Lead-lined tanks are usually set as follows: The floor is covered with a layer of glazed tile or brick, on which are placed two wooden stringers about 3 by 4 inches, carefully painted with acid-proof paint. On these are set four insulators held in place by wooden pegs kept in position by pouring melted sulphur around them. The battery tray and battery are placed on these insulators as indicated in Fig. 172.

Oil insulators were formerly used, but the oil collects dust, and as this is likely to cause leakage they are no longer employed. For very large lead-tank outfits a double system of the supporting construction shown in Fig. 172 is used, but with individual stringers for each cell. Glass cells are often set on wooden trays, which are filled with sand to distribute the strains and absorb the drip. Sawdust was also used, but it becomes carbonized by the acid drip, and as this is likely to cause leakage, it has been abandoned.

In connecting the cells, usually put in series, great care should be taken to join the positive terminal of one to the negative of the next, and so on. The color of the plate is the best indication of its polarity, the positive plate being a light brown when discharged and a chocolate color when charged, while the negative varies from a dark to a light slate color. It should be noted that the nomenclature concerning storage batteries is different from that of primary cells. The positive plate in the former is the peroxide plate (brown) and is that one from which the current flows out in discharging, whereas that would be the negative plate of a primary battery. The positive pole or terminal in a storage battery is an extension of the positive plate, and is connected to the positive terminal of the dynamo in charging; consequently there is less cause for confusion of terms than with the primary cell.

It is well to test the polarity of each cell and of the circuit before making connections. This may be done with any form of poletester, or by the definite expedient of dipping the two terminals in dilute sulphuric acid, the one from which the most bubbles arise being negative. The connections should be scraped clean and screwed up very tight, being coated with acid-proof paint to avoid corrosion. The most satisfactory way is to weld or "burn" the positive terminal of one cell to the negative terminal of the next, though soldered connections are good. This soldering is done as follows: Two strips of lead and the terminals to be connected are very

carefully cleaned; the lead strips are then clamped to the terminals, a mold placed around the joints and molten lead poured into it.

The Electrolyte.-Practice var es considerably as to the strength. of solution to use. Chemically pure sulphuric acid is carefully poured into water until its density becomes about 1.2, and then the mixture is allowed to cool before pouring it into the cells It is important to use perfectly pure acid and water, as impurities will cause local actions and ultimately destroy the plates. Water should never be poured into sulphuric acid, as it is likely to cause the liquid to be thrown out violently. The electrolyte should completely cover the plates. Cells for vehicle work use an electrolyte w th density as high as 1.3. The advantage of a strong solution is its lower resistance; but it is likely to produce the very objectionable effect of "sulphating."

The density of the electrolyte falls immediately after filling a cell, some of the acid being taken up by the plates; but it rises again in charging; for example, from 1.17 to 1.2. It is convenient to keep hydrometers in several cells to observe the density of the electrolyte, not only at the beginning, but as a permanent indicator of the amount of charge and general working conditions.

Charging. The charging should begin immediately after a new cell is filled with the electrolyte, otherwise the plates are likely to become "sulphated." The first charge differs from subsequent regular charges in that it should be at a rate (lower than normal) that will not cause the temperature of the cell to reach 100° F., but in all other respects it is the same.

Indications of Amount of Charge in a Storage Battery.1. The E.M.F. rises from 1.7 volts, the minimum value to which a lead cell should be discharged, to approximately 2.5 volts when fully charged, although this value may be a trifle higher or lower, depending upon the rate of charge and temperature of cell. The rise is gradual, but more rapid near the beginning and end of the charge, as indicated in Fig. 173. When the cell is fully charged, the E.M.F. becomes constant and the curve approaches a horizontal line as shown. The charging should then be stopped, as any more energy passed through the cell is simply wasted in producing gases. The external voltage is higher in charging than in discharging because of the internal resistance of the cell and resulting drop, which must be overcome in charging. The voltage

should be measured when the current is flowing either in charging or discharging. The E.M.F. on open circuit has lit le practical significance. The exact electrical relations are given later.

2. If a record is kept of the exact number of ampere-hours of charge and discharge, the actual amount of energy in the battery

Fig. 173. Curves showing Variations in Specific Gravity and Voltage in a Lead Storage Battery during Charge and Discharge.

at any time is known, due allowance being made for leakage and other losses. For this purpose any integrating instrument, such as the Thomson recording wattmeter, may be used.

3. The density of the electrolyte gradually r ́ses during the charging operation (Fig. 173); the density when charged being about .025 higher than when discharged. There is a lag in the change of the density of the electrolyte, the acid not being absorbed or given off at once by the plates; hence a little time should be allowed before taking any hydrometer reading as final.

4. Bubbles of gas are given off freely when the battery is fully charged, because the material of the plates is then no longer able

FULLY

CHARGED

FULLY

CHARGED

to take up the oxygen and hydrogen which tend to be set free by the electrolysis; these bubbles give the electrolyte the appearance of boiling, and often they are so fine that the liquid looks almost milky-white, particularly in a cell which has not been very long in

use.

5. The color of the positive plates varies from a light brown on active parts to a chocolate color when fully charged, and to nearly black when overcharged. The negatives vary from pale to dark slate color, but they always differ in color from the positives. This indication of the amount of charge is learned by experience, but is quite definite after one becomes familiar with a particular battery.

6. Cadmium Test.-Cadmium, when immersed in the electrolyte of a storage battery, gives reliable readings of the potential of the positive and the negative plates with respect to itself. In this way the condition of each plate of a battery can be determined. Readings are taken by inserting the cadmium (connected to one terminal of the voltmeter) into the electrolyte, but free from contact with plates, and connecting the other terminal of the voltmeter first to the positive plate, and then to the negative plate.

In making a cadmium test, care must be exercised to use either a sulphated piece of cadmium, or to wash the surface of the bright metal, after every reading, in distilled water, also when inserting the cadmium into the electrolyte, to keep it out of contact with the plates. The cadmium piece can be permanently fastened to one terminal of the voltmeter, and it should be covered by a soft rubber tube, perforated to admit the electrolyte; in this way rapid reading can be taken.

The relations between the cadmium readings and the total external voltage of the cell is fixed, that is to say, on discharge, the latter added to the minus (cadmium to negative plate) reading should equal the plus (cadmium to positive plate) reading. The voltmeter used for this work ought to be a good one and read accurately at the low end of the scale, otherwise the minus readings cannot be taken.

With normal conditions of cell, when fully charged and on open circuit, the difference of potential between the positive plate and the cadmium piece is 2.5 volts or nearly so, and between the cadmium piece and the negative plate is zero or nearly so.