6 Best Battery Hydrometer Tester for Battery Testing

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Although a voltmeter is good for measuring the state of charge of a lead acid battery, it does not provide conclusive results. The most accurate way to determine the state of charge of a lead-acid battery is to measure both the voltage and the specific gravity of the battery electrolyte. This gives you a holistic status of the battery.

You use a voltmeter/multimeter to measure voltage and battery hydrometer to measure the specific gravity. Alternatively, you can use a battery refractometer in place of a hydrometer but in this article, we shall focus on battery hydrometers.

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What is a battery hydrometer?

A battery hydrometer is a tool for measuring the specific gravity of the electrolyte in a flooded deep-cycle battery to determine the state of charge. It is a very handy measurement tool for flooded lead-acid batteries mostly found in cars, golf-carts, off-grid installations, and camping RVs.

You can perform a hydrometer test to determine how healthy the cells of a battery are before and after performing regular maintenance.

A hydrometer uses the principle of buoyancy to measure the specific gravity (SG) of a fluid. The SG reading is the measure of the number of times a fluid is denser than water.

In lead-acid batteries, SG reading can tell you the amount of charge in each battery cell. You can also determine the overall health of each cell by analyzing the specific gravity measurements taken over a period of time.

How do you measure specific gravity in a battery?

It takes three basic steps to measure the specific gravity of the battery with a battery hydrometer. First, you draw a sample of battery acid from the battery using the hydrometer. Secondly, you read and record the specific gravity readings from the hydrometer scale. Lastly, you use the specific gravity chart for battery to find out the state of charge of your battery.

Types of hydrometers

There are many different types of battery hydrometers. Some are digital others are not. Some have simple reading scales others have temperature-compensated scales. Battery hydrometers that have temperature-compensated scales are the easiest to use. You only make one final reading and that’s it!

For a hydrometer without a temperature-corrected scale, you must use a temperature compensation chart to perform temperature correction on your SG readings. Alternatively, you can use the temperature correction formula to adjust the readings. Otherwise, the raw measurements might be inaccurate. This also means that you must have a thermometer for recording the temperature separately.

In this article, we shall explore in detail the 6 best battery hydrometer kits for maintaining deep-cycle batteries. You will also learn how to use a hydrometer to check battery health. I will also share with you the best practices for extending the life of your deep cycle battery. Read on to find out more.

Comparison table for 6 Best Battery Hydrometer Kits

MidNite Solar Hydrovolt HydrometerE-Z Red SP101 Battery HydrometerOTC 4619 Professional Battery Hydrometer s RatingKTI (KTI-70212) Battery TesterPerformance Tool W1657C Deluxe Battery TesterMighty Max Battery Hydrometer
Midnite hydrovolt hydrometer
Editor's Ratings
Key Features
  • Has a temperature compensated scale
  • Compact size
  • Very high quality
  • Readings independent of vertical positioning of the instrument
  • Easy to read
  • Provides temperature compensated readings
  • Easy to take accurate readings from the pointer
  • Has a wide scale range from 1100 to 1300
  • Easy to read from the scale
  • Cheap and economical
  • Tough and unbreakable
  • Fitted with a thermometer.
  • Scale on the float has three colored sections; red,white, green.
  • Scale ranges from 1100 to 1300
  • Flexible rubber suction tube
  • One year warranty cover
  • Works on all 6-12-24 volt systems
  • Tests accurately the battery life
  • Works with a hot or cold battery
  • Detects overcharge
  • Made of plastic
  • Has a temperature compensated scale
  • Made of plastic
  • The scale on the float ranges from 1100 to 1300
  • Scale has three color coded sections (red, white, green)
  • Performs test on the battery in seconds
  • Does not have a temperature gauge.
  • Simple and easy to use
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Battery Hydrometer Reviews

Midnite Solar Hydrovolt Hydrometer Review

If you are looking for a hydrometer that you can leave behind to your grandchildren, it is non other than the Hydrovolt battery tester. It is a German-made, compact-size, very high-quality chemical-resistant hydrometer that is accurate at all electrolyte temperatures. Its advanced design allows it to display temperature-compensated readings.

Although hydrovolt hydrometer is a bit pricey, it is one of the most accurate battery testers. It has a compact size that lets you use it on batteries that are hard to get at. Furthermore, the design of the hydrometer is so unique that you do not need to hold the tool vertically or in any prescribed orientation to make your readings.

The hydrovolt battery tester features a large, easy-to-read scale with colored sectors that give a quick indication of the battery’s state of charge. It has a narrow, tightly-fitted tube that draws a bubble-free electrolyte sample. Inside the tool, there is a hydrostatic lock to prevent fluid loss and air bleed. In addition, hydrovolt hydrometer has a very sturdy, shock-resistant, and non-corrosive casing to protect the inside from damage.

Pros and Cons of Hydrovolt Battery Hydrometer

  • Large, easy to read scale
  • Has a temperature compensated scale
  • Very high quality
  • Has a free-rotating rotor that keeps the hdyrometer level always
  • Readings independent of vertical positioning of the instrument
  • Less likely to break when dropped compared to the conventional glass hydrometers
  • Its accuracy might be compromised if the rotors start failing.
  • More expensive than the conventional hydrometers.
  • Needs thorough rinsing to increase its durability
  • If you are accustomed to using the conventional hydrometer, you will find the squeeze bulb of this unit too small. That might be a little uncomfortable at first.


Although you spend a bit more on the hydrovolt tester than on the ordinary hydrometers, the unit guarantees the most accurate readings. Its free-rotating rotor makes it level always. So, you do not need to hold it upright to make readings. Additionally, you don’t have to mind about overfilling electrolyte or holding the tool upright to make accurate readings. This is a perfect choice for any technician out there!

E-Z Red SP101 Battery Hydrometer Review

Characterized by its compact and lightweight design, E-Z Red battery tester is ideal for measuring the specific gravity of all deep cycle batteries ranging from golf cart batteries to marine batteries. Because of its compact design, the battery hydrometer is particularly suitable for use on batteries that are installed in very tight spaces.

The most beautiful thing about E-Z Red unit besides the compact size and fair price is the fact that the hydrometer readings are final. You do not need to temperature-correct them. In other words, you can test your batteries at whatever temperature without the fear of making temperature-distorted readings.

Advantages and Disadvantages of E-Z Red Hydrometer

  • Provides temperature-compensated readings
  • Easy to take accurate readings from the pointer
  • Has a wide scale range from 1100 to 1300 to allow anyone to measure the specific gravity of any flooded lead-acid battery
  • Easy to read from the scale
  • Cheap and economical
  • Designed for garage use; tough and unbreakable
  • With a little more patience, the unit can be used as a battery filler
  • Includes a lifetime warranty
  • Can not test motorcycle batteries because it requires more electrolyte than is available in the small batteries
  • Its small reservoir makes it unsuitable for use as a battery filler
  • Overfilling the hydrometer will give you false readings
  • Battery Hydrometer has to be held to level for accurate readings to be made


As the name goes, E-Z Red battery hydrometer is easy to read. It has a clear scale that can be read without the user having to squint. However, it is difficult to make more accurate readings because the readings have a 25 points interval. If you are looking for a temperature compensated hydrometer that will access batteries installed in compact areas with not more than 8 inches clearance, E-Z RED is a perfect choice. It is undeniably one of the best and one of the most durable battery hydrometers you can find on the market that will also have a lifetime warranty cover.

OTC 4619 Battery Hydrometer Review

If you are looking for a tool that measures both the specific gravity and the temperature-correction factor separately, then you should consider the OTC 4619 Professional Battery Hydrometer. It is a high-quality glass hydrometer fitted with a small thermometer. However, instead of the thermometer showing degrees, it shows the temperature correction factor. The float also has a clear, easy-to-read scale that makes it easy to make readings quickly.

To use OTC 4619 hydrometer correctly, you first read and record the raw SG readings. Then, you read the correction factor and either add or subtract it from the raw hydrometer readings. This gives you temperature-corrected SG readings.

Pros and Cons of OTC 4619 Hydrometer

  • Fitted with a thermometer for displaying temperature adjustment values for the hydrometer readings
  • The float has a scale that ranges from 1100 to 1300
  • The scale on the float has three colored sections: green, white, and red
  • The rubber suction tube is flexible thus offering easy access to the battery cells
  • The device is long enough to be used for any type of lead acid battery including the motorcycle and golf cart batteries.
  • The product has a 1 year warranty cover
  • Has glass components that can be easily broken.
  • Can not be put in the same toolbox with other heavy duty tools. It needs to be handled with care.
  • Overfilling with electrolyte makes the float hit the upper stopper causing it to give false readings.
  • Some people consider the readings on the scale too small to read from a distance
  • Does not ship with a carrying case despite its fragility


If you want to record raw hydrometer readings and temperature correction factor readings separately, the OTC 4619 Professional Battery Hydrometer is a suitable choice. However, you must be extra careful with this unit because of its fragility.

KTI (KTI-70212) Battery Tester Review

The KTI-70212 battery hydrometer is similar to the E-Z Red SP101 except that they differ a bit in shape and design. It displays temperature-corrected hydrometer readings which makes it ideal for testing extremely hot or cold battery electrolyte.

This battery SG meter tool has a curved nozzle that makes it easy to access batteries in tight spaces. However, the rubber suction tube tends to kink when you draw battery fluid while holding the unit in an upright position. Nevertheless, this kinking helps in closing off the hose to retain the fluid so that you can draw out the tool from the battery for reading. KTI 70212 has a lifetime warranty, which is what any buyer wants from a product.

KTI-70212 Battery Tester Pros and Cons

  • Works on all 6-12-24 volt systems
  • Tests immediately and accurately the battery life
  • Works with a hot or cold battery electrolyte because it displays temperature compensated values
  • Made of plastic hence more durable than glass tube battery testers
  • Small and compact
  • Easy to read scale
  • Has a lifetime warranty
  • Accuracy of hydrometer readings is dependent on vertical positioning of the tool
  • Failure to rinse the tool well with fresh water may lead to faster deterioration


KTI – 70212 is a good tool kit for measuring all battery systems from 6V to 24V. It is affordable and durable and therefore a great addition to you battery maintenance kit.

Performance Tool W1657C Deluxe Battery Tester Review

Performance tool battery tester has a similar design to the KTI and E-Z red. It is made of plastic and has a temperature-adjusted scale. So, you only make one final SG reading; no additional calculations.

Advantages and Disadvantages of Performance tool battery hydrometer

  • Has a temperature compensated scale
  • Made of plastic thus more durable than glass hydrometers
  • Must be held upright for accurate readings.
  • Needs to be rinsed well with fresh water to prevent quick deterioration


Performance tool W1657C is just as good as the KTI except for the quality of its material that tends to degrade faster. If you are the kind of person who never forgets to wash tools after use, this is good for you. Otherwise, pick the KTI or E-Z red.

Mighty Max Battery Hydrometer Review

Mighty max battery tester is a simple hydrometer for measuring battery specific gravity. Its scale is not temperature-adjusted. The tool has a clear plastic tube for extended durability. Unlike other similar hydrometers, the tube on this unit does not have a rubber stopper to prevent the float from rising into the bulb. This type of design has an advantage and disadvantage. The advantage is that it eliminates the problem of false readings when overfilled. On the other hand, overfilling a hydrometer might expose your hands to the corrosive electrolyte.

Pros and Cons of Mighty Max Battery Hydrometer

  • The scale on the float ranges from 1100 to 1300
  • The scale is color-coded to make it easier to read and deduce battery state of charge quickly
  • Performs test on the battery in seconds
  • Requires a thermometer to measure temperature separately in order to compute temperature adjustments on the specific gravities
  • Simple and easy to use
  • Bulb can be pushed all way down to create more storage space in your toolbox.
  • Does not have a rubber stopper at the head of the bulb, which allows the float to rise into the bulb itself.
  • Shows raw hydrometer readings that are not temperature-compensated
  • Plastic tube is less clearer than glass tube
  • Unreliable for testing too cold or too hot battery electrolyte.
  • Allowing a hydrometer to overfill might corrode the bulb
  • Drawing too much battery acid can be dangerous to the technician.


If you live in an areas that does not experience extreme weather, your hydrometer does not need to have a temperature corrected scale. Therefore, a simple hydromer such as the Mighty Max battery tester is good enough. However, ensure you put on rubber gloves to protect yourself from electrolyte since the unit does not have an overfill protection.

Battery hydrometer

As mentioned above, a battery hydrometer is an instrument or tool that measures the specific gravity of battery electrolyte in lead-acid batteries. The specific gravity together with battery voltage help to accurately determine the state of charge and the overall health of a flooded lead acid battery

Lead-acid batteries

Before we delve further into battery hydrometers, let’s first briefly looked at lead acid batteries. Lead-acid batteries are the oldest types of rechargeable batteries. They are still very widely used for various electrical purposes. One of the most common applications is in the automotive industry. They are used in cars to store electrical charge for powering the ignition system and other electrical and electronic accessories of a car. Another application is in the renewable energy industry. Most microgrids and minigrids use flooded batteries to store electrical energy generated by PV modules. Flooded batteries in minigrids are things I am very familiar with because I have worked as an O&M engineer for a minigrid. My work involved maintaining those large lead-acid battery banks to keep the power station operating optimally.

Types of lead acid batteries

There are two broad categories of lead acid batteries: flooded type and sealed type.

Sealed Lead-acid battery – The sealed battery type is the lead acid battery that does not require regular maintenance.

Flooded lead acid batteries – Flooded type batteries have their electrodes immersed in an electrolyte made of sulphuric acid and distilled water. These batteries have vents to allow gases to escape during charging. As a result, the electrolyte levels and density usually fluctuate, requiring regular maintenance.

When the battery is fully charged, the concentration of sulphuric acid is high hence a high specific gravity. As the battery discharges, the electrolyte becomes weaker (becomes less acidic) hence a low specific gravity. There is a linear correlation between the state of charge and the specific gravity of the electrolyte. The correlation is so accurate that a hydrometer test is often considered the ultimate measure of a battery’s state of charge. You can use it to diagnose your car battery if, for example, it runs flat quite often or its performance has deteriorated significantly.

How to use a battery hydrometer

There are different types of battery hydrometers. Some are made of plastic while others are made of glass. Some are used in a vertical orientation while others don’t have to be held at a particular orientation to work. Of the different types of battery hydrometers, the glass battery hydrometer is the most common type. It has 4 parts, namely, a rubber suction bulb, a glass viewing tube, a glass float, and a rubber suction tube. The advanced glass hydrometers are also fitted with thermometers to allow you to read both the temperature and the specific gravity of an electrolyte for hydrometer correction.

The first step before using a hydrometer is to assemble all parts. This is the procedure for assembling a glass battery hydrometer:

How to assemble a battery hydrometer

  1. Confirm that all the 4 parts are in good working conditions without cracks or damages.
  2. Grab the rubber stopper and fit it onto the head of the glass viewing tube.
  3. Fit the rubber suction bulb to cover the rubber stopper.
  4. Hold the glass tube by the suction bulb and tilt it vertically.
  5. Insert the float into the tube with the scaled end first
  6. Fit the rubber suction tube.
  7. Ensure that both the suction bulb and suction tube are tightly fitted.
  8. The hydrometer is now ready for use.

All flooded batteries work on the same principle. However, since their most common retail application is in the automotive indusstry, this procedure will illustrate how to test a car battery or a golf cart battery. Please note that the same procedure applies in testing solar batteries and any other types of flooded battery.

How to test a car battery using a battery hydrometer

  1. Assemble the battery hydrometer.

    Refer to the procedure above.

  2. Open the vent cap of the deep cycle battery cell and dip the rubber suction tube

    Warning: If the level of battery electrolyte is below the plates, use a battery filler to top up with battery water until the battery plates are covered.

  3. Squeeze and release the rubber suction bulb to draw the electrolyte into hydrometer

    Submerge the suction tube into the electrolyte to avoid drawing air.

  4. Squeeze the bulb a couple of times if your hydrometer has a thermometer.

    If your hydrometer is fitted with a thermometer, draw the electrolyte several times so that the thermometer can adjust to the electrolyte temperature

  5. Draw enough electrolyte into the hydrometer

    This allows the float to rise up freely without contacting the top and the bottom.

  6. Allow the float to seek its own level

  7. Note the readings then drain the electrolyte from the hydrometer.
  8. Move on to another cell and repeat steps 2 to 7

Note: The specific gravity of the electrolyte in an aged battery tends to decrease at full charge. However, there is no reason to replace the battery as long as the specific gravity readings of each cell are within 50 points to each other.

How to read a hydrometer

After drawing electrolyte from the battery, the next step that follows is to read out the sg value. Every battery hydrometer has a specific gravity scale. Some scales range from 1100 to 1300 while others range between 1000 and 1300. The specific gravity of a fully charged battery should read 1300 while a fully discharged battery should read 1100 or below. This is how you read hydrometer:

Procedure for reading a battery hydrometer

  1. Suction battery electrolyte as illustrated in the procedure above
  2. Hold the hydrometer in an upright position and allow the float to settle.
  3. Take the reading by viewing the scale on the float through the sample electrolyte. Adjust your line of sight until it is in the plane of the horizontal electrolye surface.
  4. Take the reading of the bottom meniscus of the electrolyte.
  5. Do not take the reading if the float is touching the top or bottom of the hydrometer’s glass viewing tube.

How to Perform Hydrometer Temperature Correction


Temperature affects the specific gravity of liquids. This is due to the the fact that materials expand when heated. As a result, their volume increases subsequently causing the density to decrease. Due to the decrease in density of the liquid, the specific gravity decreases. The opposite is true; a decrease in temperature will cause the specific gravity of liquid to increase. Ideally, the specific gravity of battery electrolyte decreases by 0.002 for every temperature increase by 5 degrees Fahrenheit above 80 degrees. Similarly, it decreases by the same value for every decrease by 5 degrees Fahrenheit below 80 degrees. With this in mind, it is always important to arm yourself with a thermometer or a temperature gauge whenever you are conducting the battery hydrometer test. This allows you to measure the temperature of the electrolyte and conduct hydrometer temperature correction accordingly. You can use the two methods discussed below to temperature-correct your specific gravity readings for accuracy:

  1. Using a hydrometer correction table/chart
  2. Manual calculation using the hydrometer temperature correction formula

I. Hydrometer correction using a Hydrometer Temperature Correction Table

A hydrometer temperature correction chart or table is a very handy cheat-sheet that helps determine the correction factor. It has two columns; one has the temperature in Fahrenheit and the other column is populated with the Hydrometer Correction factor. For every 5 degrees Fahrenheit above 80 degrees F, there is an addition of 0.002 to the specific gravity readings. Similarly, for every 5 degrees F below 80 degrees F, there is a subtraction of 0.002 from the actual SG.

After checking the temperature for each cell, you can make temperature adjustments to the corresponding specific gravity reading and record. The table helps simplify the temperature correction exercise, which otherwise can be time-consuming if you have to perform the calculations manually for a large battery bank.

hydrometer temperature correction table

II. Manual calculation using the Hydrometer Temperature Correction formula

Sometimes you might not have the hydrometer temperature correction table handy when you want to make temperature adjustments to your raw hydrometer readings. In such a case, you can temperature-correct the readings manually calculating the correction factor using the hydrometer temperature correction formula, and then add or subtract it to your actual raw hydrometer readings. The example below shows how to use the formula on raw battery SG readings.

How to calculate the correction factor using the hydrometer temperature correction formula: Example

Assume it is during the summer and your hydrometer reads 1230 while your thermometer reads 100 degrees Fahrenheit. The temperature corrected specific gravity would be:

  1. Actual Specific gravity: 1230
  2. Temperature: 100 Degrees F
  3. Temperature difference: 100 deg – 80 deg = +20 degf
  4. Correction factor: {(20 deg/5 deg) * 0.002} = +0.008
  5. Corrected Specific gravity value: 1230 + 0008 = 1238

Another example is when you are in an extremely cold environment such as during the winter. Assume your hydrometer reads 1230 while your thermometer reads -10 degrees Fahrenheit. The temperature corrected specific gravity would be:

  1. Actual Specific gravity: 1230
  2. Temperature: -10 Degrees F
  3. Temperature difference: -10 deg – 80 deg = -90 degf
  4. Correction factor: {(-90 deg/5 deg) * 0.002} = -0.036
  5. Corrected Specific gravity value: 1230 – 0036 = 1194


As you can see from the two examples above, the temperature of an electrolyte can significantly distort the actual specific gravity readings. For instance, a potentially dead cell can appear good in extremely cold temperature while a good battery may seem bad at high temperatures. For that reason, it is very crucial that you have a thermometer with you when using a simple battery hydrometer such as the Mighty Max Battery Hydrometer. The thermometer will measure the temperature of the electrolyte so that you can use it to calculate the appropriate correction factor for temperature-adjusting the SG readings.

Alternatively, you can purchase the more advanced battery hydrometers that come fitted with a thermometer. The OTC 4619 Professional Battery Hydrometer is an ideal example of such hydrometers that allow you to simultaneously record both the SG and temperature readings as well as the corresponding correction factor.

You can also opt to buy a hydrometer that is internally built to provide temperature compensated readings. The E-Z Red SP101 Battery Hydrometer is one of the best types of temperature-compensated hydrometers. It displays specific gravities that are already temperature-adjusted. While using it, you don’t need to carry a temperature gauge and neither do you have to record the temperature of the electrolyte separately. It is one of the most accurate and inexpensive hydrometers for testing batteries. The hydrovolt tester also provided temperature compensated readings but it is premium type.

As it is with tools, a hydrometer will occasionally fall from a height to the floor. If the hydrometer is made of glass, it will definitely smash and that will be its untimely end. If it is plastic like the E-Z Red, you might be a little lucky. So, when buying a hydrometer for servicing your lead-acid batteries, consider the material and design just as much as you pay attention to other features and the price. A rule of thumb is that plastic battery testers such as the E-Z Red, KTI (KTI-70212), and the Performance Tool W1657C Deluxe Battery Testers are less fragile and hence will last longer than the glass hydrometers.

Video: How to use a battery hydrometer – Trojan tips

Learn how to use a battery hydrometer and the importance of measuring the state of charge of deep-cycle batteries by evaluating open circuit voltage and calculating specific gravity of deep-cycle flooded batteries.

Frequently Asked Questions (FAQ)

What does a battery hydrometer do?

A battery hydrometer is a battery testing tool used to check the specific gravity of the battery electrolyte to determine the state of charge of a battery.

How does a hydrometer work?

A hydrometer, whether for testing battery electrolyte, beer, or any other fluids works on the principle of buoyancy. The tool usually has a floating element that rises depending on the density of the fluid being tested.

How do you check the specific gravity of a battery?

When you suck the battery electrolyte with a hydrometer, the float will rise or rotate (depending on the construction of your unit). When the float settles, you will read the value from the scale where the pointer points. If your unit is the conventional model, you will have to hold it upright and read the level of fluid at the meniscus.

What is the specific gravity of a battery?

The specific gravity of a good battery may range between 1100 to 1300 depending on its state of charge. Hydrometer readings below that range indicate a dead battery.

What is the specific gravity of a fully charged battery?

The battery electrolyte of a fully charged battery should have a specific gravity above 1250. In other words, the electrolyte should be 1.25 times denser than water.

How do you know if a car battery is no good?

You can tell that a car battery is not good by checking the specific gravity of electrolyte with a hydrometer. You should start to worry if it reads anything below 1100. You can also tell that a battery is not good by how long it keeps the charge. For example, if your car battery goes flat more frequently then that is a sign of a bad battery. You can also use a battery load tester to test how long a battery lasts with a given load. If it discharges really quick then you should start planning for a replacement.

What fluid do you add in a car battery

You add distilled water in a car battery. The purpose of distilled water is to replenish lost water through gassing. By adding the correct amount of distilled water to a battery, the ratio of water to sulphuric acid needed for optimal operation is maintained.

How do you check the electrolyte level of a battery?

You can check the electrolyte level by visual inspection or by dipping the tip of a hydrometer.

How do you add electrolyte to a battery?

The most effective way to add electrolyte to a battery is using a battery filler with automatic shutoff. The filler only tops off distilled water to the required level without underfilling, overfilling, or spilling.

What level do you top up your battery up to?

For golf cart batteries that typically have a clear casing, you add water to the upper-level mark on the side. For other lead-acid batteries, it is a thumbs rule to fill up to about 1/8″ below the bottom of the vent hole.

Can you use bottled water for a car battery?

No. You cannot use bottled water because it contains impurities that may damage the battery. Only distilled water should be added to the battery.

How many volts is a 12V battery fully charged

Each cell of healthy, fully-charged lead-acid battery supplies between 2.14V and 2.2V. This is equal to 12.8V on the lower side and 13.2V on the upper side for a typical fully-charged 12V battery.

What is the SI unit for specific gravity?

Specific gravity is unitless. It is simply the measure of how heavy fluid is compared to water.

What is the sg of water?

Since water is not charged, its specific gravity on a hydrometer would read 1000 or the lowest value on the scale.


Hey there! I am an field electrical engineer by day, a blogger by night, and DIYer on weekends. Throughout my career, I have used many tools and learned that getting the right tool for the job is the first step to getting the job done right. This is why I write about tools and tests/reviews them on this blog.


  1. You are welcome Joshua! You can share it to your social space and also let me know about your experience with hydrometers. 🙂

  2. Guy at Advance told me hydrometer test is invalid. All my cells are in the red. His fancy tester just said the battery needs charging. (It said it still had sufficient cranking amps, but voltage was low.) He said the battery was “maintenenance free” even though you can pop off the cap and see the electrolyte solution (and test using hydrometer). Is there any truth to what he is saying?

  3. Hello Lee,
    First off, thanks for reaching out. Second, “maintenance free” means that you cannot add water/acid to battery. Therefore, if your battery is labelled as so, I would expect that the top is sealed to prevent topping off. Otherwise, if the top is can be opened, whether or not the battery is labelled “maintenance free”, it means to me that you can check electrolyte level and probably top it up. A hydrometer can also be a handy tool for checking the health of the cells. In your case, if the voltage is low and the electrolyte is in the “red” zone, then it is possible that it only needs charging. I would, therefore, advise that you crank up the car and leave it running or drive around for some time so that the alternator can charge up the battery. Afterwards, make sure to measure the voltage and probably do a load test. If the problem persists, you may consider replacing the battery; it may be getting old.

  4. I have yet to find ANY of the suction bulb type battery hydrometers that are accurate…NONE! ! None of the 3 I used (all new and different mfgs gave the same result…all cells in mid white to barely in green! None will read 1.000 with distilled or tap water! Fill the tube to the top and the float remains on the bottom! All 3 new batteries. Top off charged all 3 batteries…no change…no cells in the green. All 3 batteries are dated no older than 2 months. All 3 batteries hold a 12.7 charge after sitting 30 minutes or overnight (after top off charge_. All 3 batteries reach 13.5 to 13.7 volts immediately after top off charge! CONCLUSION…. all of the professional looking suction glass bulb type lead acid hydrometers WITH thermometer ARE CRAP…WILL NOT READ 1.000 WITH DISTILLED OR TAP WATER AT ROOM TEMP …. AND THEY DO NOT READ IN MID GREEN OF ANY THE CELLS ON THREE FRESH NEW BATTERIES…PURE JUNK…CANNOT BE TRUSTED!

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