Let's have a discussion on working principle of lead acid battery
This is very commonly used as storage battery or secondary battery. Before going through the working principle, we should know about materials used for lead
acid storage battery cells. The main active materials required to construct a lead acid battery are lead peroxide, sponge
lead and dilute sulfuric acid. The positive plate of lead acid battery is made of lead peroxide
This is dark brown, hard and brittle substance. The negative plate of lead acid battery is made of pure lead in soft sponge condition
Dilute sulfuric acid used for lead acid battery has ratio of water is to acid equal to 3 is
to 1. The lead acid storage battery is formed by dipping lead peroxide plate and sponge lead
plate in dilute sulfuric acid. A load is connected externally between these plates
In diluted sulfuric acid, the molecules of acid split into positive hydrogen ions and
negative sulfate ions. The hydrogen ions when reach at lead peroxide plate, they receive electrons from it and
become hydrogen atom which again attack lead peroxide and form lead oxide and water
This lead oxide reacts with sulfuric acid and forms lead sulfate and water
Negative sulfate ions are moving freely in the solution, so some of them will reach to
pure lead plate where they give their extra electrons and become radical sulfate
As the radical sulfate cannot exist alone, it will attack pure lead and will form lead sulfate
As positive hydrogen ions take electrons from lead peroxide plate and negative sulfate ions
give electrons to lead plate, there would be an inequality of electrons between these
two plates. Hence there would be a flow of current through the external load between these plates for
balancing this inequality of electrons. This process is called discharging of lead acid battery
Now we will disconnect the load and connect lead sulfate covered lead peroxide plate with
positive terminal of an external DC source and lead peroxide covered lead plate with
negative terminal of that DC source. During discharging, the density of sulfuric acid falls but there still sulfuric acid exists
in the solution. This sulfuric acid also remains as positive hydrogen ions and negative sulfate ions in
the solution. Hydrogen ions being positively charged move to the electrode connected with the negative
terminal of the DC source. Here each hydrogen ion takes one electron from that and becomes hydrogen atom
These hydrogen atoms then attack lead sulfate and form lead and sulfuric acid
Negative sulfate ions move towards the electrode connected with the positive terminal of DC
source where they will give up their extra electrons and become radical sulfate
This radical sulfate reacts with lead sulfate of anode and forms lead peroxide and sulfuric acid
Hence by charging, the lead acid storage battery cell becomes ready for discharging