254 ELEMENTS OF ELECTRICAL ENGINEERING. 



108. The use of storage batteries.* Storage batteries are 

 used for the one fundamental purpose of storing electrical energy 

 at a given time and place in order that it may be used when and 

 where it may be desired. The large first cost of storage cells 

 and their rapid depreciation, which amounts to from 5 to 10 per 

 cent, per annum even when they are properly cared for, limits 

 their commercial use to those cases in which the advantages of 

 storage are very great. 



Portable cells. Storage cells which are intended to be carried 

 about are generally made as light as possible by using thin grids 

 and hard rubber containing vessels ; and even then the weight is 

 very great. Thus a storage battery designed for car lighting, 

 and capable of operating thirty i6-candle-power incandescent 

 lamps for 8 hours, weighs about 3,240 pounds. The necessity 

 of recharging a battery promptly after it has been used is also a 

 serious matter, and storage cells which are used in small sets for 

 driving small motors and induction coils are almost never prop- 

 erly cared for, and therefore they last but a very short time. 



Stationary cells. Storage batteries are most extensively used 

 in connection with central stations : 



(a) For supplying the station output during the hours of small 

 demand. In this case the battery is charged while the station 

 is in operation, and discharged during the remainder of the day, 

 thus obviating the expense of operating the station continuously. 



(b) For equalizing a rapidly fluctuating station load. In this 

 case provision is made for the battery to charge while the station 

 load is below the average and to discharge while the station load 

 is above the average. This is the most important use of large 

 storage battery installations, and the cost of installing and main- 

 taining the battery is set over against the saving in the first cost 

 of the station and the saving in the cost of operating the station. 



(c) As a reserve. In alternating-current generating stations a 

 small direct-current generator is used to excite the field magnets 



* A very complete discussion of the uses of storage batteries is given by Lamar 

 Lyndon, in his Storage Battery Engineering. 



