« НазадПродовжити »
current means available for "forming" or preparing the lead plates was by the action of expensive primary batteries. The first chemical changes that took place were of very limited depth, and even after long service the chemical action of changed material could hardly be measured. It was only by frequent and repeated charges and discharges, extending over months of time, that it was possible to obtain cells with sufficient capacity to be of practical value. Even though laboring under these disadvantages, Planté was able to make numerous cells of this kind and perform laboratory experiments which created great interest among the scientists of that time.
The chemical change which results in the production of electricity in lead plate batteries is rather complicated, and its exact nature is not definitely known even to-day. It was advanced by Planté that the charging action, or rather chemical change when current was passed through the cell from an outside source, resulted in the formation of lead peroxide (Pb0,) on the positive plate and metallic lead on the negative. Both of these were said to be converted into lead oxide (PbO) when the current was drawn from the battery. Later investigations showed that the formation of lead sulphate also was of enough consequence to be taken into consideration. This reaction on discharge is probably about as follows:
Charged Condition Becomes Discharged Condition.
+ Therefore, during charging, the plates must be brought to their original state and the sulphate driven out of the plates into the electrolyte, as expressed chemically in the following:
When Discharged. After Charging.
H2SO4 Considering the equations previously outlined, it will be evident that the active material on both plates of a storage battery is changed into lead sulphate when the battery discharges. There are several reasons for considering this theory. The most important is that chemical analysis of a discharged plate has shown large quantities of lead' sulphate to exist. The fact that the density of the electrolyte becomes less during the discharge of the cell shows that sulphuric acid is consumed and that water remains. The specific gravity of the electrolyte is greatest when the cell is fully charged. This demonstrates conclusively that during charging the sulphate has been driven out of the plates and into the electrolyte. When a battery is discharged, the sulphate, having been absorbed by the plates, results in a lower specific gravity of the electrolyte. Then, again, considering the matter from an electro-chemist's point of view, it is known that the combination of oxygen and lead as lead oxide would not liberate sufficient electrical energy to account for the voltage of the current produced by the battery on discharge.
Planté, or Formed Plates. One of the first difficulties met with and one that militated against the development of the practical or commercial type of battery using Planté plates was the great length of time needed and the expensive means of generating the forming current. Later the plates were treated with nitric acid to facilitate the forming action. Other processes have been developed to hasten the formation. In addition to the chemical treatment, which consists of immersing the lead plates in a pickling bath to produce an oxidization before the current acts upon them, there is a mechanical action which will produce the same result and hasten formation. Laminated plates composed of ribbons of lead will form quicker than the solid lead plates, as will elements made up of lead wires or plates where the surface has been grooved with some forming-tool. An electrolytic process consists of making the plate of a lead alloy and eating the foreign matter away to leave a porous lead plate. These processes are described more in detail in the next chapter, which deals specifically with storage battery plate construction.
Faure, or Pasted Plates.—As soon as it was realized that the result of the forming current was the production of lead peroxide on one of the plates, two men, Camille Faure in France and Charles F. Brush in the United States, working independently of each other, devised a process of plate manufacture that materially reduced the cost of construction. Instead of forming the active material by expensive and time-consuming alternating charges and discharges, the common oxides of lead were applied to the surface of the plate in the form of paste, so that the work required of the electric current was reduced appreciably and considerable weight reduction obtained. Litharge, which is rich in lead, was selected for the negative plates, while red lead, which is oxidized more, was used on the plates intended to be positives. The pastes were composed of the oxides mixed with dilute sulphuric acid in the proportion about one part acid to four of water. Such a paste sets very quickly, and only small quantities can be prepared at a time. When the Faure process, as it is called, was first developed, it was believed that the Planté type of plate would be discarded. It was found by practical experience that the new structure developed faults that were not present in the older formation. Pasted plates of early development were found to bend or warp, to enlarge and to shed the active material. In order to eliminate these faults, various ingenious grid patterns were devised.
When storage cells are to be used in automobile work they are combined in a single containing member, as shown at Fig. 4, A, which is a part sectional view of storage battery. The main containing member, a box of wood is divided into three parts by cell jars of hard rubber. Each of these compartments serves to hold the elements comprising one cell. The positive and negative plates are spaced apart by wooden and hard rubber separators, which prevent short circuiting between the plates. After the elements have been put in place in the compartments forming the individual cells of the battery, the top of the jar is sealed by pouring a compound of pitch and rosin, or asphaltum, over cover plates of hard rubber, which keeps the sealing material from running into the cells and on the plates. Vents are provided over each cell, through which gases, produced by charging or discharging, are allowed to escape. These are so formed that while free passage of gas is provided for, it is not possible for the electrolyte to splash out when the vehicle is in motion.
It will be evident that this method of sealing would not be