A nickel-cadmium cell has two plates. The active material of the positive plate (anode) is Ni(OH)4 and the negative plate (cathode) is of cadmium (Cd) when fully charged. The electrolyte is a solution of potassium hydroxide (KOH) with a small addition of lithium hydrate which increases the capacity and life of the battery. The specific gravity of the electrolyte is 1.2.
Since the voltage produced by a single cell is very low, many cells are connected in series to get the desired voltage output and then this arrangement is known as the nickel cadmium battery.
Nickel Cadmium Battery Construction and Working
In these batteries, the number of positive plates is one more than that of negative plates. The container of this battery is electrically connected to the positive plates.
When the cell is fully charged, its positive plate is of Ni(OH)4 and its negative plate is of cadmium (Cd).
Discharging: When the cell discharges, the potassium hydroxide (KOH) is dissociated into potassium (K+) and hydroxyl (OH–) ions.
The hydroxyl ions go to cathode and potassium ions go to the anode. The following chemical reaction takes place during discharging.
At cathode: Cd + 2OH —–> Cd(OH)2
At anode: Ni(OH)4 + 2K ——> 2KOH + Ni(OH)2
Thus, the anode is converted from Ni(OH)4 to NI(OH)2 and cathode is converted from cadmium (Cd) to cadmium hydroxide [Cd(OH)2]. The strength of the electrolyte remains the same.
Charging: When the battery is put on charging, the hydroxyl (OH-) ions move towards the anode, whereas the potassium ions (K+) move towards the cathode. The following chemical reaction takes place during the charging:
At anode: Ni(OH)2 + 2OH —–> NI(OH)4
At cathode: Cd(OH)2 + 2K —–> Cd + 2KOH
Thus anode and cathode regain their previous chemical composition without changing the strength of electrolyte.
Electrical Characteristics of Nickel Cadmium Battery
- The EMF of a fully charged cell is 1.4 V which decreases to 1.3 V rapidly. The average EMF of the cell is 1.2 V which reduces to 1.0 V when discharged.
- The internal resistance of the cell is very less, even lesser than a lead-acid battery. Due to low internal resistance these can be operated at very high charge and discharge currents efficiently and safely.
- The ampere and watt-hour efficiencies of the cell are about 80% and 65% respectively.
Advantages of Nickel Cadmium Battery
- It has a very long (about 20 years) active life.
- Since the specific gravity of the electrolyte (KOH) does not change. It can be stored indefinitely in either a discharged or charged state without suffering any ill effects.
- It can withstand peak rates of discharge and charge up to 20 times the normal operating time.
- They have very low open circuit losses.
- The effect of high discharge rate and low operating temperature is very low on the capacity of the battery as compared to any other type of rechargeable battery.
Disadvantages of Nickel Cadmium Battery
- It is very expensive.
- The EMF developed by a fully charged cell is only 1.2 V against 2 V of the lead-acid cell.
- Its ingredients are harmful for ecosystem. Therefore, its production and utilization is banned in some countries.
Applications of Nickel Cadmium Battery
- These are used in small portable electrical appliances.
- The energy density of nickel-cadmium batteries is high. Also, these are lighter and more compact. Therefore, these are preferred in applications where weight and size of the battery are very important like in airplanes and helicopters. There these are used to start the engine.
Small Nickel Cadmium Cells
The various advantages of nickel-cadmium cells, like longer life, low maintenance, low internal resistance, etc. prompted scientists to develop these cells in small sizes.
Since EMF developed by these cells 1.2 V which is very near to the EMF developed by dry cells (1.5), therefore these cells have been developed by scientists of the same size that of small carbon-zinc or dry primary cells (called pencil cells).
Today these small sizes nickel-cadmium cells are used in cordless electric appliances such as electric shavers, hearing aids, photography equipment, etc.
The ingredients of small nickel-cadmium cells are the same as that of a larger type. The plates are woven in the form of a screen and a paste of active material is pressed into the spaces within the screen. A separator is placed between the positive and negative plates and rolled in the form of a cylinder. The complete assembly is placed into a cane for protection.
The negative plate is connected with the body of the cane which forms the negative terminal. An insulated button is placed at the top to which the positive plate is connected and forms the positive terminal.
Though the initial cost of a nickel-cadmium cell is very high as compared to a carbon-zinc primary cell (i.e. dry cell) yet it is less expensive in the long run. This is because nickel-cadmium can be recharged and has a very long life whereas the dry cell (primary cell) can not be recharged and has to be discarded when it is discharged.
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