STORAGE BATTERIES. 265 



of 75 kilowatts, and the variations of load are equalized by a 

 storage battery which has to deliver a maximum current of 250 

 amperes (=400 amperes 150 amperes) on discharge, and ab- 

 sorb the entire generator output of 1 50 amperes when the station 

 load is zero. On account of the greater values of current on dis- 

 charge than on charge, the maximum duty of the booster is les- 

 sened by choosing the number of storage cells so as to have the 

 mean voltage of the battery greater than the station voltage. 

 The storage battery in fact consists of 234 cells, which require a 

 maximum charging electromotive force of about 562 volts (2.4 

 volts per cell), and the battery is never discharged below 445 

 volts (1.9 volts per cell). The duty required of the booster 

 varies between wide limits. The heaviest duty required for 

 charging the battery comes when the battery is at nearly full 

 charge and the demand for current zero ; then the booster must 

 develop 62 volts with 150 amperes of current flowing through 

 its armature. The heaviest duty required for discharging the 

 battery comes when the battery is nearly discharged and the de- 

 mand for current a maximum ; then the booster must develop 

 55 volts with 250 amperes in its armature. The booster would 

 therefore be rated, say, as a 5 5 -volt 25O-ampere generator, inas- 

 much as the voltage of such a machine could be easily pushed 

 up to 62 volts with an armature current of only 1 50 amperes. 

 This booster differs from an ordinary generator in having a very 

 large space provided for its field windings so as to give room for 

 the two opposing windings 5 and S' of Fig. 151. This booster 

 may be driven by the same engine that drives the main gener- 

 ator, or, as is more usual, by a shunt motor which is supplied 

 with current from the station bus bars. 



