Lead Accumulator – Secondary Electrochemical Cells

An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic or galvanic cells and those that generate chemical reactions, via electrolysis for example, are called electrolytic cells.A common example of a galvanic cell is a standard 1.5 volt cell meant for consumer use. A battery consists of one or more cells, connected in parallel, series or series-and-parallel pattern.

secondary electrochemical cell

Secondary electrochemical cell is an electric cell that can be recharged and can therefore be used to store electrical energy in the form of chemical energy.

In the secondary cells, the reactions can be reversed by an external electrical energy source. Therefore, these cells can be recharged by passing electric current and used again and again. These are also called storage cells. Examples of secondary cells are, lead storage battery and nickel – cadmium storage cell.

While primary cells are single-use, secondary cells can be recharged. This means that the cell can be run as both a galvanic cell during discharge and as an electrolytic cell when being recharged. A common example is an automobile battery that can act as a galvanic cell when it powers a starter motor and behave as an electrolytic cell when it is being recharged by the car’s alternator.

This dual use of the cell is the principle behind rechargeable batteries, which require an external dc current to be applied to the cell for recharging to occur. The direct current (dc) is applied in the opposite direction to the current flow that occurs during the discharge process. Consequently, the redox reaction of the chemicals in the cell proceeds in the opposite direction, recreating the chemicals used in the initial reaction.

Lead accumulator

The lead accumulator refers to a secondary cell as the electrical energy doesn’t generate itself inside the cell, but it is stored prior to an external source.

It’s a reversible cell as the cell reactions are in the reversed state if external emf is suitably bigger than this cell’s emf which is applied. Therefore, in this cell, total cell reactions are reversed by exerting an external opposite to e.m.f which is greater than e.m.f of the cell.

This cell has the capability to store the electrical energy from charging or from the external source, and the supply process is done at the time of discharge. In chemical energy form, the energy hence is stored. So, this cell refers to an accumulator or the storage cell or the storage battery. 

Cells’ voltage is not dependent on electrodes’ size or its cell’s size, but it’s definitely dependent on the strength of the solution of sulphuric acid.

Cell Representation:

-Pb(s)| PbSO4(s)| 38% H2SO4(aq)| PbSO4(s)| PbO2(s)| Pb(s)+

Working:

Cell Discharging: When the cell starts to work, oxidation occurs on the lead plates and the reduction occurs in the lead plates with PbO i.e. Lead Oxide. This is known as the discharging of the cell.

During charging (charging circuit electrode signs):

(+) electrode:PbSO4(s) + 2H2O(l) → PbO2(s) + 4H+(aq) + SO42-(aq) + 2e–

(–) electrode: PbSO4 (s) + 2e– → Pb(s) + SO42-(aq)

Discharging (electrode signs as for cell):

(+) electrode: PbO2(s) + 4H+(aq) + SO42-(aq) + 2e– → PbSO4(s) + 2H2O(l) 

(–) electrode: Pb(s) + SO42-(aq) → PbSO4(s) + 2e–

Now we’ll look at the cell’s overall reaction as it’s being discharged.

PbO2+2H2SO4+Pb → 2PbSO4+2H2O

Charging of cell: When emf acting externally is greater than cell’s emf which is applied, then there are accurate reverse reactions taking place. The Oxidation occurs on positive electrodes and reduction at negative electrodes. This is referred to as the charging of the cell.

Lead Accumulator Uses:

• Lead accumulators have their use in motor cars and in different automobiles vehicles as a consequence, they are well-known as “car batteries”. Generally, six cells are attached in series to offer the voltage of 12V.
• Lead accumulators are used in motor cars, other automotive vehicles and therefore popularly known as car batteries. Here generally 6 cells are connected in series to give a voltage of 12 Volt
• They are used in laboratories as a source of constant DC voltage. 
• They are used in telephone and Telegraph offices.
• They are used. In electric clocks, radio set burglar alarms, etc. 
• They are also used as a source of electricity in the initial stages of the rocket.

Maintenance of the lead accumulator.

• It should be kept upright and not slanted.
• When the cell is not in use for an extended period of time, the terminal should be disconnected, the acid from the cells should be removed completely, and the cell should be kept dry.
• To achieve the best cell potential, the concentration of sulphuric acid should be kept constant.
• When the cells are in use, they should not be allowed to run down March. When the voltage is around 1.8 Volt, the EMF of the cell diminishes, and the cells should be recharged.
• The sales should not be overcharged, which means the cell’s emf should not exceed 2.1 Volt.
• By adding distilled water, the acid levels should be adequately maintained.

Conclusion :

The advantage of secondary cells is that they are rechargeable. The chemical reactions that take place in secondary cells are reversible. The active materials that are used up when the cell delivers current can be reproduced by passing current through the cell in the opposite direction.