Page:Popular Science Monthly Volume 90.djvu/965

 Popular Science Monthly

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��and then disconnected and connected in series for use.

The electrolyte is made of a mixture of sulphuric acid and water. Commercial acid of about 1.835 specific gravity should be diluted with clean rain or distilled water. Mix the solution in some kind of a glass or glazed crock, using the proportion of i

I CHARGING

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��PRIMARY BATTERIES '

Two groups of primary batteries are con- nected in parallel and eight of the storage units

part acid to 4 parts water. The resulting mixture should be about 1.180 specific gravity. Always pour the acid into the water in a very small stream, stirring vigor- ously at the same time. When the solution has cooled pour a sufficient quantity into each tube, making the level come to within ]/^ in. of the top of the tube when the elements are in place.

In usual practice 40 to 60 ampere-hours capacity is allowed for each square foot of positive plate, measuring both sides of the plate. The thickness of the plate, unless great, is disregarded. It is well to know that the electrolyte penetrates only about 1/16 in., so that any excess of 3^ in. in the thickness of a plate is solely for mechanical strengthening of the plate and is useless in an electrical capacity. As the elements of these batteries are round, only the surface can be figured. The tube being ^ in. in diameter and 5 in. long makes a surface area of about 6 sq. in. In allowing 50 amperes per square foot, the ampere- hour capacity of one cell of this battery will be about 2. The average charge or discharge voltage is about 2 and the watt- hour capacity of each cell will be 4, thus making a 24-cell battery a 96-watt-hour capacity. While it is possible to draw more current from the battery than is rated, yet it is not advisable to do so, as that ruins it quickly.

Each cell of storage-battery, regardless of its length, width and number of plates, will give after a full charge about 2)^ volts, and the voltage required to charge any number in series is equal to 2)^ times the number

��in series. Thus, the 24-series battery. Fig. I (since they are all connected in series) will require 60 volts. In the 3-series, 8- parallel battery, three cells are in series and the voltage will be 73^. Usually more voltage than is actually necessary is pro- vided — about 3^ to 5 volts per group. This extra voltage lessens the danger of having the storage cell discharge back into the charging source, the voltage of the storage cell exceeding that of the charging source. The 24-series battery requires about 62 volts and the 3-series, 8-parallel battery requires 8 volts.

To charge a storage-battery with its maximum ampere-hour capacity — that is to say, to charge an 8o-ampere hour battery with 80 amperes for one hour — would probably ruin it. The best rule to follow is to charge at the 8-hour rate. Thus a battery may be charged in 8 hours, which is the normal rate, or it may be .charged in 7, 6, 5 or even 4 hours. In charging this battery the 8-hour or normal rate is de- termined by the cell capacity divided by the 8 hours, which would equal 25. That is to say, each cell is to be charged with 14: ampere for 8 hours. The 7-hour rate is 2-f-7, and so on; but under no circum- stance should this or similar small batteries be charged at less than the 4-hour rate. Another good rule to follow is to allow a charging rate of .04 amperes per square inch of positive surface. It is apparent that the foregoing methods of calculation can be used to find the capacity and the charging rate of any size storage cell in common use.

The resistance necessary to be placed in

DISCHARGING

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��When used in a discharge, connections from the battery binding posts are made in series

series with the battery on a iio-volt direct current main to lower it to 62 volts, the amount necessary for the 24-series battery, will be 192 ohms, for the normal 8-hour rate. In using current from a 220- volt direct current main the same procedure may be carried out to determine the re- sistance necessary to reduce the current to 62 volts. This will require 632 ohms for

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