Frederick A. Draper
The wide variety of batteries described in the catalogues of electrical supply houses are a source of perplexity to the purchaser who is without technical knowledge or experience to guide him in selecting the kind best adapted for the required service. The ideal kind of battery would be one giving a strong, constant current of high voltage without polarizing, and which would permit of continuous or intermittent use without appreciable internal action when not in use and with a long life.
The accumulator, or storage battery, comes the nearest to meeting these requirements, but the use of such a battery necessitates having a charging outfit, or the sending of the cells out to be charged; the latter being the common but rather expensive custom. The construction of an efficient accumulator was described by W. C. Houghton in the June, 1904, number of Amateur Work, to which are referred any readers desiring to construct such a battery. It will be of interest, however, to consider at this time the action which takes place in a cell of this type, so that we can determine how to properly use the same and discover the cause of any trouble which may occur.
An accumulator or, more properly, a secondary cell, is one in which the fluid would not of itself set up any action between it and the other elements of the cell and so liberate energy; but when energy in the form of an electric current is applied to these elements, the fluid, or a certain portion of it, separates into its constituent gases, the electro-positive uniting with or adhering to the negative element, and the electro-negative with the positive element of the cell, thus creating a difference of potential between the two.
If communication be established between these elements by means of a conductor, but not before, these separated gases will reunite, yielding energy which is returned as electricity to an externa] circuit, but in the opposite direction. The quantity of electrical current returned does not equal, however, that originally used in the separating or charging process, the extent of the loss depending upon the efficiency of the cell and varying from 10 to 35 per cent.
To obtain an efficient cell of adequate capacity, it is evident that the elements should be those having the utmost affinity for the constituent gases of the fluid so that the operation of charging may be expeditiously done, and the output of the cell be great enough to compensate for the energy used in charging. In determining this efficiency the adaptability of the cell must be considered in relation to its uses as well as the relative cost of the elements used for a given capacity.
To more particularly learn the action of accumulators, we will consider the one described by Mr. Hough-ton, which is of the "lead" class as distinguished from the other or bimetallic class, the former being in much the greater proportion of those in common use, although the new Edison cell is claimed to have such superior efficiency as will cause it in time to replace all others. This remains to be proven, however, although there is undoubtedly great merit in the new cell, and it will have a large sale when placed generally upon the market.
The Plante lead accumulator, as improved by Faure and others, now consists of metallic lead plates in lattice-work form, firmly filled with lead oxide paste, and contained in a vessel nearly filled with sulphuric acid solution, four parts water and one part acid. The charging of this cell causes the water to become decomposed, the oxygen combines with the paste in the positive plate, changing it to peroxide, and the hydrogen uniting with the paste in the negative plate, changing it to spongy lead. Charging is continued until all the paste has been converted, which is indicated by numerous bubbles rising to the surface, and a milky appearance of the fluid, due to the minute bubbles contained therein. This is termed " gasing " and charging should not be continued beyond the point where this condition becomes pronounced as, while no harm is done the battery, it is simply a loss of current to do so. When charging, the current should flow towards the positive plate.
If, after a cell is charged, the external circuit be completed, the current will flow in the direction opposite that used for charging, the lead peroxide of the positive plate will be reconverted to lead oxide and the spongy lead to lead sulphate in the negative plate. If the discharge rate of the cell has been too rapid or too long continued, lead sulphate is liable to be found at the positive plate, which will be indicated by white crystals and is known as "sulphating." It is an extremely difficult matter to remove, as the crystals are tenacious, and bring more or less of the active material with them when taken off. Being non-conductive, the crystals increase the internal resistance and diminish the output of the cell; they are also very liable to form if the cell is nearly discharged, and then left standing for a long time before again charging. If a cell must be left without use, it should first be fully charged and not allowed to remain too long without action.
The normal voltage of a cell is about 2 volts, rising to nearly 2.5 volts just before charging ceases, and dropping during discharge to 1.85 volts, below which sulphating is very liable to occur. Another injurious effect due to too rapid or long a discharge is that known as "buckling," caused by the formation of minute sulphate crystals between the plate and the lead framework or grid, and causing the latter to expand and distort the shape of the plate. Even under normal use, the plates are liable to expand slightly, and should they from these causes, come into contact with each other, a short circuit will be formed which will cause serious trouble.
Cells of this type are rated at the ampere-hour capacity of their discharge within normal limits, the discharge rate depending upon the size of the plates, and the duration of discharge upon the quantity of active material contained in them and exposed to chemical action. With ordinary construction a normal discharge rate is from 2 to 2.5 amperes per square foot of surface of positive plate, and a discharge capacity of from 4 to 5 ampere-hours per pound of plate, both positive and negative, excluding a suitable allowance for lugs, etc.
As the chemical action taking place during charging and discharging should evenly effect all the active material, the value of a low rate of discharge will be evident, as the action will be more even and longer sustained at normal capacity, and yielding, therefore, higher efficiency.
The life of a cell depends upon the uniformity of its use. Properly charged and discharged it will last for from 1000 to 1500 chargings, with three or four renewals of positive plates. The life will diminish to less than half of the above, however, if discharged at an excessive rate, or too low.
A method of charging a small accumulator by means of gravity batteries is described in the January, 1905 number.