How To Use A Battery Hydrometer

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Using a battery hydrometer to measure the specific gravity of a battery's electrolyte has almost become a lost art because most storage batteries are sealed units. Nevertheless, you need to know how to use a battery hydrometer because there are still stor

A hydrometer is a device used to measure the specific gravity of a liquid. You may recall from your high school physic classes that the specific gravity is the ratio of the mass of a substance to that of an equal volume of water at 4°C. A battery hydrometer measures the ratio of the battery's electrolyte to an equal volume of water at 4°C or 39°F. A storage battery's electrolyte is a solution of 36 percent Sulfuric Acid (H2SO4) and 64 percent water (H2O.) The specific gravity of H2SO4 is 1.835 and the specific gravity of H2O is 1.000. The specific gravity of a battery electrolyte is 1.270. By measuring the specific gravity of the electrolyte, you can tell the battery's state of charge.

State of Charge –Vs- Specific Gravity




100 PERCENT 1.265 12.7

75 PERCENT 1.225 12.4 6.2

50 PERCENT 1.190 12.2


1.190 12.0 6.0



Safety Precautions to be observed when testing A Battery

1. Always wear eye protection, safety goggles, or a fill-face shield. Battery electrolyte is very caustic and can burn your face and even blind you if it gets in your eyes.

2. Always wear mechanic's gloves to protect your hands.

3. Place a fender cover on the car's fender to protect the car's finish from the electrolyte.

4. Do not smoke when checking a battery. The gasses emitted from a battery are highly flammable.

How to Test A Battery Using A Hydrometer

  1. Remove all the storage battery's vent caps and set the aside.
  2. Squeeze the bulb on the battery hydrometer closed before inserting the pickup tube in the battery cell nearest the positive battery post. NEVER squeeze the hydrometer's bulb with the pickup tube in the battery. The inrush of air will force the electrolyte out of the battery.
  3. By releasing the bulb slowly, let enough electrolyte into the hydrometer to make the float just rise off the bottom and drift freely.
  4. Lean over to sight across the liquid to read the specific gravity with out removing the pickup tube from the cell.

5. Record the specific gravity and the temperature of the electrolyte for that cell.

6. Holding the pickup tube just above the electrolyte, slowly release the electrolyte back in the cell.

7. Repeat steps 1 – 6 for all the remaining cells. A 12-volt cell has six cells and a 6-volt battery has three cells.

Correct The Specific Gravity Reading For The temperature.

Most battery hydrometer has a built-in temperature correction table similar to this one. To correct for temperature, all you have to do is add or subtract the indicate amount from the specific reading you obtained for that cell. Compare the corrected reading to the specific gravity is listed in the State of Charge table to determine the battery's state of charge.

A battery hydrometer will give you an estimate of the condition of a battery but a battery load tester will give the exact condition of a battery operating under load. I will cover how to use the two most popular types of battery load testers in an upcoming article.


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