Correct Answer

Concept & PrincipleElectricalBasic Electrical

Option A

increases

Quick Summary: During the charging process of a lead-acid battery, the specific gravity of the electrolyte (dilute sulfuric acid) increases. This occurs because the chemical reaction drives sulfate ions back into the electrolyte, thereby increasing the density of the solution.

💡 Explanation

During the charging process of a lead-acid battery, the specific gravity of the electrolyte (dilute sulfuric acid) increases. This occurs because the chemical reaction drives sulfate ions back into the electrolyte, thereby increasing the density of the solution.

🔢 Key Formulas

$Specific \ Gravity = \frac{\rho_{electrolyte}}{\rho_{water}}$ — represents the density ratio used to measure charge state

$PbSO_4 + 2H_2O \xrightarrow{Charge} PbO_2 + Pb + 2H_2SO_4$ — chemical equation showing sulfuric acid formation

⚙️ Working Principle

In a lead-acid battery, charging involves the conversion of lead sulfate ( $PbSO_4$ ) on both plates back into lead dioxide ( $PbO_2$ ) on the positive plate and sponge lead ( $Pb$ ) on the negative plate. As this occurs, the sulfate ions ( $SO_4°{2-}$ ) leave the plates and return to the water, reforming sulfuric acid ( $H_2SO_4$ ). Consequently, the concentration of sulfuric acid in the solution rises, which leads to an increase in the specific gravity of the electrolyte.

📌 Key Points

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Specific gravity is a direct indicator of the battery's state of charge.
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A fully charged lead-acid battery typically has a specific gravity between 1.250 and 1.280.
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During discharge, the acid is consumed, causing the specific gravity to decrease.
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Hydrometers are used to measure the specific gravity of the electrolyte.

✅ Advantages

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Provides a reliable way to monitor battery health.
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Helps prevent over-discharging.

❌ Disadvantages / Limitations

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Requires maintenance and physical access to the electrolyte.
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Acid handling poses safety risks (corrosive).

🛠️ Applications / Uses

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Automotive starter batteries.
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Uninterruptible Power Supply (UPS) systems.
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Renewable energy storage systems.

📄 Additional Information

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Specific gravity measurements should be temperature compensated if the electrolyte is significantly warmer or colder than 25°C.
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Option B (decreases) occurs during the discharge cycle, not charging.
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Option C (remains the same) is incorrect as chemical conversion involves mass transfer between the plates and the electrolyte.

📊 Diagram / Illustration

✅

A is correct — The concentration of sulfuric acid increases during charging, which directly increases the specific gravity of the electrolyte.

Core Concepts Used

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Electrochemical reactions Specific gravity Lead-acid battery chemistry

💡 EXAM TIP

Always remember: 'Charging = Acid returns to electrolyte (Gravity UP); Discharging = Acid enters plates (Gravity DOWN)'.

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