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Insight - How Pencil Cell, Primary, Carbon-Zinc Battery works


Carbon Zinc battery also known as primary cell or dry cell or pencil cell is among the cheapest reliable power sources powering a large variety of electronic equipments since the last century. It is used to provide a constant voltage source through a chemical reaction. This battery cannot be recharged and has to be disposed after usage.

Carbon Zinc Battery, Primary Cell

Fig. 1: Image of Primary Battery

A commonly used primary battery is shown in the image above. These types of batteries are available in various sizes and are used to power devices like Torches, Toys, Remote Control, Clocks etc.
Battery Terminals
Fig. 2: (Left image) Positive Terminal (Right image) Negative Terminal
The top and bottom view of the cell is shown in the image above. The battery has two terminals the top view is the positive terminal and the bottom view is the negative terminal.  

Metal Casing

Metal Casing
Fig. 3: Top-Most Layer of Batteries---Metal Casing
The topmost layer of the primary batteries is a metal casing. The metal casing is printed with the necessary information like, Company name, Logo, Manufacturing date etc. Underneath the metal casing is the Zinc anode with protective polythene covering.
How Carbon Zinc Battery works4
Fig. 4: Zinc Can and Plastic Rings
Removing the polythene covering shows the Zinc can. When the cell is in use the electrolyte reacts with the Zn container reacts with the Ammonium Chloride to form Zinc Chloride. As a result of the same the thickness of the Zn container reduces and there are chances of leakage of Zinc Chloride which may damage the device. The polythene covering works as an isolating jacket to prevent the leakage of Zinc Chloride and hence protects the device from any damage where the battery is used.  
The two plastic rings red and white in color are used as sealant. One ring is placed at the top and the other on the bottom. They prevent any leakages from the top and bottom of the battery.

Zinc Can Contd.

Zinc Can
Fig. 5: Mechanism of Positive and Negative Terminals
The bottom metal cap is placed over the Zinc container and behaves as the negative terminal of the battery which is electrically connected to the external circuitry. Similarly the positive metal cap is connected with the Carbon rod and serves as the positive terminals of the battery.
Separator in Carbon Zinc Battery
Fig. 6: Mechanics of inside of Zn Container
Cutting the Zn container shows a glimpse of the paper separator which contains the electrolyte mixture and the positive electrode.


Electrolyte in Carbon Zinc Battery
Fig. 7: Inside Parts of Zn Container---Positive Electrode, Electrolyte, and Separator
Removing the Zinc container completely shows the positive electrode, electrolyte and the separator. A paste of Ammonium Chloride or Zinc Chloride (black in color in the above image) is used as electrolyte. The electrolyte has a small amount of moisture. The positive electrode consists of a mass of manganese dioxide with a carbon rod at the center. The separator prevents the electrolyte and positive electrode to directly connect with the negative electrode (Zinc can) thereby preventing the cell from any damage. 

 Paper Seperator

Paper Separator in Carbon Zinc Battery
Fig. 8: Separator at Inside Zn Container
The separator is shown in the image above. It is made up of a paper coated with starch. It allows the movement of ions but prevents the movement of any solid particles.
Carbon Rod in Primary, Pencil Cell
Fig. 9: Carbon Rod that serves as Cathode Current Collector
The above image shows the carbon rod. It behaves as cathode current collector. This carbon electrode is electrically connected to the top metal cap which behaves as the positive terminal of the battery.