What you’ll learn
After this post, you will be able to tell a wet cell from a dry cell, name the one headline advantage of each of five battery types, and pick the right cell for a job.
Wet vs. dry: it’s the electrolyte
Batteries are classified by their electrolyte: paste = a dry cell, a solution (liquid) = a wet cell. A lead-acid battery is a wet cell. The five types in this post are all dry cells.
Carbon-zinc: cheap and everywhere
One of the oldest and most widely used dry cells. A carbon rod in the centre is the positive terminal; the zinc case is the negative electrode; a paste-like electrolyte sits between them; the cell is sealed against evaporation. Cell voltage about 1.5 volts. Its headline advantage: durable and very inexpensive to produce. The flashlight and radio battery everyone knows.
Alkaline: the longer-lasting 1.5 volts
An alkaline electrolyte of potassium hydroxide; the negative electrode is zinc and the positive is manganese dioxide; typical voltage 1.5 volts. Its headline advantage: extended life over a carbon-zinc cell of the same size — though it usually costs more. Same size, same voltage, lasts longer.
Nickel-cadmium: the rechargeable dry cell
A secondary cell with a potassium hydroxide electrolyte and a nominal voltage of 1.25 volts. Its headline advantage: a dry cell that is a true storage battery with a reversible chemical reaction — rechargeable, rugged and dependable under extreme temperature, shock and vibration, and suited to portable communications equipment. One correction to the handbook: DOE-HDBK-1011/92 prints the NiCd electrodes the wrong way round — it assigns the negative electrode to nickel hydroxide and the positive to cadmium hydroxide. Standard references (ScienceDirect, Wikipedia) agree the correct assignment is the reverse: the positive electrode is nickel hydroxide — nickel oxide-hydroxide (NiOOH) when fully charged, nickel hydroxide (Ni(OH)₂) when discharged — and the negative electrode is cadmium — metallic cadmium when charged, cadmium hydroxide (Cd(OH)₂) when discharged. This post uses the correct assignment.
Edison: lighter and tougher than lead-acid
The positive plate is nickel and nickel hydrate; the negative plate is iron; the electrolyte is alkaline. Typical output 1.4 volts, and it should be recharged when it reaches 1.0 volt. Its headline advantage: a lighter and more rugged secondary cell than a lead-acid storage battery.
Mercury: steady voltage for small instruments
Two shapes — a flat button cell, or a cylindrical cell like a flashlight battery. Each cell gives about 1.35 volts. Very rugged, with a relatively long shelf life. Its headline advantage: a fairly constant output under varying load conditions. Uses named in the handbook: electric watches, hearing aids, cameras, and test instruments.
How to pick
Match the cell to the job: cheapest — carbon-zinc; longer life — alkaline; rechargeable rugged portable power — nickel-cadmium; lighter rugged secondary — Edison; steady voltage for small instruments — mercury.
Common mistakes
- “All 1.5-volt cells are the same.” They are not. Carbon-zinc and alkaline both give about 1.5 volts, but alkaline lasts longer and usually costs more.
- “A dry cell has no electrolyte.” It does — a paste electrolyte instead of a liquid one. That paste is what makes it dry.
- “NiCd is a throwaway cell.” It is not. It is a secondary cell — a true storage battery with a reversible chemical reaction, and it can be recharged.
- “The handbook’s NiCd electrodes are right.” They are not — the handbook prints them reversed. The positive electrode is nickel hydroxide, the negative is cadmium. (Verified against standard references.)
- “Mercury cells are for big loads.” They are not. They are for small instruments — watches, hearing aids, cameras, test instruments — where a steady voltage matters.
- “Edison cells and lead-acid cells are interchangeable.” They are not the same cell. Edison is lighter and more rugged than a lead-acid storage battery, and its voltage is 1.4 volts, recharged at 1.0 volt.
Summary
Wet or dry is about the electrolyte — paste is dry, liquid is wet. Carbon-zinc is cheap, alkaline lasts longer, nickel-cadmium is the rechargeable dry cell that survives abuse (positive = nickel hydroxide, negative = cadmium — the handbook prints this reversed), Edison is lighter and tougher than lead-acid, and mercury holds its voltage steady for watches and meters. Match the cell to the job.
Quick-check questions
- What makes a cell a dry cell rather than a wet cell?
- What is the headline advantage of a carbon-zinc cell?
- What is the headline advantage of an alkaline cell over carbon-zinc?
- What are the positive and negative electrodes of a nickel-cadmium cell?
- Which cell type gives a fairly constant output for small instruments?
Answers: 1. A paste electrolyte — a solution (liquid) electrolyte makes a wet cell. 2. It is durable and very inexpensive to produce. 3. Extended life over a carbon-zinc cell of the same size. 4. Positive: nickel hydroxide (NiOOH charged, Ni(OH)₂ discharged). Negative: cadmium (Cd charged, Cd(OH)₂ discharged). 5. The mercury cell — about 1.35 volts per cell, used in watches, hearing aids, cameras, and test instruments.
Based on DOE-HDBK-1011/92, Electrical Science, Module 4, paraphrased for study. The nickel-cadmium electrode assignment in this post is corrected against standard references (see the Nickel-cadmium section); the handbook prints it reversed.
Video lesson for this module is in production — coming soon.
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What makes a cell a dry cell rather than a wet cell?
What is a carbon-zinc cell made of?
What is the headline advantage of a carbon-zinc cell?
What are the electrodes and electrolyte of an alkaline cell?
What is the headline advantage of an alkaline cell over a carbon-zinc cell of the same size?
What are the positive and negative electrodes of a nickel-cadmium cell?
What is the headline advantage of a nickel-cadmium cell?
What are the plates and voltage of an Edison cell?
What is the headline advantage of a mercury cell?
Which cell would you pick for a small instrument that needs a steady voltage, such as a watch or a test instrument?
