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The best indication about the state of charge on a lead-acid battery is given by
output voltage
temperature of electrolyte
specific gravity of electrolyte
none of the above
specific gravity of electrolyte
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.
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.
SG=╧БwaterтАЛ╧БelectrolyteтАЛтАЛ тАФ Definition of specific gravity
Pb+PbO2тАЛ+2H2тАЛSO4тАЛтЖФ2PbSO4тАЛ+2H2тАЛO тАФ Chemical reaction inside the battery
During the chemical discharge process, sulfuric acid (H2тАЛSO4тАЛ) reacts with the lead (Pb) and lead dioxide (PbO2тАЛ) plates to form lead sulfate (PbSO4тАЛ) and water (H2тАЛO). 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.
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.
Provides a direct quantitative measure of chemical health.
Independent of temporary surface charge effects that skew voltage readings.
Requires physical access to the liquid electrolyte (not possible in sealed/VRLA batteries).
Measurement is invasive and requires handling of caustic sulfuric acid.
Maintenance of flooded lead-acid stationary batteries.
Automotive battery health diagnostics.
Industrial UPS systems.
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.
C is correct тАФ specific gravity of electrolyte provides a direct measurement of the sulfuric acid concentration, which is chemically tied to the battery's state of charge.
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.