Determining Battery State of Charge

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

Specific gravity (SG) is defined as the ratio of the density of a material to the density of a reference material—with liquids, this is usually pure water. For lead-acid (LA) battery electrolyte, the amount of acid dissolved in the water determines the SG. A battery electrolyte’s density is a direct indicator of the battery cell’s state of charge (SOC)—and can also reveal the relative health of that cell compared to others.

With flooded LA (FLA) batteries, each battery cell’s SG should be measured and recorded at the time of installation, but only after an initialization charge. These initial readings will serve as a reference for comparison over time, and will also verify that the battery is in good condition. If there is a significant imbalance between the SG of the cells in a newly initialized battery or if the electrolyte does not reach a “full” SG reading after charging, contact the battery manufacturer or supplier.

Once the batteries are in service, each cell’s SG should be measured annually to identify problems like unequal charging or if the battery is not getting fully recharged. These identified problems can then lead to discovering their causes, such as incorrect setpoints; high-resistance connections in battery strings; an inadequate charging source; or even a loss of electrolyte due to overfilling cells during watering.

Charging the battery to a full SOC before testing the SG will result in more accurate readings, as the electrolyte will be better mixed and cell voltages more balanced. After this charge, but prior to taking the SG reading, do not top off the battery with water in an attempt to replace the electrolyte lost to gassing or your SG reading will be inaccurate.

Stratification of a battery, where the electrolyte becomes denser at the bottom of a cell and less dense at the top, occurs frequently on tall, high-capacity LA cells that are not fully recharged. This condition can only be detected by testing the SG. If left to persist, stratification will damage the battery by corroding the lead plate at the bottom while allowing the top of the plate to become sulfated and inactive.

Typically, the SG of a fully charged LA battery’s electrolyte is 1.265 to 1.275. (Batteries with other types of chemistries will have different SG values.) That is, the electrolyte is 26.5% to 27.5% denser than water. Often, the measurement is multiplied by 1,000 to eliminate the decimal point. This also reduces the tendency of people to round off the value when recording it.

Measuring SG Using a Hydrometer

To measure SG, you need a hydrometer or refractometer. Low-cost hydrometers are readily available, but refractometers and higher-quality hydrometers may need to be specially ordered.

A hydrometer consists of a glass float inside a glass tube reservoir. A rubber bulb at the top is used to draw the electrolyte from the battery. A section of the float is calibrated with a scale so that when it is submerged in water it will read a value of 1.000 (or 1,000). When the liquid being tested has a higher density, the float sits higher in the liquid, indicating a higher SG value on the scale. When reading the scale on a hydrometer, be sure that the scale is read using the bottom level (the meniscus) of the liquid’s surface.

When drawing the battery electrolyte into the hydrometer, first squeeze the rubber bulb and then insert the hydrometer into the vent opening on the battery. This reduces introducing air into and bubbling the electrolyte. Keep the tip of the hydrometer in the cell while slowly releasing the rubber bulb, drawing the electrolyte into the glass tube. There is enough electrolyte in the tube once the float is lifted off the bottom. Hold the hydrometer completely vertical when taking the reading.

Comments (1)

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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