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Measure the Specific Gravity of Battery Acid in Flooded Lead Acid Batteries

Power - by Joe Weber - updated on 4/24/2024

Using a tool to measure the density and gravity of battery acid

To the everyday person, a car battery is a heavy hunk of metal that provides the power to start your car. It either works or it doesn't. As you read through the many articles we've posted about car batteries, you'll learn quickly that there is much more to them than you think.

We've repeatedly mentioned that testing batteries is an important part of battery maintenance. Keep reading to learn more about another test you can complete on your flooded lead-acid batteries to check on the battery's state of health. But this time, at the individual cell level.

What is Battery Acid?

But first: science. When we talk about lead-acid batteries, "battery acid" refers to the electrolyte solution used in the battery. In lead-acid batteries, this is a mixture of distilled water (pure H₂O) and sulfuric acid (H₂SO₄). Sulfuric acid can be dangerous because it is odorless, colorless and strongly acidic so take precautions when working around batteries, especially if the electrolyte is leaking.

What is Specific Gravity?

Specific gravity (SG) is a measurement of the relative density of electrolyte in a flooded lead acid battery's cell. Specific gravity refers to the ratio of the weight of a solution (sulfuric acid) to the weight of water. As the water-to-sulphuric acid ratio inside the battery cell changes, the density of the electrolyte also changes, this is what the SG test measures.

Why Measure Specific Gravity?

The specific gravity of a battery's electrolyte solution is similar to a fuel gauge. It provides insight into how much energy is left by measuring the density of the mixture. As the battery discharges, the acid loses density and the SG decreases.

What Tools Do I Need to Test Specific Gravity?

Completing the specific gravity test is a pretty simple procedure and doesn't require many tools.

When working with batteries, especially when working directly with the electrolyte solution, you should always wear safety glasses and rubber gloves to avoid any contact with your skin or eyes.

As far as tools go, you'll only need two things: a screwdriver to remove the filler caps, and a hydrometer with a built-in thermometer. Don't have a hydrometer? They can be purchased for around $20. It's an excellent tool to add to your toolbox to make battery maintenance a little easier.

How To Measure Specific Gravity in Lead Acid Batteries

Now you are ready to begin! This process is pretty much the same for all lead-acid batteries, whether that's in your car or truck, golf cart or other equipment.

  1. Prepare your vehicle or battery - This test can most of the time be completed while the battery is still in the vehicle. If you are going to run the test without removing the battery, make sure you park the vehicle in a well-ventilated area.

    If you have removed the battery from the vehicle, place it on a sturdy workbench that is in a well-ventilated area with good light.

    Pop the hood (if it's a car or truck battery) and remove the filler caps on all the cells.

    Ensure the battery has been charged before testing. This is very important to receive accurate readings.

  2. Insert the hydrometer into the cell(s) - Carefully insert the tube of the hydrometer into the cell and slowly squeeze the bulb and release to suck electrolyte into the device. Allow the solution to fill the device to the "maximum" line.
  3. Read and test each cell - With the device full of electrolyte, take note of the measured value for the specific gravity of the electrolyte in that cell. Hold the hydrometer vertically at eye level and note the reading where the electrolyte meets the scale on the float. Do this for all cells in the battery. Jot down the SG for each cell.

    Important Notes

    The correct reading is adjusted for electrolyte temperature. High-quality hydrometers will include a temperature conversion scale to correct the reading.

    If you are measuring the specific gravity in flooded golf cart batteries, depending on the voltage of the battery, you may have 3 (6V), 4 (8V) or 6 (12V) cells.

  4. Note the results - Once all cells have been tested, re-install the filler caps on the cells to prevent any debris from falling in.

    Take a look at your written down specific gravity levels of each cell.

    • If your readings are between 1.275 and 1.300 the cell has a good level of SG and is still properly holding a charge.
    • If a cell shows between 1.210 to 1.250 the cell is holding a charge but at a reduced level.
    • If a cell reading is under 1.200, it indicates the cell is discharged.

Important Note

Don't run this test immediately after filling your battery with distilled water. It will not give you an accurate reading. After filling your battery allow it to be discharged and recharged at least once before testing the specific gravity with a hydrometer.

One Or More Cells Have Low Specific Gravity, What Does That Mean?

All of the cells in your battery should have similar specific gravity. So what does it mean when you have a cell significantly lower than the rest?

If you test all of the cells and find that you have some significantly lower than the other cells, this is a good sign that the battery is getting closer to its failure point. A variation of fifty points between any two cell readings indicates a problem with the low-reading cell(s). You should bring the battery to your nearest Batteries Plus to have it professionally tested.

If All Cells Have Low Specific Gravity, Is the Battery Bad?

Not necessarily. As a battery ages, the specific gravity of the electrolyte will decrease at full charge. This is not a reason to replace the battery providing all cells are within fifty points of each other. If all of the cells are showing less than ideal levels, it could be a couple of things that are easily fixable and don't require you to buy a new battery.

Did you fill them with distilled water and then test them right away? That would easily affect the readings as the new water hasn't had time to thoroughly mix with the sulfuric acid. If this is the case, you need to let your newly filled battery go through at least one full charge cycle before testing the specific gravity to get accurate results.

It could also be a charging issue. In automobiles, the charging system is pretty stable and designed specifically for the battery your vehicle takes. If there are issues, your mechanic will want to investigate your vehicle's charging system for faulty parts. When you bring your vehicle into Batteries Plus and have us test your battery, we will test your starter and alternator to give you a jump on any issues that might exist.

Make sure you are using the appropriate charger for your battery if that battery is used in golf carts, sump pumps or other applications that aren't your car or truck. Pay attention to the chemistries that it is rated for and that the voltage matches your battery.

Apply an equalization charge

Lead-acid batteries are prone to something called sulfation that affects the lead plates inside the battery. It's not like corrosion that can be cleaned away. An equalization charge is something that should be done periodically to reverse the effects of sulfation in a flooded lead acid battery. Sulfation buildup on one or more of the plates could be a reason for the reduced specific gravity of a cell.

What is an equalization charge? It's essentially a deliberate overcharge that helps remove those pesky sulfate crystals on the lead plates that are created during the sulfation process as the battery discharges.

To accomplish this you must apply an overcharge to a fully charged battery using a regulated current of between 200 and 500mA (milliAmps) for an extended time, up to 24 hours. This process allows the battery terminal voltage to rise between 2.50 and 2.66 volts per cell while also increasing the battery's temperature. Combined, these two factors will help dissolve the lead sulfate crystals that have built up on the plates and hopefully bring your battery back to a healthy state.

Bring Your Batteries to Batteries Plus to Test

If we've said it once, we've said it a hundred times: bring us your batteries to test. Whether it's your car or truck, boat, golf cart or sump pump battery, we would be happy to test your batteries, for free I might add, to give you the insight you need on how your batteries are performing.

After we test your batteries, our battery experts will walk you through the results of the test and if necessary, point you in the right direction for a suitable replacement. We carry an impressive selection of batteries for whatever equipment you are powering. We carry the brands you know and love like Duracell Ultra, Optima, X2Power and more, we've got the batteries you need for the devices that demand power.

Shop online, stop in or call your local Batteries Plus store today to see all we have to offer.

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