is lead peroxide (PbO2), a chemical combination of lead and oxygen. The active material of a negative plate is form of pure grey lead (Pb). Separators are an essential part of a multiplate cell. They are insulators, and their primary objective is to prevent metallic contact between the alternate positive and negative plates, while permitting free circulation of the electrolyte. They are chemically inactive, being made from various insulating materials. The plates are immersed in an electrolyte- which is a dilute solution of sulphuric acid in water.

How a battery works

Action of a battery on discharge,

When a load is connected to the terminals of the battery, chemical action takes place. In a lead-acid battery the sulphuric acid in the electrolyte combines with the active material of the positive and negative plates, forming lead sulphate in each. The amount of active material converted to lead sulphate is directly proportional to the amount of current flowing. The battery is discharged when there is not enough sulphuric acid left in the electrolyte.

Action during charging

Recharging is brought about by passing a current of electricity through the battery in the reverse direction to the flow on discharge. The charging current causes a reversal of the chemical action, the positive plates being reformed to lead peroxide and the negative plates back to pure grey lead. When all the active material has been reformed, the battery is fully charged.

Measuring the state of charge of a battery

Because sulphuric acid is 1.835 times as dense as water, the electrolyte of a charged battery will be denser than that of a discharged one. Hence, the density of the electrolyte in each cell of a battery provides measure of the batteries state of charge. The relative density of the electrolyte (specific gravity) is measured by an instrument known as a hydrometer. The working principle of the hydrometer is that an object which floats in water will float higher in a liquid of greater specific gravity, such as sulphuric acid. For battery testing the hydrometer is contained as a float in a special syringe which enables the electrolyte to be drawn into a glass tube and its level observed in relation to the scale marked on the narrow steam of the float.