This terminal is now to be connected to a charging current (not over 1 ampere), as described in the directions for charging batteries, for eight hours, and then discharged at a rate not over 1 ampere for six hours. Then the connections are to be reversed and the cell charged backward, as it were, and discharged. This has to be repeated for a long period, perhaps a month, before the cell is in good condition; on the final charge it is to be connected positive to positive of charging source. This operation is called "forming," and the result is to change the metallic lead of the positive plate into red-brown peroxide of lead, and the lead negative plates into spongy lead.
In modern commercial cells this operation is no longer pursued, the plates are variously constructed of lead frameworks holding plugs of litharge or lead oxide, which is "formed" with great facility. For many purposes other than operating Ruhmkorff coils, a few simple cells made, as described, are handy to have and easy to make. In sealing the cells up for portability, care must always be taken to leave a small hole in the cover for the escape of the sulphurous acid gas.
Charging Storage Batteries.
Although the charging of a storage or secondary battery is by no means a difficult operation, yet it requires care, and one unaccustomed to the work will meet many slight difficulties which may seriously affect the results. Pre-eminently the best charging source is a direct current, constant potential electric-light circuit. The amount of current required varies according to the type and make of the cell, but we will select one of a capacity of 50 ampere hours for illustration.
By 50 ampere hours is meant a delivery of 1 ampere per hour for fifty hours, or a rate of discharge equal to the above, as 2 amperes per hour for twenty-five hours. In practice a secondary cell will not be found to act exactly as above, the total amount of current decreasing as the discharge is greater. Each cell is constructed to discharge at a certain rate, above which it is not safe to go. Five amperes per hour is a suitable rate for a fifty-hour cell, and should not be greatly exceeded. The Chloride type, however, is one which can be discharged at a higher rate than normal without any serious results, the latter being generally a bulging or "buckling," as it is called, of the plates whereby they short circuit or fall apart. The voltage of the charging source should be at least 10 per cent over that of the battery when fully charged. The voltage of a cell of storage battery varies from about 2.3 at commencement of discharge to 1.7, at which latter voltage discharge must be stopped and charging recommenced.
Fig. 64 shows the connections to charge a storage battery from an electric-light circuit. The latter must be direct current and of low tension. The circuit from the negative lead runs to the rheostat handle R, thence through as many coils as are in circuit (varied by moving the handle over the contact pieces in connection with the resistance coils). The positive of the cell is connected to the positive main.
In connecting storage cells to the mains the utmost care must be taken that the terminals are correctly attached. It happens in isolated plants that some change is made in the wiring or the switchboard, which reverses the current without warning being given to the battery charger. It is the safest way to test the polarity of the terminals of both battery and mains each time charging is commenced. For polarity tests see Chapter I. It is immaterial on which side of the battery the rheostat or similar device is placed.
Fig. 64.