Wet and Dry Cell Batteries
Definitions
Primary Cells - A cell that will discharge an electric current resulting from a chemical reaction but not one that is capable of being reversed ie. recharged.
 
Secondary Cells - Cells that are capable of being reversed to almost original condition so that the Eo potential is restored.
 
Storage Batteries - Car, Truck, Motorcycle etc.
The 12 V battery consists of 6 Voltaic cells connected in series so that their voltages are additive.
The anode and cathode are immersed in an electrolytic solution containg sulphuric acid.  The anode is spongy lead and the cathode is lead dioxide.
 
The anode reaction is:
 
      Pb(s)  +  HSO4-1  <===>  PbSO4  + H+1 + 2e-1    oxidation
 
The  cathode reaction is:
 
      PbO2(s)  +  HSO4-1  + 3H+1  + 2e-1  <====>  PbSO4(s)  + 2 H2O  reduction
 
The total reaction is:
 
     Pb(s)  +  PbO2  + 2 H+1  + 2 HSO4-1  <===>  2 PbSO4  + 2 H2O
 
As the battery discharges the density of the sulphuric acid solution decreases.  In older times the concentration of this could be checked at a garage.  The older batteries also had a gas fitting at the top of each cell that allowed any H2 gas created to escape.  this is course is an open system and the H2 that escaped is unavailable to recombine in the reverse reaction.
 
Todays' batteries are sealed. The H2 gas is trapped adn an equilibrium set of conditions is in place.  this means that the battery should last longer, be able to supply more consistent current and be recyclable once finished it life expectancy.
 

 

 
Dry cells

 
The reactions that are believed to occur in a dry cell are these:
 
Cathode:   2 MnO2  +  2 NH4+1   + 2e-1   ---->  Mn2O3   +  2 NH3  + H2O
 
Anode:     Zn  +  4 NH3   ----->   Zn(NH3)42+   + 2 e-1