Basic Electrical Engineering (3110005)

Discharging process

• 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 the acid split into positive hydrogen ions ( ${\mathrm{H}}^{+}$ ) and negative sulfate ions ( ${{\mathrm{SO}}_4}^{-2}$ ).
• The hydrogen ions when reach at ${\mathrm{PbO}}_2$ plate, they receive electrons from it and become hydrogen atom which again attack ${\mathrm{PbO}}_2$ and form $\mathrm{PbO}$ and ${\mathrm{H}}_2\mathrm{O}$(water). This $\mathrm{PbO}$ reacts with ${\mathrm{H}}_2{\mathrm{SO}}_4$ and forms ${\mathrm{PbSO}}_4$ and ${\mathrm{H}}_2\mathrm{O}$ (water).
  
  
  ${\mathrm{PbO}}_2 + 2\mathrm{H} \to \mathrm{PbO} + {\mathrm{H}}_2\mathrm{O}$
  ${\mathrm{PbO}}_2 + {\mathrm{H}}_2{\mathrm{SO}}_4 + 2\mathrm{H} \to {\mathrm{PbSO}}_4 + 2{\mathrm{H}}_2\mathrm{O}$
  
  
• ${{\mathrm{SO}}_4}^{-2}$ ions are moving freely in the solution so some of them will reach to pure $\mathrm{Pb}$ plate where they give their extra electrons and become radical ${\mathrm{SO}}_4$.
• As the radical ${{\mathrm{SO}}_4}^{-2}$ cannot exist alone it will attack Pb and will form ${\mathrm{PbSO}}_4$. As ${\mathrm{H}}^{+}$ ions take electrons from ${\mathrm{PbO}}_2$ plate and ${{\mathrm{SO}}_4}^{-2}$ ions give electrons to $\mathrm{Pb}$ 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. The lead sulfate ( ${\mathrm{PbSO}}_4$ ) is whitish in color.

 

Charging process

• Both of the plates are covered with ${\mathrm{PbSO}}_4$. Specific gravity of sulfuric acid solution falls due to formation of water during reaction at ${\mathrm{PbO}}_2$ plate.
• As a result, the rate of reaction falls which implies the potential difference between the plates decreases during discharging process.
• Now we will disconnect the load and connect ${\mathrm{PbSO}}_4$ covered ${\mathrm{PbO}}_2$ plate with positive terminal of an external DC source and ${\mathrm{PbO}}_2$ covered $\mathrm{Pb}$ 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 ${\mathrm{H}}^{+}$ and ${{\mathrm{SO}}_4}^{-2}$ ions in the solution. Hydrogen ions being positively charged, move to the electrode (cathode) connected with negative terminal of the DC source.
• Here each ${\mathrm{H}}^{+}$ ion takes one electron from that and becomes hydrogen atom. These hydrogen atoms then attack ${\mathrm{PbSO}}_4$ and form lead and sulfuric acid.
  
  
  ${\mathrm{PbSO}}_4 + 2\mathrm{H} \to {\mathrm{H}}_2{\mathrm{SO}}_4 + \mathrm{Pb}$
  
  
• ${{\mathrm{SO}}_4}^{-2}$ ions move towards the electrode (anode) connected with positive terminal of DC source where they will give up their extra electrons and become radical SO4. This radical ${\mathrm{SO}}_4$ cannot exist alone.
• Hence reacts with ${\mathrm{PbSO}}_4$ of anode and forms lead peroxide ( ${\mathrm{PbO}}_2$ ) and sulfuric acid ( ${\mathrm{H}}_2{\mathrm{SO}}_4$ ), 
  
  
  ${\mathrm{PbSO}}_4 + 2{\mathrm{H}}_2\mathrm{O} + {\mathrm{SO}}_4 \to {\mathrm{PbO}}_2 + 2{\mathrm{H}}_2{\mathrm{SO}}_4$
  
  
• Hence by charging the lead acid storage battery cell,
  1. Lead sulfate anode gets converted into lead peroxide
  2. Lead sulfate of cathode is converted to pure lead
  3. Terminal potential of the cell increases
  4. Specific gravity of sulfuric acid increases.