When operating serviced lead-acid batteries, it is periodically necessary to measure the density of the electrolyte. Using it you can find out the degree of charge, determine the end of charge time, detect sulfation and assess the general condition of the battery. To measure the density of electrolyte, two tools are available to car enthusiasts and professionals - a hydrometer and a refractometer. From this material you will learn: how they are structured and on what principle they work; advantages and disadvantages; how to use correctly; why a device for measuring electrolyte density can lie.

Physical methods for measuring electrolyte density

An electrolyte is a solution of sulfuric acid in distilled water. A conductive liquid that, at its nominal density, does not freeze down to -74 degrees Celsius. It is an integral part of a car battery. In conventional batteries, the electrolyte is liquid, which, in the presence of plugs in the case, makes it possible to measure its density. In AGM and GEL batteries, the electrolyte density cannot be measured.

The density of an electrolyte is the ratio of its mass to its occupied volume. It is measured in grams per cubic centimeter (g/cm3). There is a separate material on this topic on the website Auto without a service station. I recommend checking it out. The density of the electrolyte is higher, the more sulfuric acid is dissolved in water. When the battery is discharged, the density is minimal (down to 1.01 g/cm3), since the electrolyte consists mostly of water. In a fully charged, serviceable battery, the electrolyte density is in the range of 1.27...1.30 g/cm3.

There are three convenient and fairly accurate ways to measure the density of an electrolyte:

  1. Archimedes' law.
  2. Limit angle of refraction.
  3. Calculation by mass and volume.

Let's look at these methods briefly.

Archimedes' Law

According to this law, a buoyant force acts on a body immersed in a liquid. All other things being equal, it depends precisely on the density of the liquid that interests us. The higher the density, the greater the buoyancy force. Surely many of you have already guessed the operating principle of which device for measuring electrolyte density is based on Archimedes' law.

Archimedes' Law
Archimedes' Law

Limit angle of refraction

This method is based on the fact that light passes through liquids at different speeds, which depend on density. By placing the liquid between two prisms, we obtain the limiting angle of refraction, which is easy to see by focusing the light on a transparent scale. Unlike Archimedes' law and the buoyant force of a liquid, this method is much more difficult to understand. But only the principle is important to us, and we’ll leave the subtleties to lovers of optical physics.

Speed of light in liquids of different densities
Speed of light in liquids of different densities

Density by mass and volume

Density by definition is the ratio of mass to occupied volume. Knowing the mass and volume of the electrolyte, we can easily calculate its density. The accuracy, in this case, will depend on the accuracy of the instruments that we used to measure mass and volume. At home, this can be done with fairly high accuracy using jewelry scales and a small measuring cylinder (you can use a medical syringe).

Jewelry scales
Jewelry scales

Mastering this method will also be useful for those who independently prepare electrolyte or antifreeze from concentrate. To find out the density, weigh a known volume of liquid and divide the resulting mass by the volume. For example, if you take 10 ml of electrolyte from a fully charged battery, its weight will be 12.8 grams.


A hydrometer is the simplest and most accessible device for measuring the density of an electrolyte. Our grandfathers and fathers used it. We also use a hydrometer. Because it is cheap, convenient and understandable.

Classic car hydrometer
Classic car hydrometer

Design and principle of operation of a hydrometer

A classic car hydrometer is a cylinder made of transparent material (glass or plexiglass) with a bulb and a spout. Inside there is a glass float with a weight and a scale. To measure the density of the electrolyte, you need to insert the spout into the battery and draw up the liquid using a bulb. When the float floats, the liquid level coincides with the corresponding density value on the scale.

Hydrometer scale
Hydrometer scale

The more sulfuric acid in the electrolyte, the higher its density and buoyancy force. This means that the float floats higher, “moving” the scale relative to the upper level in the cylinder. Despite the simple principle of operation of a hydrometer, correct readings can only be obtained using this device under certain conditions. This is discussed below.

Pros and cons of a hydrometer

Advantages of a hydrometer:

  • low price from 3 dollars;
  • ease of use;
  • undemanding to cleanliness;
  • accuracy sufficient for battery maintenance;
  • high measurement speed;
  • convenience.

Note. Even if the hydrometer is broken, the surviving float can be temporarily used to measure the density of the electrolyte. To do this, place it in a transparent measuring cylinder, into which, using a medical syringe, pour electrolyte from the battery.

Disadvantages of a hydrometer:

  • fragility of models with a glass cylinder;
  • reduced transparency of models with a plastic cylinder;
  • to recognize the readings you need to look closely;
  • readings are difficult to photograph;
  • no calibration.

The biggest disadvantage of a hydrometer is its scale. Most often it is applied to paper, which is rolled into a tube and not secured in any way to the float. During operation, this scale tends to shift, due to which the instrument readings are irrevocably distorted. The hydrometer cannot be calibrated.

Note. The hydrometer shown in the photo below does not have two disadvantages of the classical design. In this device the scale is more visual and convenient. But the most important thing is that it does not get lost during operation. Such a device maintains its original accuracy throughout its entire service life. In addition, it does not contain fragile glass parts that can easily break in a garage environment.

Non-standard hydrometer
Non-standard hydrometer

Despite the shortcomings and the presence of more advanced instruments for measuring density, the good old hydrometer remains objectively the best tool for working with battery electrolyte.

How to use a hydrometer correctly?

To obtain correct readings of electrolyte density using a hydrometer, the following conditions are necessary:

  1. The electrolyte temperature should be +20 degrees Celsius. A deviation from this value of 10 degrees distorts the density by approximately 0.01 g/cm3. When the electrolyte is cold (the battery is outside in the cold), the hydrometer will show an overestimated density. When the electrolyte is overheated (for example, when you try to increase its density by boiling the battery), the device displays a low density.
  2. The float should float up completely. If you have not collected enough electrolyte from the battery, the float will remain at the bottom of the cylinder and its readings will be unreliable.
  3. The scale is aligned with the lower meniscus of the liquid level. If you look closely at the upper level of the electrolyte in the hydrometer, you will notice that this line has a quite visible height. This is due to the surface tension of the liquid. Many people estimate the density of the electrolyte by looking at the upper meniscus, while it is more correct to look at the lower one.
  4. The hydrometer and float must be in a strictly vertical position. When the hydrometer is tilted, the liquid level intersects the scale at an angle, which distorts the reading. You also need to make sure that the float does not “rest” on the cylinder walls. It often sticks because sulfuric acid is oily. All these factors distort the instrument readings.
  5. The upper level of the electrolyte in the hydrometer should be at the same level as your eyes. Often they look at the device without lifting it from the battery, from an extremely uncomfortable position. From this angle it is impossible to view the border of the lower meniscus and obtain correct readings.

Bottom line. To correctly measure the density of an electrolyte with a hydrometer, first bring it to room temperature. Then carefully draw in the liquid until the float floats to the surface. Place the hydrometer scale at eye level and take readings along the lower meniscus.

Why is the hydrometer lying?

A good hydrometer can easily measure the density of an electrolyte with an accuracy of 0.005 g/cm3. This is more than enough for full battery maintenance. However, there are a number of reasons why this very accurate device can lie:

  • electrolyte temperature is much higher or lower than +20 degrees Celsius;
  • you are looking at the scale from the wrong position;
  • the float lies at the bottom of the cylinder or is stuck to its walls;
  • due to careless use, the paper with the scale has moved;
  • You initially received a defective device.

Unfortunately, the hydrometer can only be checked and calibrated using another device for measuring the density of the electrolyte.


An automobile refractometer is a device for measuring the density of electrolyte, as well as the freezing point of antifreeze, windshield washer fluid and urea. On average, it is 7 times more expensive than a hydrometer. Very accurate, but more sensitive to external factors than a hydrometer. This is if we talk about this device briefly. Let's take a closer look at it.

Design and principle of operation of a refractometer

In laboratory conditions, stationary tabletop refractometers are used. Not long ago a compact hand-held device became available to car enthusiasts, which looks like this.

Handheld refractometer
Handheld refractometer

The design of a handheld refractometer is shown in the following image.

Refractometer device
Refractometer device

The device consists of the following main elements:

  1. Frame.
  2. Protective glass (lighting prism).
  3. Measuring prism.
  4. Focusing lenses.
  5. Scale.
  6. Eyepiece.
  7. Screw for calibration.

The liquid being tested in a volume of two drops is placed on a measuring prism and pressed against protective glass. As a result, a thin layer of electrolyte is formed, through which light passes easily. The protective glass has irregularities. Thanks to this, the light passes through the frosted glass, is evenly scattered, passes through the layer of liquid and hits the measuring prism.

Due to the fact that the speed of light depends on the density of the liquid, it hits the measuring prism at a certain angle. In the measuring prism, light is refracted, forming a clear boundary between light and shadow. Next, the light rays are focused on a scale on which the above-mentioned boundary is clearly visible through the eyepiece. It usually looks like this.

Automotive refractometer scale
Automotive refractometer scale

The higher the density of the electrolyte, the lower the speed of light passing through it and the greater the angle of refraction. From the user's point of view, the blue area is smaller than the white area. It seems to “rise” higher on the scale, showing a higher density. And vice versa.

Pros and cons of a refractometer

Advantages of a refractometer:

  • ease of use;
  • high accuracy (subject to operating rules);
  • you don’t need to look closely to recognize the readings (the eyepiece can be focused to suit your particular vision);
  • readings are easy to photograph;
  • calibration is provided.

Disadvantages of a refractometer:

  • the device is less convenient than a classic hydrometer;
  • low measurement speed;
  • fragility;
  • demands for cleanliness;
  • strong dependence on ambient temperature and electrolyte transparency.

The last point is considered the biggest drawback of the refractometer, which is discussed in detail below.

How to use a refractometer correctly?

To obtain accurate readings of the density of the battery electrolyte using a refractometer, measurements must be carried out under the following conditions:

  1. The device must be calibrated before use. For this purpose, the design provides an adjusting screw. Instead of an electrolyte, two drops of distilled water are placed on the prism. There is a line on the scale corresponding to the density of water. By rotating the adjustment screw, you need to align the border of light and shadow with this line. Calibration must be performed before using the refractometer for the first time. In the future, it is advisable to check it periodically.
  2. Measurements must be carried out at the same ambient temperature at which the instrument was calibrated. The refractometer has extremely sensitive optics. When overheated or overcooled, the characteristics of the elements of the optical circuit change. The instrument readings are significantly distorted. The optimal ambient and electrolyte temperature is +20 degrees Celsius. If it is less or more by 5-7 degrees, then it’s okay - the device will be quite accurate.
  3. After placing the liquid, you must wait 30 seconds. This is necessary so that the temperatures of the electrolyte and the refractometer are equal. Otherwise, the device will show incorrect data.
  4. The electrolyte should be transparent. The operating principle of the device is based on optical phenomena. Light does not pass through a cloudy and dirty electrolyte at the speed at which the refractometer is calibrated. Therefore, the readings may differ greatly from the actual ones.
  5. Before each new measurement, the device must be thoroughly wiped. Residue from previous fluid can significantly affect the refractometer readings. If you do not take care of the main prism and protective glass, the instrument will soon become completely unusable.
  6. Measurements should be taken in good natural light. If LED or fluorescent lamps are used as a light source, the readings may be significantly distorted.

Note. If the refractometer scale does not show a clear boundary between the blue and white areas, this may be due to dirty electrolyte or inappropriate lighting. The presence of dust, dirt, hair and other microscopic debris on the prism is also not allowed.

Fuzzy light-shadow boundary
Fuzzy light-shadow boundary

Why does the refractometer lie?

The scale of a classic automobile hydrometer has a resolution of 0.01 g/cm3. However, if you follow the operating rules and are careful, you can easily consider the density of the electrolyte with an accuracy of 0.003 g/cm3.

However, this is all meaningless if:

  • the device is not calibrated;
  • water of questionable quality was used for calibration (tap, from a well, and so on);
  • the measuring surfaces of the device are dirty or wet;
  • the electrolyte is cloudy, brown, with debris and dirt;
  • electrolyte temperature is much higher or lower than +20 degrees Celsius;
  • the refractometer is not used at the same ambient temperature at which it was calibrated;
  • measurements are taken in inappropriate lighting.
Scale when calibrating a refractometer
Scale when calibrating a refractometer

Unlike a hydrometer, a refractometer can always be put in order and returned to high accuracy. To do this, simply wipe it thoroughly and calibrate it using distilled or ionized water.

Hydrometer or refractometer - which is better for measuring electrolyte density?

If you professionally service batteries, then you should have both devices in your arsenal. This will be an indicator of your professionalism, which is important for business. You can always double-check the density of a “suspicious” electrolyte with several devices and make sure your actions are correct. It is easier to photograph refractometer readings before and after service to show the client. When servicing batteries in large quantities, of course, a hydrometer will be much more practical. It will simply save you a lot of valuable time.

A simple car enthusiast rarely measures the density of the electrolyte. Usually before the onset of cold weather and again in the spring. Another device for measuring electrolyte density is useful when recharging the battery, during the desulfation process, and also to understand why the battery charge indicator does not turn green. For all these cases, an inexpensive but high-quality hydrometer is quite sufficient.

Does the average car enthusiast need a refractometer? This is a very interesting device that is nice to use a couple of times a year. You can also check your hydrometer for accuracy using a calibrated refractometer. Otherwise, like the professionals, most often it will be in your case. Whether you need this, given the price of this device, is up to you to decide.