The Storage Battery

A storage battery is composed of one or more secondary, or storage, cells. This type of cell is rechargeable—that is, the chemical reactions that produce electricity in the cell can be readily reversed to restore the materials in the cell to their original condition. (The chemical reactions in a primary cell either are difficult to reverse or cause irreversible changes in the cell's internal structure.) A secondary cell is recharged by forcing an electric current through the cell in a direction opposite to that of the current produced by the cell itself. Devices called rechargers make it possible to recharge some secondary cells with household electrical current.

For some uses, secondary cells are recharged frequently to keep the battery at full charge. For others, the battery is used essentially like a primary battery and is allowed to run down" before it is recharged. Over time, a storage battery will fail because of deterioration of its parts or decomposition of its materials. Two major kinds of storage batteries are the lead-acid and nickel-cadmium batteries.

The Lead-Acid Battery

is used in most automobiles, trucks, and other vehicles for the starter motor, ignition system, and accessory electrical equipment. Each secondary cell produces approximately 2 volts, and the total voltage of the battery (6 or 12 volts in the case of automobiles) equals the number of cells multiplied by two.

The most common lead-acid cell consists of two types of plates immersed in a solution of sulfuric acid and water. Perforated separators made of fiber glass, wood, or rubber help insulate the plates from each other. The negative plates are made of spongy (porous) lead. The positive plates are a lead grid filled with lead dioxide. As the battery discharges (gives off current), the surfaces of the lead dioxide and spongy lead plates gradually become lead sulfate and the sulfuric acid becomes increasingly diluted with water formed during the process. As these chemical reactions occur, the battery becomes progressively weaker.

An automobile battery is recharged by an electric current produced by an alternator or generator. A device called a voltage regulator controls the charging process and prevents overcharging. Overcharging causes the plates in the battery to deteriorate.

The state of charge of some lead-acid batteries can be checked by testing each cell with either a hydrometer or a voltmeter. The hydrometer measures the specific gravity of the sulfuric acid in a cell. The specific gravity indicates a cell's state of charge. A voltmeter indicates the state of charge of a cell by measuring the cell's voltage. The failure of one or more cells to reach an adequate state of charge may be an indication that the battery needs replacing.

Some batteries lose water in the electrolytic solution through evaporation or decomposition and have removable caps for their individual cells so that water can be added periodically. Calcium-lead batteries, commonly known as maintenance-free batteries, do not require additional water; the cells usually do not have caps but are permanently sealed.

The Nickel-Cadmium Battery

has a positive electrode composed of nickelic hydroxide, a negative electrode composed of cadmium, and an electrolyte of potassium hydroxide. Nickel-cadmium cells are available in many of the same sizes and shapes as primary cells. In one flashlight nickel-cadmium cell, the positive and negative electrodes are long plates wound in a spiral within the metal cylinder. Wound with the plates is a separator that lies between the plates.