STORAGE BATTERIES. 



245 



mean concentration of the entire electrolyte ; and while the cell 

 is discharging the concentration of the electrolyte in contact with 

 the active material is less than the mean. It is for this reason 

 largely that a greater electromotive force is required to charge a 

 lead storage cell than the cell maintains while discharging. 



2.6 



Time in hours 



Fig. 143. 



The two curves in Fig. 143, show the variation of the voltage 

 between the terminals of a lead storage cell while it is discharg- 

 ing and while it is being charged. The shapes of these charge 

 and discharge curves depend greatly upon the thickness of the 

 electrodes, the temperature of the cell, the concentration of the 

 electrolyte, and the value of the charging or discharging 

 current. 



Practical limit to discharge. The extent to which a lead 

 storage cell may be discharged in practice is determined by 

 the following considerations : (a) The lead peroxide and the 

 spongy metallic lead which constitute the active materials of a 

 fully charged lead storage cell, are fairly good electrical condirc- 

 tors ; whereas the lead sulphate into which these materials are 

 converted by discharge is an extremely poor electrical conductor, 

 much too poor to act as electrode material even in small pieces 



