Definition: A primary cell is a cell in which the chemical action cannot be reversed. A primary cell cannot be recharged.
One might think that the chemical action of the zinc-carbon primary cell would continue to produce a voltage as long as the active ingredients of the cell were present.
In studying the equation for the discharge of the cell, you will observe the formation of free hydrogen gas. Since the carbon electrode does not enter into chemical action, the hydrogen forms gas bubbles. These collect around the carbon electrode.
As the cell continues to discharge, an insulating blanket of bubbles forms around the carbon. This reduces the output and terminal voltage of the cell. The cell is said to be polarized. The action is called polarization.
To overcome this defect in the simple voltaic cell, a depolarizing agent can be added. Compounds that are rich in oxygen, such as manganese dioxide (MnO2), are used for this purpose.
The oxygen in the depolarizer combines with the hydrogen bubbles and forms water. This chemical action appears as:
The free hydrogen has been removed, so the cell will continue to produce a voltage.
One might assume that when current is not being used from the cell, the chemical action would also stop. However, this is not true.
During the smelting of zinc ore, not all impurities are removed. Small particles of carbon, iron, and other elements remain. These impurities act as the positive electrode for many small cells within the one large cell. This chemical action adds nothing to the electrical energy produced at the cell terminals. This action is called the local action. It can be reduced by using pure zinc for the negative electrode, or by a process called amalgamation.
With amalgamation, a small quantity of mercury is added to the zinc during manufacturing. As mercury is a heavy liquid, any impurities in the zinc will float on the surface of the mercury, causing them to leave the zinc surface. This process increases the life of a primary cell.
Types of Primary Cells
There are many different primary cells. What follows are details on some of the most common primary cells you might encounter.
Although the primary cell has been described as a liquid cell, the liquid type is not in common use. Rather, the primary cell is often a dry cell. In a dry cell, the electrolyte is in a paste form as opposed to a liquid form. A dry cell averts the danger of spilling liquid acids.
Flashlight batteries (cells) are examples of dry cells. The dry cell consists of a zinc container that acts as the negative electrode. A carbon rod in the center is the positive electrode.
Surrounding the rod is a paste made of ground carbon, manganese dioxide, and sal ammoniac (ammonium chloride), mixed with water. The depolarizer is the MnO2.
The ground carbon increases the effectiveness of the cell by reducing its internal resistance. During discharge of the cell, water is formed.
You may recall having difficulty removing dead cells from a flashlight. This is because the water produced caused the cells to expand. Although this problem has been solved by improved manufacturing techniques, it is still not advisable to leave cells in your flashlight for long periods of time. You should keep fresh cells in your flashlight, so it will be ready for emergency use.
The alkaline battery uses manganese dioxide for the positive activating substance. Zinc powder is used as the negative activating substance. A caustic alkali is used for the electrolyte.
Recent progress in electronic product design has demanded more compact supply sources. The number of products needing a large current and a long battery life has increased. This required the development of more advanced batteries.
Cylindrical alkaline batteries are now widely used to supply power for electronic products. They can be used with common manganese dioxide batteries, Figures 1 and 2.
Figure 1. AA size alkaline cells
Figure 2. Cutaway of AA size alkaline cell. (Panasonic Battery Sales Division)
A relatively new type of dry cell is shown in Figure 3. It is called a mercury cell. It creates a voltage of 1.34 volts from the chemical action between zinc (–) and mercuric oxide (+). It is costly to make. However, the mercury cell is better in that it creates about five times more current than the conventional dry cell.
It also maintains its terminal voltage under load for long periods of operation. The mercury cell has found wide use in powering field instruments and portable communications systems.
Figure 3. Mercury cell. It creates voltage by chemical action between zinc and mercuric oxide.
Lithium has the highest negative potential of all metals. It is, therefore, the best substance for an anode. Many battery makeups are possible by mixing lithium with various cathode substances.
Energy densities of these batteries can be computed by respective reaction equations. Figure 4 shows the energy densities of lithium batteries compared with those of conventional batteries.
Lithium is the most suitable anode for the production of high voltage and lightweight batteries. Refer to Figure 5.
Figure 4. Theoretical energy densities of lithium batteries compared with conventional batteries. (Panasonic Battery Sales Division)
Figure 5. A cross-sectional view of a cylindrically shaped lithium battery. (Panasonic Battery Sales Division)
Features of lithium batteries, such as voltage and discharge capacity, are determined by the type of cathode substance used. Fluorocarbon is an intercalation (inserted between or among existing elements) compound.
It is produced through reaction of carbon powder and fluorine gas. It is expressed in (CF)n.
Silver Oxide Cell
Silver oxide cells have several advantages over other types of cells. These advantages include:
- Very stable discharge voltage.
- Excellent high discharge characteristics.
- High energy density per unit volume.
- A wide range of operating temperatures.
- Compact, thin size.
Compact silver oxide batteries have the highest electrical volume and leakage resistance of any battery of that size. They are commonly used in watches.
Two types of silver oxide batteries are made for use in watches. One type uses caustic potash for an electrolyte. The other uses caustic soda.
The caustic potash battery has the symbol W on the bottom of the battery. It is for high drain use, where more power is needed. It is used in wristwatches with liquid crystal displays and multifunction analog watches.
The caustic soda battery has the symbol SW on the bottom of the battery. It is for low drain use. It is used mostly in single function analog watches. Figure 6 shows a cutaway of a silver oxide cell.
Figure 6. Cutaway view of a silver oxide cell. (Panasonic Battery Sales Division)