How To Use A Battery Hydrometer To Test The Battery (Specific Gravity Tests)

How To Use A Battery Hydrometer To Test The Battery

This is a step-by-step guide for the absolute beginner on how to use a battery hydrometer to test your vehicle's battery.

If you find testing the battery with a hydrometer daunting, let me assure you that it's not a difficult process, and with the step-by-step instructions provided here, you'll be able to easily determine if the battery is good or bad.

What Type Of Battery Do You Have?

This tutorial only applies to lead-acid batteries with removable caps and not to maintenance-free batteries. Here's how you can tell them apart and find out which type you have installed in your vehicle:

Lead-acid battery:

  • A lead-acid battery has removable caps that allow access to the battery's electrolyte for maintenance and testing.
  • A lead-acid battery needs to periodically be topped off with distilled water to replace any water that has been lost due to evaporation or electrolysis.
  • A lead-acid battery does not come equipped with a charge indicator on its case.

Maintenance-free battery:

  • A maintenance-free battery has a sealed case, which means that it does not have removable caps and does not require maintenance of the electrolyte.
  • A maintenance-free battery will have a labeled that clearly indicates it's a maintenance-free battery.
  • A maintenance-free battery usually (but not always) has a charge indicator located on the top of the battery that shows the battery's state of charge.
    • The charge indicator on a maintenance-free battery typically shows a green dot to indicate a fully charged battery, a dark dot to indicate a low charge, and a light yellow dot to indicate a bad battery.
    • The charge indicator on a maintenance-free battery is a type of hydrometer that gauges the battery's state of charge, and is not removable or replaceable.

Symptoms Of A Bad Battery

On average, a lead-acid battery usually lasts about 3-5 years. The one factor that generally causes the battery to fail early are frequent short trips that don't allow the alternator to fully charge the battery.

Here are the most common symptoms that may indicate a bad battery:

  • Slow engine crank: If the engine cranks slowly when the key is turned, it could be a sign that the battery is weak or failing.
  • Engine does not crank: If the engine does not crank at all when the key is turned, it could be a sign that the battery is weak or failing.
  • Swelling or leakage: If the battery is visibly swollen or leaking, it is likely that the battery is failing and needs to be replaced.

It's important to note that these symptoms can also be caused by other issues, such as a faulty alternator or starter, so it is recommended to check the battery first before replacing other parts.

What Tools Do I Need To Test The Battery?

To clean the battery and to perform a battery gravity test on a lead-acid battery, the following tools and items are needed:

  • Baking soda - used to neutralize any acid on the battery's surface.
  • Distilled water - used to rinse the battery after cleaning and to fill the battery cells if the electrolyte levels are low.
  • Safety goggles - to protect the eyes from any potential splashes of battery acid.
  • Rubber gloves - to protect the hands from battery acid and to prevent contamination of the battery.
  • Battery terminal cleaner - used to scrub away any corrosion or debris from the battery terminals and connectors.
  • Battery hydrometer - tool that measures the specific gravity of the battery acid in each cell.
  • Voltmeter - to measure the voltage of the battery before and after the gravity test, to confirm the battery is holding a charge.

It is important to note that when working with lead-acid batteries, it is crucial to take safety precautions and handle the battery with care to avoid injury or damage.

Where To Buy Battery Testing Tools

The following links are for known automotive brand testers and tools

STEP 1: Clean The Battery

Cleaning the battery is the first step in prepping it for the hydrometer test and in this section I'll explain how.

IMPORTANT: Do not use any acidic solutions, such as vinegar or lemon juice, to clean a battery as it can damage the battery.

CAUTION: When cleaning the battery, wear eye protection and rubber gloves.

Here are the steps to clean a vehicle battery safely:

  1. 1

    Disconnect and remove the battery from the vehicle.

    Although not absolutely necessary, removing the battery from the vehicle can make it easier to clean it thoroughly in preparation for the hydrometer test.

  2. 2

    Mix a small amount of baking soda with water to create a paste.

  3. 3

    With the baking soda solution, clean the battery terminals and the top and sides of the battery.

    Use a wire brush to gently scrub the battery to remove any dirt or corrosion.

    Be sure to pay attention to the small crevices and the posts of the battery.

  4. 4

    Rinse the battery with distilled water to remove any remaining baking soda paste.

    IMPORTANT: It's essential to clean off all the traces of the baking soda solution using liberal amounts of distilled water, particularly around the battery caps.

  5. 5

    Thoroughly dry the battery with a clean, dry cloth.

Now that the battery is cleaned, the next step is to fully charge it.

STEP 2: Charge The Battery

Before performing a specific gravity test, the battery must be fully charged to get the most accurate specific gravity readings of the battery's electrolyte.

Now, it's important to note that since you suspect that the battery may be bad, you must avoid fast-charging the battery. Fast-charging a bad battery can cause it to overheat, leak or even explode.

Here are some pointers:

  • Use a slow (trickle) charge to charge the battery for the most optimal results.
  • The 2-amp charge setting on a charger is the slow (trickle) charge setting.
  • Slow charging the battery typically takes a few hours (yeah, I know, that sucks).

The two main advantages of slow charging a battery are to prevent overcharging, which can cause battery damage (leaks/explosion), and to ensure a full charge, which is necessary for accurate specific gravity readings.

If the battery does not take the charge or cannot hold a charge after being slow charged, it indicates that the battery is bad and needs to be replaced.


Every battery charger is different and come with unique features and settings. But most will have three charge settings for charging an automotive battery:

  • Slow (2A).
  • Medium (10A).
  • High (50A - Engine Start).

It's common for new (modern) battery chargers not to have a simple switch to select between the three different charge rates.

With these chargers, it may be necessary to press a button in a specific sequence to choose the 2A trickle charge. If you own one of these, refer to the charger's manual and carefully follow the instructions provided to select the appropriate charging rate.

Also, different chargers may have different indicators or methods for determining when the battery is fully charged. To determine when the battery is fully charged, consult the charger's manual and follow the instructions provided.

Remember, avoid using the Medium (10A) and High (50A) charge settings, as these settings can cause the battery (if its bad) to overheat, leak, or even explode.

STEP 3: Hydrometer Test

Now that the battery is prepped (clean and charged), it's time to check it with a hydrometer.

CAUTION: Wear eye protection and use acid proof gloves when performing a specific gravity test.

Here are the steps to perform a battery specific gravity test:

  1. 1

    Remove the battery vent caps. You can use a screwdriver to remove them.

    IMPORTANT: Be careful and make sure dirt or debris doesn't fall into the cells.

    IMPORTANT: Make sure the battery is clean, fully charged, and is at room temperature.

  2. 2

    Squeeze the hydrometer rubber bulb and submerge the tip into the electrolyte of the first battery cell you're testing.

  3. 3

    Release the rubber bulb to draw a sample of electrolyte into the hydrometer.

    NOTE: Make sure to draw enough electrolyte into the hydrometer.

  4. 4

    Note and write down the specific gravity reading on the hydrometer's scale after drawing the sample.

  5. 5

    Release the electrolyte back into the battery cell.

  6. 6

    Repeat steps 2 through 5 on the remaining cells.

  7. 7

    Reinstall the vent caps securely on the top of the battery.

OK, now that the test is done, let's interpret your results in the next section.

IMPORTANT: It's essential to rinse the hydrometer thoroughly after measuring the specific gravity of your battery's electrolyte, as the corrosive electrolyte residue can cause damage to the instrument itself and anything else it may come into contact with over time. It's also recommended to store the hydrometer in an acid-resistant container.

STEP 4: Interpreting The Results Of The Hydrometer Test

Let's interpret the results of your hydrometer readings:

  • Good Battery: The specific gravity reading of each cell is close to or around 1.265. This result also indicates that the battery is fully charged.
  • Battery Needs Charging: The specific gravity reading of each cell is below 1.265, but the readings are within 25 points of each other. Battery is still recoverable through proper charging.
  • Bad Battery (Bad Cell): A significantly lower specific gravity reading (more than 25 points) in a cell compared to the rest of the cells is an indication of a bad cell. A battery with a bad cell will need to be replaced as it cannot be repaired.

NOTE: A "point" represents one thousandth of a unit of specific gravity. So 25 points is equal to 0.025 of a unit of specific gravity.

A bad cell can be caused by several factors, including sulfation, which occurs when lead sulfate crystals build up on the lead plates in the cell, reducing its capacity. A bad cell can also be caused by physical damage (such as a cracked or broken case) or overcharging.

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