Lead–Acid Battery Behavior — Effect of Temperature on Electrolyte Specific Gravity As the temperature of a lead–acid starter battery increases, how does the specific gravity (SG) of its sulphuric acid electrolyte change (for the same state of charge)?

Introduction / Context:
Battery testing and maintenance often involve hydrometers to read electrolyte specific gravity, a proxy for state of charge. However, SG is temperature-dependent, so correct interpretation requires understanding how temperature skews readings.

Given Data / Assumptions:

• Chemistry: lead–acid with sulphuric acid electrolyte.
• State of charge is fixed; only temperature changes.
• Hydrometer reading is not temperature-compensated unless corrected.

Concept / Approach:
Density of liquids generally decreases as temperature rises due to thermal expansion. Sulphuric acid solutions follow this trend: at higher temperatures, the same amount of acid occupies a larger volume, reducing density and therefore measured SG. This is why hydrometers include temperature correction charts.

Step-by-Step Solution:
Hold state of charge constant; vary only temperature.Increased temperature → decreased density → lower SG reading.Therefore, the SG decreases with increasing temperature.

Verification / Alternative check:
Service manuals provide correction factors (e.g., add or subtract points per 10 °C) to normalize readings to a reference temperature.

Why Other Options Are Wrong:
“Increases” contradicts the known thermal expansion behavior.“Remains the same” ignores temperature dependence of density.“None/oscillates” have no physical basis here.

Common Pitfalls:
Failing to temperature-correct hydrometer readings leads to misjudging state of charge, especially in hot or cold climates.

Final Answer:
decreases