A lead acid battery will have a ration of sulfuric acid and water as the electrolyte. The process of charging forces any sulfate from the battery plates back to the electrolyte which increases it's specific gravity. The specific gravity of pure water is set at 1. By dissolving substances in water makes if heavier per volume. The common amount of acid brings electrolyte specific gravity (SG) to 1.128 approx.
Measuring the SG with a hydrometer can show if the acid is in the plates and they are sulphated.
The two times this would occur is a) in the normal process of discharging and b) in the longer term build up of sulphated plates that do not respond to a normal charge and the battery is termed 'sulphated'
The SG will also vary on the level of electrolyte as the water evaporates. As water evaporates the SG increase along with ration of acid to water. Adding water will lower the SG and show the battery is not as fully charged as it may seemed when the electrolyte was lower than the designed level.
Created by • Last edit by on 22 Mar 2019