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How To Measure Specific Gravity

What Is Specific Gravity and How Should I Check It?

Specific gravity is used to check the state of charge of a battery, essentially it's the ratio of a the weight of a solution to the weight of an equal volume of water. Testing the specific gravity of a cell is done using a hydrometer, the best of these automatically compensate for temperature. As the ratio of water to sulphuric acid inside the battery changes with activity the density of the electrolyte also changes, this is what the hydrometer measures. Another test that should be performed along with the Specific Gravity tests is looking at the Open Circuit Voltage of your battery. These are the steps that Trojan Battery recommends for performing specific gravity tests on their batteries:

  1. Specific Gravity Test

  2. (Used For Flooded batteries only)
    1. Do not add water at this time.
    2. Fill and drain the hydrometer 2 to 4 times before pulling out a sample.
    3. There should be enough sample electrolyte in the hydrometer to completely support the float.
    4. Take a reading, record it, and return the electrolyte back to the cell.
    5. To check another cell, repeat the 3 steps above.
    6. Check all cells in the battery.
    7. Replace the vent caps and wipe off any electrolyte that might have been spilled.
    8. Correct the readings to 80° F:
      1. Add .004 to readings for every 10° above 80° F
      2. Subtract .004 for every 10° below 80° F.
    9. Compare the readings.
    10. Check the state of charge using the table below
    The readings should be at or above the factory specification of 1.277 +/- .007. If any specific gravity readings register low, then follow the steps below.
    1. Check and record voltage level(s).
    2. Put battery(s) on a complete charge.
    3. Take specific gravity readings again.
    If any specific gravity readings still register low then follow the steps below.
    1. Check voltage level(s).
    2. Perform equalization charge. Refer to the Equalizing My Batteries section for the proper procedure.
    3. Take specific gravity readings again.
    If any specific gravity reading still registers lower than the factory specification of 1.277+/- .007 then one or more of the following conditions may exist:
    1. The battery is old and approaching the end of its life.
    2. The battery was left in a state of discharge too long.
    3. Electrolyte was lost due to spillage or overflow.
    4. A weak or bad cell is developing.
    5. Battery was watered excessively previous to testing.
    Batteries in conditions 1 - 4 should be taken to a specialist for further evaluation or retired from service.
  3. Open-Circuit Voltage Test

  4. For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.
    1. Disconnect all loads from the batteries.
    2. Measure the voltage using a DC voltmeter.
    3. Check the state of charge with Table 1.
    4. Charge the battery if it registers 0% to 70% charged.
    If battery registers below the Table 1 values, the following conditions may exist:
    1. The battery was left in a state of discharge too long.
    2. The battery has a bad cell.

    Batteries in these conditions should be taken to a specialist for further evaluation or retired from service.

TABLE 1. State of charge as related to specific gravity and open circuit voltage


Percentage of Charge Specific Gravity Corrected to
80° F
Open-Circuit Voltage
6V 8V 12V 24V 36V 48V
100 1.277 6.37 8.49 12.73 25.46 38.20 50.93
90 1.258 6.31 8.41 12.62 25.24 37.85 50.47
80 1.238 6.25 8.33 12.50 25.00 37.49 49.99
70 1.217 6.19 8.25 12.37 24.74 37.12 49.49
60 1.195 6.12 8.16 12.24 24.48 36.72 48.96
50 1.172 6.05 8.07 12.10 24.20 36.31 48.41
40 1.148 5.98 7.97 11.96 23.92 35.87 47.83
30 1.124 5.91 7.88 11.81 23.63 35.44 47.26
20 1.098 5.83 7.77 11.66 23.32 34.97 46.63
10 1.073 5.75 7.67 11.51 23.02 34.52 46.03