How to Read a Battery Hydrometer

Written by jennifer leighton
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How to Read a Battery Hydrometer
A hydrometer test helps determine the amount of sulphuric acid in a car battery. (dead battery image by Katrina Miller from

Used in most nonelectric cars, the lead acid battery is composed of many parts, including plates, lead, and lead oxide. A solution of 65 per cent water and 35 per cent sulphuric acid makes the electrolyte solution that helps power the battery. A battery hydrometer is used to test the amount of sulphuric acid present in the electrolyte solution. If the battery hydrometer shows a low or high sulphuric acid reading, liquid amounts can be adjusted to ensure that the battery is properly powered again.

Skill level:

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  1. 1

    Put on protective eye goggles and protective gloves.

  2. 2

    Disconnect the battery. Carefully remove the battery vent cap.

  3. 3

    Insert the battery hydrometer into the battery cell. Do not use force or push on to the battery plates.

  4. 4

    Pull electrolyte solution into the hydrometer. If you have a syringe hydrometer, pull the plunger up, filling it with liquid. With a bulb hydrometer, lightly squeeze the bulb, filling the tube with liquid. Move the hydrometer as little as possible during this process. Stop pulling the electrolyte solution into the hydrometer when the float (typically a bright ball) starts rising.

  5. 5

    Repeat the process at least two more times. This helps to achieve an accurate reading later.

  6. 6

    Fill the hydrometer a final time, using the same process used in step 4.

  7. 7

    Look directly at the float marker. Note the number the fluid level is most closely aligned with.

  8. 8

    Check manufacturer's data (information normally provided when you purchase the battery) to obtain the number recommended for a fully charged specific gravity reading. A proper fully charged battery reading may be as much as 1.30. According to Arcon batteries, a lead acid battery needs recharging when it has lost 20 per cent of its power, which is typically a hydrometer reading of 1.140 to 1.180. If a lead acid battery has a hydrometer reading of 1.120 or less, the battery should not be used until repaired or replaced.

  9. 9

    Add liquid or remove liquid as needed, until the hydrometer tests read correctly for your particular battery.

Tips and warnings

  • Cold or warm weather can cause inaccurate hydrometer readings. For temperatures below 21.1 degrees Celsius, subtract 0.03 points for each 10 degrees below 70. For temperatures above 21.1 degrees Celsius, add 0.03 points for each 10 degrees above 70.

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