A secondary cell is that cell in which the electrical energy is first stored up as a chemical energy and when the outside circuit is closed to draw the current from the cell, the chemical energy is reconverted into electrical energy. The chemical reactions are reversible in this cell.
The secondary cells are also called storage cells or accumulators because they act in such a way as if they were reservoir of electricity, i.e. the current can be drawn from them whenever required and when they are discharged, they can be recharged. The commonly used secondary cells are Lead-Acid accumulator and Edison cell.
Lead-Acid accumulator: It consists of a glass or hard rubber vessel containing dilute
sulphuric acid (20% conc.), which act as electrolyte. There are two sets of perforated lead plates arranged alternately parallel to each other inside the vessel (Fig. 1.23). These plates are held apart by strips of wood or celluloid. Alternate plates are soldered together to one lead rod forming one electrode while remaining once soldered to another common lead rod forming another electrode. The holes or perforations in the lead plates are filled with red lead or lead oxide (PbO2).
Fig. 1.23: Secondary cell (lead-acid accumulator)
Charging: Charging means storing of electrical energy. To charge this accumulator a
source of steady current or battery charger is connected across the two terminals of two electrodes. The electrode which is connected to positive terminal of external source serves as anode and the other electrode serves as cathode. The dissociation of H2SO4 gives the H+and SO
42–. When current is passed through the cell by the help of external source,
hydrogen ions move to the negative electrode (called cathode) and the sulphate ions go to positive electrode (called anode).
During charging electron moves from the anode to cathode, thus raising the potential difference between the electrodes. In charging process, water is consumed and sulphuric acid is formed. When the specific gravity of sulphuric acid solution becomes 1.25, the cell is fully charged. The emf of the cell at this stage is 2.2 volts.
Discharging: If the cell is connected to the external circuit, the current is drawn from the
charges, they react with the electrodes and reduce the active material of each plate to lead sulphate.
In discharging process, the electrons moves from the cathode to anode, thus lowering the potential difference between electrodes. Hence, the emf of cell falls. In this process, sulphuric acid is consumed and water is formed. Therefore, the specific gravity of sulphuric acid also falls. If the specific gravity of sulphuric acid falls below 1.18, the cell requires recharging.
Alkali accumulator (Ni-Fe) or Edison cell: It is also known as alkaline secondary cell or
Edison cell. It consists of a steel vessel containing 20% solution of KOH in distilled water (as electrolyte) and 1% Lithium hydroxide to make it conducting. Here anode is a perfo- rated steel plate in the form of a grid. Its holes are packed with nickel hydrochloride and trace of nickel to make it conducting. The cathode is also made of a steel grid. Its holes are packed with a iron hydrochloride and trace of mercury oxide for lowering its internal resistance (Fig. 1.24).
Fig. 1.24: Alkali accumulator or Edison cell
Working: Potassium hydroxide solution breaks up into positive potassium ions and
negative hydroxyl ions due to ionization.
Charging: On passing the current from an external source, the anode attracts negative
hydroxyl ions and cathode attracts positive potassium ions. These ions on reaching the respective electrodes lose their charge and react with them. Thus, when accumulator is charged Ni(OH)4 is formed on the anode and a spongy Fe on the cathode. In this process, electrons moves from anode to cathode, raising the potential difference between the two electrodes of cell. When this potential difference becomes 1.36 V, the cell is fully charged.
Discharging: When the two electrodes of the cell are connected together through a resistor,
there is discharging of the cell, i.e. the cell is giving the current. Now the anode attracts the potassium ions and cathode attracts hydroxyl ions. These ions on reaching the respective electrodes give their charges and react with them. The electrons moves from cathode to anode, thus lowering the potential difference between two electrodes, due to which emf of the cell falls. When the emf becomes less than 1.1 V, then the cell requires recharging.
The emf of Ni-Fe cell is 1.36 V. Its internal resistance is low but is higher than net storage cell.
advantages
1. It can withstand rough handling.
2. It is lighter, stronger and more durable than the lead accumulator. 3. It is not damaged or over recharged.
4. It is not spoiled even if left uncharged for a long time.
disadvantages
1. Its initial cost is high.
2. Its emf is smaller and internal resistance is greater than that of lead accumulator. Therefore, it cannot give us very strong currents.
3. It absorbs carbon dioxide when exposed to atmosphere and thus its capacity is considerably reduced.