How to Test a Battery With a Hydrometer

Written by amy hogue
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How to Test a Battery With a Hydrometer
Checking your car battery with a hydrometer can prevent you from being left stranded with a dead battery. (dead battery image by Katrina Miller from

A hydrometer is an instrument used to check the concentration of acid in a battery by measuring the density of the fluid. The higher the concentration of battery acid, the higher the specific gravity. Also use a hydrometer to measure the state of charge of a car battery or deep-cycle batteries in a solar-power system. Check batteries regularly with a hydrometer, as opposed to a voltmeter, to ensure accurate readings.

Things you need

  • Safety glasses
  • Rubber gloves
  • Glass jar (recycled)
  • Hydrometer
  • Battery
  • 1 tbsp baking soda
  • Rag
  • 1/2 cup water

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  1. 1

    Gather your equipment in one place and add the 1/2 cup of water to your plastic container. Add 1 tbsp of baking soda and stir. Keep this mixture handy in case of acid spillage.

  2. 2

    Put on all safety equipment, including safety glasses and gloves. The acid in a battery can damage skin and clothing, so ensure that you are not wearing anything that you do not want to replace.

  3. 3

    Unscrew the cap from the battery. Squeeze the bulb on the end of the hydrometer and insert it into the battery opening below the level of the battery acid.

  4. 4

    Release the bulb to allow the liquid to rise into the hydrometer until the interior bulb begins to float. Return the hydrometer to the glass jar so the end of the hydrometer is resting in the jar.

  5. 5

    Read the level of the hydrometer by holding it up so the fluid is level while keeping it in the jar. Refer to your battery's specifications to determine the charge level of your battery.

  6. 6

    Return the fluid to the cell you withdrew it from and repeat this process with all other cells in your battery. Record your findings. Compare readings from different cells to ensure they are within .05 of each other; a greater difference indicates a battery that is sulfated (has a build-up of sulphates on the plates) and needs attention.

  7. 7

    Clean all equipment with a rag soaked in your baking soda and water mixture. Baking soda neutralises battery acid. Store your hydrometer tip-down in the glass jar.

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