Discharging.—In discharging a battery its voltage should never be allowed to fall below 1.8 volts, under load, thus leaving about 30 per cent. of the total capacity unused. The normal discharging current may be equal to the normal charging current, but a discharge equal to 3 or 4 times the normal may be given without injury to the plates. Some types may be discharged at even six or seven times the normal rate. In such cases, however, the capacity will be reduced in the same proportion, as before explained in the paragraph dealing with battery capacities.
Figs. 1,137 to 1,151.—Parts of the Witherbee battery. 1, jar; 2, inside cover; 3, cover; 4, handle; 5, vent cap; 6, cover, screws, nuts and washers; 7, handle eyes, nuts and washers; 8, rubber covered nut; 9, spannernut; 10, plate strap for positive plates; 11, plate strap for negative plates; 12, rubber separator; 13, wood separator; 14, positive group of plates; 15, negative group of plates; 16, positive plate; 17, negative plate; 18, cell connector. An element consists of a complete set of plates bound together on strap, with wood and rubber separators for a single cell. Positive plates are brown, negative plates, gray.
Ques. What is the effect of discharging too rapidly?
Ans. It tends to break the plates, and in the case of pasted plates, a very sudden discharge will dislodge the paste.
Ques. How is the discharge capacity of a storage battery stated?
Ans. In ampere hours. This, unless otherwise specified, refers to its output of current at the eight hour rate. Most manufacturers of automobile batteries specify only the amperage of the discharge at three and four hours. Thus, at the eight hour rate, a cell which will discharge at ten amperes for eight hours is said to have a capacity of eighty ampere hours. It does not follow that eighty amperes would be secured if the cell were discharged in one hour. It is safe to say that not more than forty amperes would be the result with this rapid discharge.
As a general rule, the one hour discharge rate is four times that of the normal, or eight hour discharge, and considerations of economy and prudence suggest that it should never be exceeded, if, indeed, it ever be employed. The three hour discharge, which is normally twice that of the eight hour, is usually the highest that is prudent, while the four hour discharge is the one most often employed in vehicles for the average high speed riding.
Ques. What should be the maximum rate of discharge?