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

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specific gravity of electrolyte

Explanation: The specific gravity of the electrolyte, which is a sulfuric acid ($H_2SO_4$) solution, is the most accurate indicator of a lead-acid battery's state of charge. As the battery discharges, sulfate ions from the electrolyte are consumed to form lead sulfate ($PbSO_4$) on the plates, causing the density of the electrolyte to decrease.

Working Principle: During the chemical discharge process, the reaction $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$ occurs. This reaction consumes sulfuric acid and produces water, thereby reducing the specific gravity of the electrolyte. By measuring this density using a hydrometer, one can determine the precise state of charge, whereas output voltage can be deceptive due to surface charge or internal resistance variations.

Key Formulas: $Density (\rho) = \frac{Mass}{Volume}$; $State of Charge (SOC) \propto (\rho_{measured} - \rho_{discharged})$

Key Points: A fully charged lead-acid battery typically has a specific gravity around 1.270 to 1.285.; A discharged battery drops to approximately 1.150.; The hydrometer is the instrument used for this measurement.; Voltage measurement is affected by load current (voltage drop) and state of health.

Advantages: Direct correlation to chemical state; Independent of temporary surface charge effects

Disadvantages: Requires access to the electrolyte (cannot be used in sealed/VRLA batteries); Temperature dependent (requires calibration)

Applications: Automotive lead-acid battery maintenance; Solar power storage systems; Industrial UPS systems with vented cells

Additional Info: The relationship between specific gravity and temperature is governed by the electrolyte expansion; measurements should be corrected to a standard temperature (usually 27\u00B0C).. Option A (output voltage) varies significantly under load and is not a stable indicator of capacity during operation.

Answer: C is correct — The specific gravity of the electrolyte decreases proportionally as the sulfuric acid is consumed during the battery discharge process.

Exam Tip: Always remember that in sealed maintenance-free (SMF) batteries where specific gravity cannot be measured, the open-circuit voltage (OCV) is the primary fallback for SOC estimation.

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