How to Test a Battery With a Hydrometer
A hydrometer is an instrument used to check the concentration of acid in a battery by measuring the density of the fluid. The higher the concentration of battery acid, the higher the specific gravity. Also use a hydrometer to measure the state of charge of a car battery or deep-cycle batteries in a solar-power system. Check batteries regularly with a hydrometer, as opposed to a voltmeter, to ensure accurate readings.