Hydrometer

A battery hydrometer is an inexpensive measuring tool to find the state of charge of flooded lead acid batteries. 
Here is how it works: When the flooded lead acid battery accepts charge, the sulfuric acid is heavier than the distilled water, causing the specific gravity (SG) to increase. Thus as the state of charge decreases through use, the sulfuric acid removes itself from the electrolyte and binds to the plate, forming lead sulfate material. The specific gravity or density of the electrolyte becomes lighter and more like water, causing the (SG) the specific gravity to get lower. Table 3 shows SG vs % SOC