A battery hydrometer is an instrument that checks the charge of a battery by measuring the density of the electrolyte in the cells. Although not a substitute for a battery performance, a hydrometer reading gives a valuable first indication that there may be a charging-system failure.

A battery hydrometer consists of a rubber squeeze bulb, a floar chamber, and an accurately weighted float. The amount of sulfuric acid in a lead-acid battery indicates the state of charge. The greater the concentration of acid in the electrolyte, the denser the electrolyte and the greater the level of charge. Water has a specific gravity of 1.000; sulfuric acid has an s.g. of 1.830. Each cell in a fully-charged battery has an s.g. in the range 1.240 to 1.280. The electrolyte in a totally discharged battery has an specific gravity of about 1.100. Remember that specific gravity falls with increasing temperature.

To use a battery hydrometer draw enough electrolyte into the device to set the float adrift, without it touching the sides. Sight across the main level and ignore the meniscus that rises up the sides of the instrument.