A lead-acid battery is the most inexpensive battery and is widely used for commercial purposes. It consists of a number of lead-acid cells connected in series, parallel or series-parallel combination.
A lead-acid cell basically contains two plates immersed in electrolyte (dilute sulphuric acid i.e. H2SO4 of specific gravity about 1.28). The positive plate (anode) is made up of lead-peroxide (PbO2) and the negative plate (cathode) is made up of sponge lead (Pb).
When the cell is delivering electrical energy to the external circuit (load), the process is known as discharging of the cell. Whereas, when it is taking electrical energy from the external DC source, the process is known as charging of the cell.
Lead acid battery charging and discharging equations
Lead acid battery discharging equations
When the sulphuric acid is dissolved, its molecules are dissociated into hydrogen ions (2H+) and sulfate ions (SO4– –) which moves freely in the electrolyte.
When the load resistance is connected to terminals of the battery; the sulfate ions (SO4– –) travel towards the cathode and hydrogen ions (2H+) travel towards the anode. The chemical reactions at the cathode are as under:
SO4– – – 2e = SO4 (radical)
Pb + SO4 —–> PbSO4
Each hydrogen ion (H+) on reaching the anode, takes one electron from it to become hydrogen gas. This electron is the same electron which is given by the sulfate ion at the cathode and has come to the anode via external load resistor R.
2H+ + 2e = 2H
The hydrogen gas liberated at the anode acts chemically on the anode material (PbO2).
PbO2 + 2H —-> PbO + H2O
This PbO2 is reduced to PbO which reacts with H2SO4 and forms PbSO4.
Thus during discharging
- Both the plates are transformed into lead sulfate (PbSO4).
- Sulphuric acid is consumed and water is formed which reduces the specific gravity of electrolyte from 1.28 to 1.18.
- The terminal voltage of each battery cell falls to 1.8V.
- Chemical energy is converted into electrical energy which is delivered to load.
Lead acid battery charging equations
The lead-acid battery can be recharged when it is fully discharged. For recharging, positive terminal of DC source is connected to positive terminal of the battery (anode) and negative terminal of DC source is connected to the negative terminal (cathode) of the battery.
During recharging, hydrogen ions (2H+) travel towards the cathode and sulfate ions (SO4– –) travel towards the anode. The chemical reactions are as under:
SO4– – – 2e —> SO4 (radical)
SO4 + H2O —> H2SO4 + O
The oxygen (in atomic stage) reacts chemically with anode material (PbSO4):
PbSO4 + O + H2O —> PbO2 + H2SO4
Each hydrogen ion (H+) on reaching the cathode, takes one electron from it to become hydrogen gas. This electron is given by sulfate ion at the anode and has come to cathode via external circuit (from the DC source)
2H+ + 2e —> 2H
The hydrogen gas liberated at the cathode acts chemically on the cathode material (PbSO4)
PbSO4 + 2H —> Pb + H2SO4
Thus, PbSO4 is transformed to Pb. Water (H2O) is consumed and H2SO4 is formed in above chemical reactions. This increases the specific gravity of the electrolyte. Ultimately, the cell regains its previous (before discharging) chemical compositions.
Thus during charging
- The anode is transformed into lead peroxide (PbO2) and cathode into the spongy lead (Pb).
- Water is consumed and sulphuric acid is formed which increases the specific gravity of electrolyte from 1.18 to 1.28.
- The terminal voltage of each battery cell increases to 2.2 to 2.5V.
- Electrical energy is converted into chemical energy which is stored in the cell.
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