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 Specific Gravity - The Hydrometer Reading Chart

Specific gravity is a unit of measurement that determines the sulphuric acid content of the electrolyte solution.

The built-in hydrometer in sealed maintenance free is mounted with a reflective rod fitted in a rig, a caged green ball attached at the lower end of the reflective rod. The green ball is free to move into the cage and floats when the battery reaches at least 70 % state-of-charge.

Typical Hydrometer
The SG in Low Maintenance or conventional batteries can be measured by a bulb suction floating-hydrometer.

Built-in Hydrometer

 Red Dot White Dot Green Dot Red Dot

 Temperature Correction
 The standard temperature electrolyte for use in this method is 27oC, at different values use the following factor of correction to calculate the true specific gravity. SG27 = SGR + 0,00076 (T – 27) Where: SG27 = Corrected Specific Gravity SGR = Hydrometer Reading T = Electrolyte Temperature Reading Example: SGR = 1.250 T = 44oC Calculating: SG27 = 1.250 + 0,00076 (44-27) SG27 = 1.250 + 0,00076 (17) SG27 = 1.250 + 0.013 SG27 = 1.263 g/cm3

The bulb suction floating-hydrometer is used to measure the specific gravity in low maintenance and/or vented type batteries.

In order to use this method follow this procedures:

1. Introduce a bulb thermometer into the cell, wait a minute and record the electrolyte temperature.
2. Draw electrolyte from the first battery cell into the hydrometer. The float will lift to the surface.
3. Read the specific gravity level on the float and record the value. Return the electrolyte to the cell by pressing the bulb.
4. Repeat steps 1 and 2 for all remaining cells.
5. According to the specific gravity range and if this range between the highest and lowest values is greater than 0.050, or the lowest value is less than 1.235, recharge the battery.
6. After recharging recheck the specific gravity for all cells. If the differences between the cells remain greater than 0.050, replace the battery. If the readings between the cells are lower than 0.050 and the specific gravity is higher than 1.250, proceed the load test (described later in the next pages).

 Specific Gravity g/cm3 Voltage Volts Charge Level % Charge Level Description What to Do 1.100 - 1.120 11.8 V < 20 % OVER DISCHARGED RECHARGE AND LOAD TEST TO SPECIFICATION 1.120 - 1.140 11.9 V 30 % 1.140 - 1.160 12.0 V 40 % 1.160 - 1.180 12.2 V 50 % DISCHARGED RECHARGE AND LOAD TEST TO SPECIFICATION 1.180 - 1.200 12.3 V 60 % 1.200 - 1.220 12.4 V 70 % 1.220 - 1.240 12.5 V 80 % 1.240 - 1.260 12.6 V 90 % CHARGED LOAD TEST TO SPECIFICATION 1.265 - 1.280 12.8 V 100 % > 1.285 > 12.9 100 % OVERCHARGED REPLACE BATTERY