STORAGE BATTERY. An accumulation of energy which is able to produce directly an elec tric current. The term is a misnomer, as elec tricity itself cannot be stored. A so-called stor age battery or accumulator when acted upon by a current undergoes certain chemical changes. The chemicals which are thus separated recom bine again when the circuit of the battery is closed, and in uniting give off a current of elec tricity about equal to that by which they were decomposed. Lead is the metal most commonly used in accumulators, the positive plate having a coating of lead peroxide, Pb0,., and the negative plate a surface of spongy lead.
The idea of the storage cell may be traced back to 1801, at which time Gautherot showed that platinum wires used in the electrolysis of saline solutions developed secondary currents. Later, in 1S03, Ritter constructed a secondary pile of copper disks separated by cloths moistened with a solution of sal ammoniac. By charging this a few moments with a powerful galvanic battery the pile gave a strong shock. Volta, Becquerel, and others discovered that platinum and other metals—gold. silver—gave secondary electric currents when subjected to electrolytic action in certain solutions. In 1842 Grove pro duced his celebrated gas battery. which gave a current by means of the difference in polarity of oxygen and hydrogen, the constituents of water. In Faraday's Researches he mentions the high conductivity of peroxide of lead at the negative pole. Gaston was the first to apply this principle, and constructed in 1860 his cell with coiled plates. the first practical storage battery, which was afterwards developed and modified by Faure, Metzger, Brush. and others.
Storage batteries may be divided into two gen eral classes: Those in which the active material (peroxide of lead) is formed on the surface of the plates by chemical or electrochemical action, and those in which some easily reducible salt of lead is applied mechanically. The former are known as the Plante type, and the latter as the 'pasted' or Faure type.
The Plante cell is the simplest form of storage battery. The earliest cells were formed of two lead plates immersed in dilute sulphuric acid in water. The solution should have a specific grav ity of 1.17 before charging, and as the charge proceeds the specific gravity increases to I.195 at full charge. At each successive charge the peroxide formed on the positive plate sinks deeper into the metal, and this action continues until the metal is covered to a sufficient thick ness to protect the lead from electrolytic action. There is no difficulty in forming the positive plate in a Plante cell, but with the negative plate the action is very slow. The latter is the great difficulty with all Plante cells. The usual meth od of forming the spongy lead is to charge the cell, allow it to rest, then reverse the charge through the cell. At each reversal of current the peroxide is liberated at the surface, leaving me tallic lead in a very finely divided state. The
voltage of a lead-sulphuric acid cell is about two volts. The above description is applicable in a general way to all cells of the Plante type, of which there are a great many varieties. Most of the modifications introduced by the different manufacturers are mechanical changes with a view to exposing more surface of lead to the action of the electrolyte. The Plante accu mulator is a very efficient cell when once formed, but the great amount of time it requires for forming is its chief drawback. To avoid the great loss of time consumed in forming the Plante cells, Faure in 1S80 devised the method of pasting a layer of chemically prepared oxide of lead to the surface of the plates. This was done by spreading the plates with minium, or litharge, made into a thick paste by the addition of acidulated water. After drying these plates were placed in a bath of dilute sulphuric acid, and then subjected to an electric current. A piece of felt was placed between the plates to prevent the lead salts from disintegrating. After charging, the salt on the positive plate is reduced to peroxide of lead, while that on the negative plate is converted into porous lead. The chief fault of the early Faure cells was the dis integration of the active material, which would drop away from the plate. Many methods have been devised for holding the active material on the plates, the most common of which is to cast a grid, or plate with cells or perforations, into which the active material is pressed. Ali the modern cells are made with perforated plates of this description. Besides these two types of ac cumulators, which are the most important ones, there are a number of others, in which the ele ments are composed of lead, zinc. copper, etc., none of which are in very extensive use.
Storage batteries are employed in many cen tral stations to aid the dynamos at the time of the maximum output and to act as equalizers or reservoirs of electrical energy. The efficiency varies from SO to S5 per cent. in laboratory tests, but in commercial practice it is about 70 per cent. or less. The depreciation, however, may be very rapid if the batteries are not op erated with care. It is very important not to discharge accumulators faster than the rate for which they are built, as it results in the speedy destruction of the plates. They are not well adapted to traction purposes, as the motion of the car jars out the active material from the plates, and in starting the car a high rate of discharge is required. The weight and bulk of accumulators is also against their use on cars. With light \ chicks, as well as with delivery wagons and trucks, aceumulittors have been em ployed very successfully, while for telephone and telegraph work in large cities they are rapidly supplanting primary cells, the change being at tended not only with economy, but also with in creased cleanliness and available space. Consult: Benjamin, Voltaic Cell (New York, 1S93) ; Treadwell, The Storage Battery (ib., 1898).