An electric cell is a single unit designed to convert stored chemical energy into electrical energy through a controlled chemical reaction. Most devices use a combination of these cells, known as a battery. A primary cell, like a standard alkaline battery, is designed for a single use because its internal chemical changes are permanent. A secondary cell, by contrast, is engineered so its chemical reactions can be reversed, allowing it to be used and refreshed multiple times.
The Defining Feature: Rechargeability
The designation “secondary” stems from the cell’s ability to undergo an electrically reversible chemical process. This reversibility means the flow of energy is not unidirectional, unlike primary cells where components are permanently consumed upon discharge. When supplying power, the secondary cell converts chemical energy into electrical energy through an oxidation-reduction reaction. Conversely, when recharged, an external electrical current is forced through the cell, reversing the chemical process. This restores the cell’s chemical structure to its original, charged state, replenishing the stored energy.
Internal Anatomy and Operational Cycle
Every secondary cell requires three fundamental components: two electrodes (anode and cathode) and an electrolyte. The electrodes are solid materials where chemical reactions occur, and the electrolyte is the medium that allows ions to move between them. During discharge, the anode releases electrons through the external circuit, creating electrical current. Simultaneously, positively charged ions travel through the electrolyte to the cathode, completing the chemical reaction.
The charging cycle works by applying an external voltage higher than the cell’s own voltage, forcing the reaction to run in reverse. This external power source pushes electrons back into the anode and pulls ions out of the cathode, reversing their flow through the electrolyte. This process restores the electrochemically active materials at both electrodes to their original, high-energy state.
Common Secondary Cell Types and Applications
Modern technology relies heavily on several common chemistries of secondary cells, tailored for different applications based on energy density and power output. Lithium-ion (Li-ion) cells are dominant in portable electronics, such as smartphones and laptops, and are the power source for electric vehicles due to their high energy density and lightweight nature. Another common type is the Nickel-Metal Hydride (NiMH) cell, often found in high-drain devices like digital cameras and rechargeable household batteries.
For large-scale, high-power needs, the long-established Lead-Acid battery remains widely used. It is the standard for starting, lighting, and ignition systems in conventional automobiles. These cells are also deployed in backup power systems and various industrial applications where weight is not a major constraint.