In this article we will discuss about the working of lead-acid battery with the help of diagram.
When the sulphuric acid is dissolved, its molecules break up into hydrogen positive ions (2H+) and sulphate negative ions (SO4– –) and move freely. Now if two lead electrodes are immersed in this solution and connected to dc supply mains, the hydrogen ions being positively charged move towards the electrode connected to the negative terminal of the supply mains (i.e., cathode) and SO4– – ions being negatively charged move towards the electrode connected to the positive terminal of the supply main (i.e., anode).
Each positive hydrogen ion (H+) reaching the cathode gets one electron from it, becomes atom of hydrogen and escapes out as hydrogen gas in the form of bubbles. Each of the negatively charged sulphate ion (SO4– –) reaching the anode gives two electrons to it, reacts with water and forms sulphuric acid and oxygen according to chemical reaction SO4 + H2O → H2SO4 + O. The oxygen produced attacks the lead anode and forms lead peroxide PbO2. Thus during charging lead cathode remains as lead but lead anode gets converted into lead peroxide (PbO2), chocolate in colour.
Now if the battery is disconnected from dc source of supply and voltmeter is connected between the electrodes, it will show potential difference between them and on connecting these electrodes, by a wire, current will flow from positive plate to the negative plate through external circuit, i.e., the cell is now capable of supplying electrical energy.
Chemical Action during Discharging:
When the battery is fully charged anode is of lead peroxide (PO2) and cathode is of sponge metallic lead (Pb). When the electrodes are connected through a resistance, the battery discharges and electrons flow in a direction opposite to that during charging. Now sulphuric acid molecules again break up into (2H +) and (SO4– –) ions.
Each hydrogen ion now moves to the anode and reaching the anode receives one electron from the anode (coming from cathode through external circuit) and becomes hydrogen atom. Since it is directly in contact with anode of PbO2, so it attacks and forms lead sulphate (PbSO4), whitish in colour and water according to chemical equation.
Each sulphate ion (SO4– –) moves towards the cathode and reaching there gives up two electrons (moving to the anode through external circuit) becomes radical SO4, attacks the metallic lead cathode and forms lead sulphate, whitish in colour according to chemical equation Pb + SO4 = PbSO4.
Thus during discharging:
(i) Both of the electrodes are converted into lead sulphate, whitish in colour.
(ii) Terminal potential of the cell falls.
(iii) Density of the electrolyte decreases due to formation of water during chemical action, and
(iv) Chemical energy stored is converted into electrical energy and is supplied to an external circuit.
Chemical Action during Recharging:
When the battery gets discharged as explained above both of the electrodes are converted into lead sulphate (PbSO4), whitish in colour. On recharging of the battery, the anode and cathode are connected to the positive and negative terminals, of the dc supply mains respectively. The molecules of sulphuric acid (H2SO4) in solution again breaks up into (2H+) and (SO4– –) ions.
Hydrogen ions (2H+) being positively charged move to the cathode, receives two electrons from there (coming from anode through external circuit), form hydrogen atom and react with lead sulphate cathode forming lead and sulphuric acid according to chemical equation-
PbSO4 + 2H = H2SO4 + Pb
SO4– – ion moves to the anode, gives up its two additional electrons, (moving to the cathode through external circuit) becomes radical SO4, reacts with lead sulphate anode and forms lead peroxide (PbO2) and sulphuric acid according to chemical equation-
PbSO4 + 2H2O + SO4 = PbO2 + 2H2SO4
Hence during recharging:
(i) Lead sulphate anode gets converted into lead per-oxide (PbO2) dark chocolate brown in colour.
(ii) Lead sulphate cathode gets converted into lead, grey in colour.
(iii) Terminal potential of the cell increases.
(iv) Density (concentration) of the electrolyte (H2SO4) increases (due to formation of H2SO4 during chemical reaction).
(v) Electrical energy received from the external source is stored in the form of chemical energy.
The charging and discharging of cell can be represented by a single reversible equation given below:
The equation should be read downward for discharge and upward for recharge.