During charging the specific gravity of the electrolyte of a lead-acid battery

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increases

Explanation: During the charging process of a lead-acid battery, the specific gravity of the electrolyte increases. This happens because the chemical reaction converts lead sulfate ($PbSO_4$) on both plates back into lead peroxide ($PbO_2$) and sponge lead ($Pb$), releasing sulfuric acid ($H_2SO_4$) back into the electrolyte.

Working Principle: During discharge, sulfuric acid is consumed to form lead sulfate on the plates, which dilutes the electrolyte and lowers its density. When charging, the reverse chemical reaction occurs: the lead sulfate is broken down, and the sulfate ions return to the solution to recombine with water, thereby increasing the concentration of sulfuric acid and raising the specific gravity of the electrolyte.

Key Formulas: $PbSO_4 + 2H_2O + PbSO_4 + Electrical Energy \rightarrow PbO_2 + Pb + 2H_2SO_4$ — chemical reaction during charging; $\rho = \frac{m_{electrolyte}}{V_{electrolyte}}$ — definition of density which governs specific gravity

Key Points: Specific gravity is a direct indicator of the battery's state of charge.; A fully charged lead-acid battery typically has a specific gravity between 1.250 and 1.280.; The hydrometer is the instrument used to measure the specific gravity of the electrolyte.; During discharging, the specific gravity decreases.

Advantages: Reliable indicator of charge state; Easy to monitor using a hydrometer

Disadvantages: Requires access to the electrolyte; Corrosive nature of the acid

Applications: Automotive starter batteries; Uninterruptible Power Supply (UPS) systems

Additional Info: As the battery reaches full charge, the evolution of hydrogen and oxygen gases occurs (gassing).. Option B is incorrect because a decrease occurs during the discharge process.. Option C is incorrect because the chemical composition of the electrolyte changes dynamically during charging.

Answer: A is correct — The concentration of sulfuric acid increases during charging, thereby raising the specific gravity of the electrolyte.

Exam Tip: Remember that specific gravity and voltage are proportional to the state of charge in a lead-acid battery; both increase during charging and decrease during discharging.

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