Hi friends,
 
In this article, I am going to discuss the nickel iron battery construction, working principle, and compare its features with a lead-acid battery. So keep reading.
 
The Nickel-Iron alkaline cell was developed by an American scientist Thomson A. Edison in 1909. Therefore it is also known as Edison-cell.

Nickel Iron Battery Construction

The nickel-iron battery construction is shown in Figure. A Nickel-Iron cell has two plates. The active material of the positive plate is Ni(OH)₄ and the negative plate is of iron (Fe). The electrolyte is a solution of potassium hydroxide (KOH) with a small addition of lithium hydrate (LiOH) which increases the capacity of the cell.

The specific gravity of the electrolyte is 1.2. The container is also made of nickel-plated iron. Negative plates are connected to the container. Since a small quantity of electrolyte is used, therefore, it is quite compact.
 
Since a single cell produces a very low amount of current and voltage, many cells are connected in series and parallel to increase current and voltage rating of a nickel-iron battery respectively.

Nickel Iron Battery Working Principle

When the battery is fully charged, its positive plate is of Ni(OH)₄ and its negative plate is of iron (Fe). The electrolyte used is potassium hydroxide (KOH).  
 
Discharging: When the battery discharges, the potassium hydroxide (KOH) is dissociated into potassium (K⁺) and hydroxyl (OH^–) ions. The hydroxyl ions go to cathode and potassium ion go to the anode. The following chemical reaction takes place during discharging:
 
At cathode: Fe + 2OH –> Fe(OH)₂
At anode: Ni(OH)₄ + 2K —-> 2KOH + Ni(OH)₄
 
Thus, the anode is converted from Ni(OH)₄ to Ni(OH)₂ and cathode is converted from iron (Fe) to iron hydroxide [Fe(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 (K⁺) ions move towards the cathode. The following chemical reaction takes place during charging:
 
At anode: Ni(OH)₂ + 2OH —> Ni(OH)₄
At cathode: Fe(OH)₂ + 2K —> Fe + 2 KOH
 
Thus, anode and cathode regain their previous chemical composition without changing the strength of electrolyte.
 
The electrolyte does not take part in the chemical reaction. It performs only functions a conveyor. Its specific gravity changes only due evaporation of water and changes in temperature. Variation in specific gravity affects battery efficiency considerably. After 5 to 10 years, the electrolyte of the battery should be changed.

Electrical Characteristics of Nickel-Iron Cell

The EMF of a fully charged cell is 1.4 V which decreases to 1.3 V rapidly. The average EMF of a cell is 1.2 V and reduces to 1.0 V when fully discharged. The internal resistance of this cell is quite high nearly 5 times to that of the lead-acid cell.

Nickel-Iron Battery Vs Lead Acid

 Advantages

• Its life is more (about 40 years approximately) than that of a lead-acid battery (about 10 years approximately).
• Spilling of electrolyte (KOH) is not harmful. Whereas in the case of lead-acid battery, it is harmful.
• Since the specific gravity of the electrolyte (KOH) does not change while charging and discharging, therefore a nickel-iron battery is not damaged if it is left in a fully discharged condition for a considerable time of period. Whereas in the case of lead-acid battery, it may damage the battery permanently.
• Its weight is quite less (nearly half) than a lead-acid cell of equivalent capacity.
• It can be charged and discharged at a higher rate for a longer period without any danger to the battery.
• It can withstand higher temperatures as compared to a lead-acid battery.
• It is more rugged and can withstand more mechanical and electrical stresses.

Disadvantages

• It is costlier than a lead-acid battery of the same capacity.
• The EMF developed in a nickel-iron cell is only 1.2 V against 2 V of the lead-acid-cell. It means, if a supply voltage of 6 V is required, 5 nickel-iron cells have to be connected in series against 3 lead-acid cells.

• It has five times higher internal resistance as compared to a lead-acid battery and therefore can not supply a large value of current. Therefore, they are unsuitable for automobile starting.

• Its ampere-hour and watt-hour efficiencies are 75 to 80% and 60 to 65% respectively. Whereas in the case of lead-acid battery these are 85 to 90% and 70 to 80% respectively i.e. its efficiency is much lower.

Applications of Nickel-Iron Battery

Long long ago these batteries were used in sufficient quantity. Later, due to their high manufacturing cost and poor electrical characteristics, their production stopped.
 
They have poor energy density and poor specific power. Also, they have a higher self-discharge rate and lower charge-discharge efficiency. Due to these reasons, their production is very costly and applications are very limited.
 
However, there are some advantages also. These are their longer life span, higher mechanical strength and ability to withstand rough use, nowadays, research is going on to use them as a storage device for solar and wind power systems. By comparing their life span with that of lead-acid battery, in the long run, they may prove cheaper.
 
Thanks for reading about “nickel iron battery construction and working principle”.