Key Difference – Primary and Secondary Cells
Batteries are used when storage of electric power is needed. They accumulate and give away electric charges as an electric current when it is required. Batteries consist of either primary or secondary cells. The key difference between primary and secondary cells is the reusability. Secondary cells can be reused again and again while the primary cells can only be used once. The purpose and the load connected to the battery depend on which type of cells inside. There may be one or more cells of a single type in a battery; so that decides the voltage, or in other words, the electromotive force (EMF) of that battery. Any cell consists of 3 major parts; namely, Anode, Cathode, and Electrolyte.
What are Primary Cells?
Primary cells can be used once and discarded. They cannot be recharged and reused. Label of a primary cell always states that it should not be recharged because it is harmful to attempt recharge and may be exploded, if do so. Dry cell and Mercury cell are examples for Primary cells. Primary cell is essentially a chemical cell and produces electrical current by an irreversible chemical reaction. Once the reaction is done, it cannot be re-established. For an instant, a dry cell is consisted of a Carbon Cathode surrounded by NH4Cl in a Zink container. A paste of NH4Cl and ZnCl2 serves as the electrolyte while the Zinc container acts as the Anode. A small amount of MnO2 is also mixed with the electrolyte. The chemical process of a dry cell can be summarized as follows;
Zn——>Zn2++2 electron (Anode reaction)
NH4+ + MnO2 + electron ——>MnO(OH) + NH3 (Cathode reaction)
Primary cells are commonly found and used in most of the electric toys, clocks, wrist watches and domestic remote controllers.
What are Secondary Cells?
Secondary cell is also a chemical cell but, can be recharged to use again. The chemical reaction that produces electricity is reversible, and the cell can be used as a new one after the recharging process. The cell can be reused but the lifetime is shortened. Lead-acid and LiFe cell are some examples of secondary cells. In a Lead-acid cell, Lead acts as the anode and a grid of lead packed with lead dioxide acts as the cathode. Sulfuric acid is filled to serve as the electrolyte. Chemical reactions inside a Lead-Acid cell are given below. They are reversible processes.
Pb+So42- ——->PbSO4 + 2 electron (Anode reaction)
PbO2 + 4H+ + SO42- + 2 electron ——> PbSO4 + 2H2O (Cathode reaction)
Modern hybrid vehicles are powered by both petroleum and electric power. The battery charges when the car is moving, and then the stored electric power can be used to run. All of the battery packs inside those automobiles are made of secondary cells. Another common use for secondary batteries is for starting, lighting, and ignition in vehicles. Also, they are used in uninterruptible power supplies (UPSs), telecommunications, and portable tools.
What is the difference between Primary and Secondary Cells?
Cost Effectiveness:
Using primary cells is cost effective compared to secondary cells, initially.
But using secondary cells would be a long term investment since primary cells are to be replaced by another set after some time.
Self-discharge Rate:
Primary cells have lower self-discharge rate hence they are suitable for standby functioning devices which need small currents continuously for a long time. It is an important fact on behalf of safety equipment such as smoke/ Fire detectors, burglar alarms and clocks.
Secondary cells have a higher self-discharge.
Cost and Use:
Primary cells are cheap and easy to use.
Secondary cells are expensive and more complex in usage.
Image Courtesy:
1. “Alkaline-battery-english” by Tympanus [Public Domain] via Commons
2. Secondary cell diagram By Original Author: Barrie Lawson. [CC BY-SA 3.0], via Wikimedia Commons
