- A cell or battery is a device that is used to convert stored chemical energy to electrical energy.
- A cell is made up of two metal plates of different materials called electrodes which are immersed in a suitable solution called an electrolyte.
The magnitude of developed electromotive force (EMF) of a cell depends upon:
- The nature of the material of electrodes
- The nature of the electrolyte
In actual practice the current and voltage rating of the cell is very less, to overcome that rating we have to use the numbers of cells in a particular arrangement that is series or parallel.
This arrangement of the cell is known as Battery.
Primary Cell:
- A cell in which chemical action is irreversible is called a primary cell.
- It is the simplest voltage cell.
- It supplied energy until the electrolyte is exhausted or the negative electrode is completely dissolved.
- The life of the primary cell is about one year.
- Application: Torch, clock, TV remote, etc
Example of Primary Cell:
1. Leclanche Cell:
- Its anode or positive electrode is made up of Carbon and the cathode or negative electrode is made up of a Zinc container.
- Electrolytes consist of an aqueous solution of ammonium chloride.
- It has an open circuit EMF of 1.5 V.
2. Clark Cell:
- Its anode or positive electrode made up of Mercurous sulfate and cathode or negative electrode made up of Zinc sulfate.
- Saturated zinc sulfate is used as the electrolyte.
- It has an open circuit EMF of 1.43 V.
3. Weston Cell:
- Its anode or positive electrode is made up of cadmium sulphate and the cathode or negative electrode is made up of mercurous.
- The solution of cadmium sulfate is used as an electrolyte.
- It has an open circuit EMF of 1.3183 V.
Secondary Cell:
- A cell in which chemical action is reversible is called a primary cell.
- It operates on the phenomena of charging and discharging.
- When a secondary cell is charged, electrical energy is converted into chemical energy which is stored in the cell. When the cell discharges, the stored chemical energy starts converting into electrical energy. For this reason, a secondary cell is sometimes called a storage cell.
- It is also known as Dry cell.
- Lead-Acid cell, Nickel-iron-alkaline cell (Edison cell), Nickel-cadmium-alkaline cell are an example of the secondary cell.
1. Lead Acid cell:
Component of Lead-Acid cell:
- The positive electrode consists of lead peroxide (PbO2) which is deposited on the frame of antimony lead alloy. Under full charge conditions, the positive electrode is dark brown.
- The negative electrode consists of porous lead (Pb) which is deposited similarly to the negative electrode on the frame.
- Both the positive and negative electrode is separate electrically through the separator.
- Dilute sulphuric acid (H2SO4) is used as the electrolyte.
- Electrode, electrolyte, and separator are placed in a container made of hard glass or hard rubber.
Discharging of Lead-Acid cell:
- When the cell is fully charged then the anode (PbO2) is brown and the cathode (Pb) is grey.
- When the cell is connected to the load then it starts discharging.
- Under discharging conditions H2SO4 is dissociated into H2 and SO4 ions.
- H2 ion move to the positive plate and SO4 move to the negative plate.
- Once the cell completely discharges both electrodes (PbO2) and (Pb) converts into PbSO4 and electrolyte H2SO4 converted to H2O.
It can be represented by the following chemical equation:
PbO2 + 2H2SO4 + Pb Discharging → PbSO4 + 2H2O + PbSO4 + electrical energy
Charging of Lead-Acid cell:
- For charging of Lead-acid cell, we have required DC supply which fed in opposite direction to that in which cell provided current.
- Under that condition the H2 ion move to the cathode and the SO4 ion move to the anode.
It can be represented by the following chemical equation:
PbSO4 + 2H2O + PbSO4 + electrical energy Charging → PbO2 + 2H2SO4 + Pb
EMF Characteristics:
- EMF of the fully charged lead-acid cell is a maximum of 2.2 volts.
- When the cell is connected to the load then discharging starts and EMF decreased concerning discharging.
- The EMF of a cell also depends on the last time when it was charged, the specific gravity of the electrolyte, and temperature.
Internal resistance characteristics:
- Internal resistance offered opposition to the current in the cell.
- Internal resistance in the cell is due to the resistance of electrodes and the resistance of electrolytes.
- The wattage rating of internal resistance of the lead-acid cell is about 0.01 W.
The internal resistance of a lead-acid cell depends on the following factor:
- The cross-section area of electrodes.
- Spacing between the electrodes.
- Specific Gravity of electrolyte (H2SO4)
The capacity of Lead-acid cell:
The capacity of the cell is measured in ampere-hours (Ah) and it is defined as the quantity of electricity that can be given out from fully charged condition to fully discharge condition.
Ah = Energy EMF
Efficiency:
- The ampere-hour efficiency of a lead-acid cell is about 90%.
- The watt-hour (or energy) efficiency of a lead-acid cell is about 75%.
The efficiency of a lead-acid cell depends upon the following factors:
- Rate of charging and discharging
- The internal resistance of the cell
- Temperature
- The time interval between the charging and discharging
Applications of Lead-acid cell:
- The lead-acid cell is considered as most economical DC source after the DC generator because of its reliability and low cost of installation.
- The major application of Lead-acid battery is following:
- In automobiles for additional power supply such as light, horn, and start.
- In traction for supplying supply to light and fan circuit.
- In Separately excited DC generator and Alternator for providing field excitation.
- In commercial and residential places for continuity of supply with the inverter.
- In lighting purpose in remote area etc
2. Alkaline Cell:
- The lead-acid cell has many disadvantages such as shorter life, sulphation of plates if left
- discharged for a long period and need much care and maintenance.
- To overcome the disadvantages of Lead-acid cells we have to use an Alkaline cell or battery.
Generally, two types of Alkaline cell is widely used:
- Nickel-iron cell or Edison cell
- Nickel-cadmium cell
1. Nickel-Iron Cell or Edison Cell:
- It was first developed by an American Scientist Thomas A. Edition in 1909.
- It has the advantage that lesser weight, longer life, therefore, it is very suitable for portable used.
- The EMF of the Edition cell is about 1.36 volts.
Component of Edition cell:
- The positive electrode consists of Nickel Oxyhydroxide Ni(OH)4
- The negative electrode consists of Iron (Fe).
- Both the positive and negative electrode is placed in a nickel-plated steel container separate electrically through hard rubber strips.
- Potassium Hydroxide (KOH) is used as the electrolyte.
- The molecules of electrolyte (KOH) dissociate into K+ and OH– ions.
Discharging:
- When the cell is connected to the load under that condition K+ ion move toward the positive plate and OH- ion move toward the negative plate.
The chemical reaction during discharging can be represented by the following chemical equation:
Positive Plate: Ni(OH)4 + 2K → Ni(OH)2 + 2KOH
Negative Plate: Fe + 2OH → Fe(OH)2
Ni(OH)4 + KOH + Fe → NiOH2 + KOH + FeOH2 + electrical energy
Charging:
- For charging of Edition cell, we have required DC supply which fed in opposite direction to that in which cell provided current.
- When the cell is connected to the external DC supply K+ ions move toward the negative plate and OH- ions move toward the positive plate.
The chemical reaction during discharging can be represented by the following chemical equation:
Positive plate: Ni(OH)2 + 2OH→ Ni(OH)4
Negative plate: Fe(OH)2 + 2K → Fe + 2KOH
NiOH2 + KOH + FeOH2 + electrical energy → Ni(OH)4 + KOH + Fe
Advantages:
- The specific gravity of electrolytes does not change.
- It can be left in full discharge condition for a long period.
- For the same Ah capacity, the weight of the Edition cell is about 50% of the weight of the Lead-acid cell.
- It can withstand more electrical and mechanical abuse than a lead-acid cell.
- It has a much more useful life than the lead-acid cell.
- It can withstand more temperatures than a lead-acid cell.
Disadvantages:
- For the same Ah capacity, the cost of the Edition cell is more than the Lead-acid cell.
- The EMF of a nickel-iron cell is about 1·2 V against 2 V of the lead-acid cell.
- The efficiencies of an Edition cell are less than that of a lead-acid cell.
Applications:
Due to its high mechanical strength, lightweight, ability to sustain atmospheric conditions, it can be used in traction purposes, electrical vehicles, submarines, etc.
2. Nickel-Cadmium Cell:
- Nickel-Cadmium cell was first developed by Waldermar Junger in 1899.
- It has the property of both the Edition cell as well as Lead-acid cell.
- The EMF of the Edition cell is about 1.2 volt.
Component of Nickel-Cadmium cell:
- The positive electrode consists of Nickel Oxyhydroxide Ni(OH)4
- The negative electrode consists of Cadmium (Cd)
- Both the positive and negative electrode is placed in a nickel-plated steel container separate electrically through hard rubber strips.
- Potassium Hydroxide (KOH) is used as the electrolyte.
- The molecules of electrolyte (KOH) dissociate into K+ and OH– ions.
Discharging:
- When the cell is connected to the load under that condition K+ ion move toward the positive plate and OH- ion move toward the negative plate.
The chemical reaction during discharging can be represented by the following chemical equation:
Positive Plate: Ni(OH)4 + 2K → Ni(OH)2 + 2KOH
Negative Plate: Cd + 2OH → Cd(OH)2
Ni(OH)4 + KOH + Cd → NiOH2 + KOH + CdOH2 + electrical energy
Charging:
- For charging of Edition cell, we have required DC supply which fed in opposite direction to that in which cell provided current.
- When the cell is connected to the external DC supply K+ ions move toward the negative plate and OH- ions move toward the positive plate.
The chemical reaction during discharging can be represented by the following chemical equation:
Positive plate: Ni(OH)2 + 2OH→ Ni(OH)4
Negative plate: Cd(OH)2 + 2K → Cd + 2KOH
NiOH2 + KOH + CdOH2 + electrical energy → Ni(OH)4 + KOH + Cd
Comparison of Acid Cell and Alkaline Cell:
Particulars | Acid cell | Alkaline cell |
Positive plate | Lead-peroxide | Nickel hydroxide |
Negative plate | Spongy lead | Iron or Cadmium |
Electrolyte | H2SO4 | KOH |
Average EMF | 2.0 V/cell | 1.2 to 1.36 V/cell |
Internal resistance | Comparatively low | Comparatively higher |
Amp-hour efficiency | 90-95% | nearly 80% |
Watt-hour efficiency | 72-80% | about 60% |
Cost | Less | More |
Life | Gives nearly 1250 charges and discharges | 5 to 20 years |