The FPL cell, the construction of which is shown in Figs. 295, 300, 301, 302 and 303, has a sealed glass jar with a lead antimony cover. The cover construction is shown in detail in Figs. 301 and 302. Insulation between the posts and cover is provided by a hard rubber bushing, a hard rubber washer, and a soft rubber washer. The bushing is shaped like a "T" with a hole drilled in the stem. The stem of the bushing fits down into the post hole in the cover, the flange at the top testing on the raised portion of the cover around the post hole. The post has a shoulder a little less than halfway up from its lower end. Upon this shoulder is placed the hard rubber washer, and upon the hard rubber washer is placed the soft rubber washer. This assembly is fastened to the cover by the "peening" process used in Prest-O-Lite automobile batteries as described on page 386. This forces the soft rubber washer tightly against the cover so as to make a leak proof joint-between the bushing and cover. The ring of lead formed around the posts by the peening process supports the posts, plates, and separators, which therefore are suspended from the cell cover. The plate straps extend horizontally across the tops of the plates, and thus also act as "hold-downs" for the separators. The separators are held up by two rectangular rubber bridges which fit Mito slotted extension lugs cast into the lower corners of the outside negative plates. An outside negative having these extension lugs is shown in Fig. 303.
Specific Gravity of Electrolyte. The values of the specific gravity of Prest-O-Lite farm lighting batteries are as follows:
Battery fully charged reads 1.250 Battery three-fourths charged reads 1.230 Battery one-half charged reads 1.215 Battery one-fourth charged reads 1.200 Battery discharged completely reads 1.180
These readings are to be taken with the electrolyte at a temperature of 80° Fahrenheit. Readings taken at other temperatures should be converted to 80°. To convert readings at a lower temperature to the values they would have at 80°, subtract one point for every two and one-half degrees temperature difference. For example, suppose a cell reads 1.225 gravity at 60°. To find what the gravity would be if the temperature of the electrolyte were 80° divide the difference between 80° and 60° by 2-1/2, or 80° minus 60° divided by 21/2 equals 8. The gravity at 80° would therefore be 1.225 minus .008, or 1.217, which is the value of specific gravity to use. If the specific gravity is read at a higher temperature than 80°, divide the difference between 80° and the temperature at which the gravity reading was taken by 21/2, and add the result to the actual gravity reading obtained. If, for example, the gravity were 1.225 at 100°, the gravity at 80° would be 1.225 plus .008, or 1.233.
Charging Rates. The normal charging rate to be used in giving Prest-O-Lite batteries a regular charge or overcharge are as follows: