Battery testers and antifreeze testers are both hydrometers, but they're calibrated differently. Sulfuric acid in a fully-charged battery will be somewhere between 1.250 and 1.265 specific gravity, depending on the manufacturers' specifications. All the cells should be approximately equal in acid concentration. Better-quality battreries usually have stronger acid. A discharged battery will have acid concentration down around 1.100 or less, as most of the acid has reacted with the plates to form lead sulfate. Recharging the battery increases the acid strength and reconstitutes the plates into lead (negative plate) and lead dioxide(positive plate). By measuring the acid strength, the state of charge is easy to figure out.
Jerry
