The best indication about the state of charge on a lead-acid battery is given by

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Option C: specific gravity of electrolyte

Explanation & Concepts:
The specific gravity of the electrolyte is the most accurate indicator of a lead-acid battery's state of charge. As the battery discharges, the sulfuric acid is consumed and the specific gravity of the electrolyte decreases, whereas during charging, the concentration of sulfuric acid increases, raising the specific gravity.

Working Principle: During the chemical discharge process, sulfuric acid ($H_2SO_4$) reacts with the lead ($Pb$) and lead dioxide ($PbO_2$) plates to form lead sulfate ($PbSO_4$) and water ($H_2O$). This dilution of the electrolyte lowers its density. By measuring the specific gravity with a hydrometer, we determine the concentration of the remaining acid, which directly correlates to the remaining capacity.

Key Formulas: $SG = \frac{\rho_{electrolyte}}{\rho_{water}}$ — Definition of specific gravity; $Pb + PbO_2 + 2H_2SO_4 \leftrightarrow 2PbSO_4 + 2H_2O$ — Chemical reaction inside the battery

Key Points: A fully charged lead-acid battery typically has a specific gravity between 1.250 and 1.280.; A discharged battery generally shows a specific gravity below 1.150.; Specific gravity measurements must be temperature-compensated as liquid density changes with temperature.; Hydrometers are the standard instrument used to measure this density.

Advantages: Provides a direct quantitative measure of chemical health.; Independent of temporary surface charge effects that skew voltage readings.

Disadvantages: Requires physical access to the liquid electrolyte (not possible in sealed/VRLA batteries).; Measurement is invasive and requires handling of caustic sulfuric acid.

Applications: Maintenance of flooded lead-acid stationary batteries.; Automotive battery health diagnostics.; Industrial UPS systems.

Additional Info: Option A (output voltage) is misleading because it varies significantly with the load current (due to internal resistance).. Option B (temperature) is an environmental variable, not a direct indicator of electrochemical energy storage.

Answer: C is correct — specific gravity of electrolyte provides a direct measurement of the sulfuric acid concentration, which is chemically tied to the battery's state of charge.

Exam Tip: Always remember that terminal voltage is a poor indicator of charge under load, whereas specific gravity remains a reliable chemical indicator for flooded-cell technologies.

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Option A: output voltage

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Option B: temperature of electrolyte

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Option D: none of the above

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