Determining Battery State of Charge: Page 2 of 2

By Measuring Specific Gravity

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Determining Battery State of Charge By Measuring Specific Gravity
Determining Battery State of Charge By Measuring Specific Gravity
glass-type hydrometer
A technician checks the batteries’ specific gravity using a glass-type hydrometer.
Hydrometer close-up
A color-coded and numeric scale on the float indicates the cell’s state of charge.
A refractometer
A refractometer uses only a drop of electrolyte to measure specific gravity, and is more accurate and easier to use.
using a refractometer
A technician checks the specific gravity using a refractometer and reading its illuminated internal numeric scale.
Determining Battery State of Charge By Measuring Specific Gravity
glass-type hydrometer
Hydrometer close-up
A refractometer
using a refractometer

If there is not enough electrolyte drawn into the hydrometer, squeeze the bulb slowly to remove the liquid and try to compress the rubber bulb more completely to draw up more electrolyte from the cell. Once the reading of the SG is complete, be sure to return the electrolyte to the same cell that it was removed from.

Since the density of a liquid changes with its temperature, the SG measured with a hydrometer needs to be adjusted, or compensated. Quality hydrometers have a built-in thermometer with an easy-to-use scale for making this adjustment. The thermometer’s temperature is referenced to a value that is either added or subtracted from what is indicated on the float. Verifying a hydrometer’s accuracy should be done regularly by taking a reading of distilled water—which should measure 1.000. Be sure to adjust for differences in water temperature.

Sometimes, a battery cell’s electrolyte level is too low to allow a hydrometer to draw up enough electrolyte. Distilled water will need to be added but, before taking a SG reading, you will need to charge the battery for several hours to ensure that the electrolyte is fully mixed. Otherwise, the acidless water will float on the top of the more dense electrolyte, giving an inaccurately low SG reading.

After completing all readings, be sure to rinse off the hydrometer in clean water and allow it to dry before putting it away. Be especially careful of setting down the hydrometer on a dirty surface, since dirt in the hydrometer can interfere with an accurate reading and contaminate the battery electrolyte. Keeping the hydrometer inserted into a bottle of distilled water between uses is the best way to keep it clean, protect it from being dropped or knocked over, and to reduce the amount of acid that gets on your clothing.

Using a Refractometer to Measure SG

While a hydrometer is an off-the-shelf tool to measure SG, it has some shortcomings. First, the scale on the float can be difficult to see in poor light. Second, because the surface of the liquid is curved, it’s possible to get inaccurate readings depending on what part of the meniscus is used when reading the scale. The float can also be tilted in the tube, causing it to hang up on the side and making it hard to read accurately. And finally, the relatively large volume of electrolyte required to fill the hydrometer makes it easier to end up with acid on your hands and clothing, or on the outside of the battery.

Refractometers, however, only require a small drop of the electrolyte to measure the SG, making them much less messy and easier to use. The level on the scale is much easier to read and they can be used in low-light conditions. They also have excellent accuracy and provide consistent results, unlike float type hydrometers.

A refractometer uses the “angle of refraction” of light through a small drop of the liquid being tested. The angle will vary depending on the density of the solution.

To measure SG, use the sampling dropper to obtain some electrolyte from the cell. Then, open the refractometer’s illuminator flap and put a drop or two of electrolyte on the measurement prism’s surface. After closing the flap, look through the eyepiece. Read the value where the shadow boundary line meets the scale. It may be necessary to point the refractometer toward a light source (the sun or a lamp), but during the day, ambient light is usually sufficient. When you’re done, wipe the prism and flap with a clean, soft cloth and rinse the dropper with distilled water. Store the refractometer in its protective case as it’s a somewhat fragile device that can have its calibration affected if dropped or jostled. Use all of the same safety precautions and equipment as previously discussed in the hydrometer section.

The refractometer will retain its accuracy as long as it is kept close to the nominal temperature used as its reference (77ºF). Many refractometers also include a built-in temperature compensation mechanism. However, allow the instrument to equalize with the environment before using and protect it from temperature extremes.

Like a hydrometer, you can verify the accuracy of the measurement of a refractometer in the field—all you need is some distilled water. The reading should equal 1.000 or 1,000, depending on the scale of the instrument.


Christopher Freitas is an engineer and project manager for international RE projects. He was a cofounder of OutBack Power Systems and was the director of engineering at Trace Engineering.

Comments (3)

BlindSquirl's picture

Would this type of Pulse Tech Battery Tester serve the same purpose as the hydrometer?

And correct me if I'm wrong but doesn't desulfating battery plates restore the SG of a battery's electrolyte by putting the sulfur back into solution?

My back up system is cheap (by necessity) and my battery "bank" consists of 2 car batteries that I swap out. I also do what I can to keep them conditioned by using a Pulse Tech Xtreme charger that both does the diagnostics and desulfating while it's charging.

It admittedly takes a long time to bring back a heavily sulfated battery but I've been able to accomplish a lot without even having to lay out the cost for deep cycle batteries.

Michael Welch's picture

No, you cannot accurately determine battery state of charge by looking at its instantaneous voltage, which is what that "tester" does. If a battery is fully at rest and been charged completely, you can get a rough idea of its state of charge, but not as accurately as measuring specific gravity.

The main thing that desulfation does is remove the crystals that prevent the electrolyte from fully contacting the lead plates. I do not know how much of it goes back into the electrolyte and how much ends up at the bottom of the cell.

It's good that you are hands-on enough to get by with refurbished car batteries, but most people want more reliability and less maintenance, and to be able to more deeply cycle a battery with less impact on the battery's life span.

RMichael Curran's picture

Christopher - You say an SG reading is the only way to detect stratification, but don't say how. I'm assuming a low SG (may) indicate stratification. Correct?
Also, I've found it extremely important when taking an acid sample up into the hydrometer to make sure the glass bulb floats freely and doesn't "drag" on the sides as acid is introduced. Otherwise you get a low reading.
Thanks for the great article on an important but simple tool for FLA battery maintenance.

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