What is a hydrometer and how do I read one?

A hydrometer is used to determine the state of charge of each cell of a battery. The hydrometer will determine the cells specific gravity of electrolyte (its weight compared to water).

A hydrometer is a bulb or tear drop syringe which will extract electrolyte from the cell. A float in the hydrometer is calibrated to read in terms of specific gravity. A common range of specific gravity used in these floats is 1.160 to 1.325. A common specific gravity reading is 1.250 to 1.280 in most climates for a fully charged battery, with a reading of 1.210 to 1.230 in tropical climates. A tropical climate is considered one in which water never freezes. In extremely cold climates, most batteries use a stronger electrolyte solution. A specific gravity reading for this climate could be 1.290 to 1.330. A higher specific gravity reading can, in most cases, decrease the service life of the battery.

Should a reading occur in which the float does not float, you can assume that the cell is bad or that the battery has been fully discharged at the time of the reading. In this case, it may take several hours on a charger to reach the minimum specific gravity level of 1.160 (approximately 1/2 charged). Remember, the lower the float drops in the electrolyte, the lower its specific gravity, the lower the charge.

The following chart shows the approximate specific gravity values of batteries at various states of charge. Note the two columns, one showing a temperate climate the other showing a tropical climate. The values shown are for a cell in various states of charge with its ability to crank an engine at 80 degrees F.
State of Charge Specific Gravity as used in Cold and Temperate Climates Specific Gravity as used in Tropical Climates
Fully Charged 1.265 1.225
75% Charged 1.225 1.185
50% Charged 1.190 1.150
25% Charged 1.155 1.115
Discharged 1.120 1.080

Jul 3rd 2010 The Battery Genius

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