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.

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.

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

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.

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.

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.

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

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

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.

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.