What instrument should be used to measure the state of charge of a lead acid storage battery?

A hydrometer is the appropriate instrument for measuring the state of charge of a lead acid storage battery because it assesses the specific gravity of the electrolyte solution within the battery. The specific gravity indicates the concentration of sulfuric acid in the electrolyte, which varies according to the state of charge. By measuring the density of the liquid, a hydrometer can effectively determine how fully charged or discharged the battery is.

In contrast, a voltmeter measures the voltage across the battery terminals but does not provide a direct indication of the electrolyte’s specific gravity or the state of charge in a lead acid battery. An amperemeter measures the current flowing into or out of the battery, which does not directly correlate to the state of charge. A multimeter can also measure voltage and current, but it would not give the specific gravity reading necessary to ascertain the battery's charge level effectively, making the hydrometer the most suitable tool for this particular task.