What is the fundamental difference between a primary and a secondary battery?

The fundamental difference lies in the reversibility of their electrochemical reactions. Primary batteries undergo irreversible chemical reactions, meaning once the reactants are consumed during discharge, they cannot be regenerated by applying an external current. They are single-use. Secondary batteries, on the other hand, have reversible chemical reactions. They can be recharged by passing an external current, which reverses the discharge reactions and restores the original reactants, allowing for multiple cycles of use.

Why do mercury cells provide a constant voltage throughout their operational life?

Mercury cells maintain a constant voltage (1.35 V) because the overall cell reaction does not involve any ions in the electrolyte whose concentrations would change significantly during discharge. The reactants (Zn(Hg) and HgO) and products (ZnO and Hg) are all solids or liquids, and the electrolyte (KOH) concentration remains relatively constant. According to the Nernst equation, if reactant/product concentrations remain stable, the cell potential remains constant.

What is the 'memory effect' in batteries, and which type is most affected?

The 'memory effect' is a phenomenon where a rechargeable battery, if repeatedly recharged after only being partially discharged, 'remembers' the shallower discharge point and subsequently delivers only that reduced capacity. This leads to a temporary reduction in the battery's effective capacity. This effect is most prominently associated with Nickel-Cadmium (Ni-Cd) batteries. Modern Lithium-ion batteries are virtually free from the memory effect.

Explain the role of sulfuric acid in a lead-acid battery during discharge and charge.

During discharge, sulfuric acid ($H_2SO_4$) acts as the electrolyte and is consumed in the reactions at both the anode and cathode, forming lead sulfate ($PbSO_4$) and water ($H_2O$). This leads to a decrease in the concentration and density of the sulfuric acid. During charging, an external current reverses these reactions, regenerating sulfuric acid and consuming water. Consequently, the concentration and density of sulfuric acid increase, which is why a hydrometer can be used to check the state of charge of a lead-acid battery.

Why are Lithium-ion batteries preferred for modern portable electronics and electric vehicles?

Lithium-ion batteries are preferred due to several key advantages: they offer a very high energy density (more energy stored per unit mass/volume), a high cell voltage (typically 3.7V), a long cycle life (can be recharged thousands of times), and they do not suffer from the 'memory effect.' These characteristics make them ideal for devices requiring compact, lightweight, and long-lasting power sources.

Primary and Secondary Batteries

Chemistry

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Version 1Updated 22 Mar 2026

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Batteries, fundamentally electrochemical cells, are devices that convert chemical energy into electrical energy through spontaneous redox reactions. They are broadly classified into two main categories: primary batteries and secondary batteries. Primary batteries are non-rechargeable, meaning their chemical reactions proceed in one direction until the reactants are consumed, rendering the battery …

Quick Summary

Batteries are devices converting chemical energy to electrical energy via redox reactions. They are broadly categorized into primary and secondary types. Primary batteries are non-rechargeable, meaning their chemical reactions are irreversible, leading to single use.

Examples include the Dry Cell (Leclanché cell) with a zinc anode and $MnO_2$ cathode, providing about 1.5 V, and the Mercury Cell, featuring a zinc-mercury amalgam anode and $HgO$ cathode, known for its stable 1.

35 V output. Secondary batteries are rechargeable, as their chemical reactions are reversible, allowing them to be repeatedly discharged and charged. Key examples include the Lead-Acid Battery, used in automobiles, which involves lead and lead dioxide electrodes in sulfuric acid, producing 2 V per cell.

Nickel-Cadmium (Ni-Cd) batteries, with cadmium and nickel oxyhydroxide electrodes, provide 1.2 V but suffer from a 'memory effect.' Lithium-ion (Li-ion) batteries, utilizing intercalation of lithium ions in graphite and metal oxides, offer high energy density and 3.

7 V, dominating modern portable electronics and electric vehicles. The choice between primary and secondary depends on application, cost, and environmental considerations.

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Key Concepts

Redox in a Leclanché (Dry) Cell

In a Leclanché cell, the zinc casing acts as the anode. Here, zinc metal ( $Zn$ ) undergoes oxidation, losing…

Charging a Lead-Acid Battery

The charging process in a lead-acid battery is an example of electrolysis, where an external electrical…

Memory Effect in Ni-Cd Batteries

The 'memory effect' is a historical issue primarily associated with Nickel-Cadmium (Ni-Cd) batteries. It…

Primary Batteries: — Non-rechargeable, irreversible reactions. Examples: Dry Cell, Mercury Cell.

* Dry Cell: Anode ( $Zn$ ), Cathode ( $MnO_2$ , C rod), Electrolyte ( $NH_4Cl$ , $ZnCl_2$ paste). Voltage $approx 1.5,\text{V}$ . * Mercury Cell: Anode ( $Zn(Hg)$ ), Cathode ( $HgO$ , C), Electrolyte ( $KOH$ , $ZnO$ paste). Voltage $approx 1.35,\text{V}$ (constant).

Secondary Batteries: — Rechargeable, reversible reactions. Examples: Lead-Acid, Ni-Cd, Li-ion.

* Lead-Acid: Anode ( $Pb$ ), Cathode ( $PbO_2$ ), Electrolyte ( $H_2SO_4$ ). Voltage $approx 2,\text{V}$ /cell. * Discharge: $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$ * Charge: $2PbSO_4 + 2H_2O \rightarrow Pb + PbO_2 + 2H_2SO_4$ * Ni-Cd: Anode ( $Cd$ ), Cathode ( $NiO(OH)$ ), Electrolyte ( $KOH$ ).

Voltage $approx 1.2,\text{V}$ . Suffers from 'memory effect'. * Li-ion: Anode (Graphite), Cathode ( $LiCoO_2$ ), Electrolyte (non-aqueous Li salt). Voltage $approx 3.7,\text{V}$ . High energy density, no memory effect.

Primary Never Recharges, Secondary Reverses Reactions.

Primary: Dry Mercury (Dry cell, Mercury cell) Secondary: Lead Ni-Cd Li-ion (Lead-acid, Nickel-Cadmium, Lithium-ion)

Lead-Acid: Lead Lead Dioxide Sulfuric Acid (Anode, Cathode, Electrolyte) Mercury: Maintains Constant Voltage (1.35V)