The motion picture projectionist (Oct 1927-Sept 1928)

September, 1928 The Motion Picture Projectionist 13 Storage Batteries and Charging Circuits By Nat H. Hewitt Local 306 LET us first consider the general construction and action of a storage battery. It is not really electricity which is stored up in a storage cell, but the flow of current from a direct current dynamo through the cell from plate to plate performs a certain amount of chemical work. Whenever required, this stored-up chemical energy can be released in the form of an electric current which will pass from plate to plate through an external circuit. The common type of lead cell comprises a set of prepared lead plates immersed in a dilute solution of sulphuric acid, but a certain electrochemical process known as charging must be gone through in order that the cell may deliver a current of electricity. There are two general methods by which the lead plates for a storage cell may be prepared: (1) A paste of litharge or oxide of lead mixed with a dilute solution of sulphuric acid may be applied to perforations in a lead grid, and then by means of a current of electricity and a suitable electrolyte the surface of some of these plates may be coated with peroxide of lead, while the remaining plates become simply spongy. Second, large lead plates may be immersed in a certain electrolyte and connected to the terminals of a dynamo. By repeated charge and discharge some of the plates may be coated with peroxide of lead, while the remaining plates become simply spongy. Development of Battery Reviewing the bevelopment of the storage battery we find (1) that the earlier types of storage cells comprised two lead plates immersed in a dilute solute solution of sulphuric acid. The terminals of the plates were connected to a direct current dynamo for a period of several weeks. By repeated charges and discharges the surface of the plates received a coating of so-called active material. (2) Later it was determined that the formation of the plates could be hastened by chemical means prior to the charging process. Thus, the cost of manufacturing the plates was accordingly cheapened. (3) In certain types of present day cells — for instance, the Exide lead cell — the active material is applied to the plates mechanically in the form of a paste. The Charging Process In general the charging process of a storage cell is as follows : When two ordinary lead plates or sets of plates are placed in a dilute solution of sulphuric acid of the correct proportion and a di Cell Connector Seal Nut Cover Positive Strap Vent Plug .Filling Tube Post Gasket Level of Electrolyte Negative 3 Strap Separator Positive Plate Negative JPlate Fig. 1. — Sectional view of battery, showing construction rect current of electricity from a dynamo passed from one plate through the solution to the other, the resultant chemical decomposition deposits a coating of peroxide of lead on one plate the while the other plate becomes gray and spongy, or porous. When one set of plates is fairly .well coated with lead peroxide and the other set becomes spongy, the cell is said to be charged. If the terminals of these plates are now joined together by a conductor (the charging generator having been disconnected) a current of electricity will flow from plate to plate. The plate coated with lead peroxide is known as the positive plate of the storage cell, and the other, the negative plate. When joined together by a copper conductor, current flows in the external circuit from the positive to the negative plate and the resultant chemical change undoes the work of charging, part of the peroxide of lead of the positive plate and the active material on the negative plate is converted to lead sulphate which covers the surface. When the plates are fairly well coated with sulphate the cell is said to be discharged. In order that current may be drawn from the cell again, the plates must be connected to a source of direct current and the charging process repeated. The process involved in ■ the charge of storage cells may be better understood from the explanation which follows : Fundamental Actions When a lead storage cell is put on discharge, the current is produced by the acid of Ihe solution going into and combining with the porous part of the plate called the active material. As stated before, in the positive plate the active material is lead peroxide, and in the negative plate it is metallic lead in a spongy form. When the sulphuric acid in the solution combines with the lead in the active ma terial a compound known as lead sidphate is formed. As the discharge progresses, the solution becomes weaker by the amount of the acid used in the plate which, incidentally, produces the compound of acid and lead called lead sulphate. This sulphate continues to increase in quantity and bulk, thereby filling the pores of the plate. As the pores of the plate become filled with sulphate, the free circulation of acid in the plate is retarded, and since the acid then cannot get into the plate fast enough to maintain the normal action, the battery becomes less active, a condition indicated by a rapid drop in voltage. During the charging period direct current must pass through the cells in the direction opposite to that of discharge. This current will reverse the action which took place in the cells during the discharge. It will be remembered that during the discharge the acid of the solution went into and combined with the active material, filling its pores with sulphate and causing the solution to become weaker. Reversing the current through the sulphate in the plate restores the active material to its original condition and returns the acid to the solution. Thus, during charge, the solution gradually becomes stronger as the sulphate in the plate decreases, until no more sulphate remains and all the acid has been returned to the solution, when it will be of the same strength as before the discharge, and the same acid will be ready to be used over again during the next discharge. Since there is no loss of acid by this process, none should ever be added to the solution. The whole object of charging, therefore, is to drive from the plates the acid which is now absorbed by them during discharge. The liquid in a storage cell is known (Continued on page 21)