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Dry Cell Batteries — Scliuete 455
has or has not a low resistance. The short circuit Hash of a cell is no true measure of its available capacity. To a buyer, or an engineer who is using the same batteries every day, the flash may indicate a variation from an average.
The voltage test is a reliable, practical test that can be made, but it likewise must be used with discretion. To rate a battery by voltage, it is necessary to know the number of cells that are contained in it so that the engineer can determine how closely the battery tests to the usual 11/2 volts per cell.
Unless high resistance voltmeters are used, there will be a variation in indicated voltage between batteries of different sized cells, which usually shows the large and low resistance cells as having a higher voltage.
In an installation of two or more batteries, it w^ould alw^ays be worthwhile to make a voltage test on each battery at the time of installation and record these data. At frequent intervals, other voltage measurements should be made with the same voltmeter and the voltage recorded. By keeping records of each battery in this manner the engineer or operator can easily tell if his batteries are falling uniforml}^ If there is a non-uniformity of voltage drop with respect to time, it will indicate that certain of the batteries are below average and should probably be replaced before the others.
The cut-off voltage of course is determined by the characteristics of the operating circuit, and the operator can determine this for his own information by making a measurement of it when the batteries are being replaced. The open circuit voltage of the batteries, when they are replaced, might be noted, and on subsequent battery installation this figure will be of a help in determining the condition of the batteries. It is impossible to say what the open circuit voltage of a usable battery will be, but it is generally higher than 11/4 volts. In other words, the open circuit voltage range of a ''B" battery, between the time it is installed and w^hen it is discarded, is only from II/2 to I14 volts.
Cut-off Voltages When a cell is discharged, the working voltage drops rapidly at first. Then it slows down to a constant rate, but as the end is reached, it again drops rapidly. In comparing the capacities of