STORAGE BATTERIES. 255 



of the alternators, and it is desirable either to duplicate this 

 direct-current generating machinery or install a storage battery so 

 that the station may not be thrown out of service by a slight 

 accident to a comparatively insignificant part of the station 

 machinery. 



Controlling Devices. When a storage battery is used for 

 operating motors, as in case of automobile batteries for example, 

 no attempt is made to compensate for decrease of battery voltage 

 during discharge. When, however, a battery is used for oper- 

 ating incandescent lamps, the decrease of battery voltage during 

 discharge must be compensated so as to give a constant voltage 

 between the lighting mains. 



When a storage battery is used for equalizing a rapidly fluc- 

 tuating station load, provision must be made for automatically 

 causing the battery to discharge when the station load is large 

 and to charge when the station load is small. 



109. The use of the storage battery for supplying the station 

 output during the hours of small demand. When a storage 

 battery is used for this purpose it is nearly always required to 

 deliver current at constant volt- 

 age. A sufficient number of 



D 



storage cells is used to give 



the required voltage when the ^ J Main 



battery is discharged and has 1.8 

 volts per cell, and the control- 

 ling device is arranged to take 

 up the excess of voltage when 

 the battery voltage is higher than 

 the desired value. 



Control of voltage by rheostat. 

 The current, 2, delivered by the 



V Main 



battery is made to flow through 



Fig. 146. 



an adjustable resistance, R, Fig. 



146, so that the excess of battery voltage may be used up as the 



voltage drop, Ri t in the resistance. When the lamp load is con- 



