STORAGE BATTERIES. 253 



of positive grid area, where positive grid area is equal to 2lbu y 

 I being the length, b the breadth of each grid, and n the number 

 of positive grids. The reason for referring the discharge rate to 

 the area of the positive grids rather than to the area of the nega- 

 tive grids is that the positive grids are always one less in number 

 than the negative grids, so that both sides of every positive grid 

 are active. 



The discharge rate of a storage battery in amperes should 

 never exceed four or five times the- normal rate of discharge 

 (eight-hour basis). If a greater current must be taken from the 

 battery it should be for a few minutes, only, and the battery 

 should be at full charge. 



The rate of charge may be high when the battery is nearly 

 discharged, but it should be low when the battery approaches 

 full charge, especially after the evolution of gas begins. A good 

 rule for rapid charging is to deliver to the battery 3 5 per cent, of 

 the total ampere-hours during the first hour ; 52 per cent, during 

 the next two hours, and 14 per cent, during the fourth hour. 

 Thus a 100 ampere-hour cell may be completely charged in 

 four hours by using a charging current of 35 amperes during the 

 first hour, 26 amperes during the second and third hours, and 

 14 amperes during the fourth hour. 



Very slow charging is injurious. The charging current should 

 never be less than about one fourth of the normal discharge rate 

 of the cell (on an eight-hour basis). 



The variation of the ampere-hour capacity with the rate of dis- 

 charge depends greatly upon the thickness of the active material 

 and the freedom of circulation and diffusion of the electrolyte. 

 The values for the stationary batteries of the Electric Storage 

 Battery Company are as follows : A cell that has 100 ampere- 

 hours capacity on an eight-hour discharge (12.5 ampere dis- 

 charge rate) has a capacity of 87.5 ampere-hours on a five-hour 

 discharge (17.5 ampere discharge rate), 75 ampere-hours on a 

 three-hour discharge (25 ampere discharge rate), and 50 ampere- 

 hours on a one-hour discharge (50 ampere discharge rate). 



