220 



ELECTRICAL ENGINEERING 



The capacity of a cell is proportional to the area of the plates 

 exposed to the electrolyte and for an eight-hour discharge rate a 

 current density of from 40 to 60 amperes per square foot of 

 positive plate is common practice. 



Voltage. The voltage of a cell depends on the character of the 

 electrodes, the density of the electrolyte and the condition of the 

 cell but is independent of the size. The variation of the terminal 

 voltage of a cell during charge and discharge is shown by the 

 curves in Fig. 185. On charge the voltage begins about 2 volts 



01234567 

 Hours 



FIG. 185. Voltage characteristics of a storage cell. 



and rises to 2.5 volts or a little above. When the charging cir- 

 cuit is opened the voltage falls to 2.1 volts and during discharge 

 falls off gradually to about 1.9 volts. Beyond this point the fall 

 of voltage is very rapid and discharge should not be continued 

 after the voltage has fallen to 1.7 volts. 



The required battery voltage is obtained by connecting a num- 

 ber of cells in series and the required current is obtained by con- 

 necting a number of plates or cells in multiple. 



134. Applications. Batteries are installed in direct-current 

 power stations to store energy during periods of light load and to 

 deliver energy in parallel with the generators during periods of 

 heavy load. When the load is light the generators charge the 

 battery and when the load is heavy the charge is given up and so 

 the load on the generators is maintained nearly constant and they 

 can be operated at maximum efficiency. The result is that the 

 voltage regulation of the system is improved. 



