Electrolyte, Hydrometers and Specific Gravity – The March Blog

After conducting quite a few training courses and seminars over my career, I’ve found more than several battery-related maintenance topics that are frequently misunderstood. I like to communicate information I think will be useful to the battery user community. I hope you enjoy this month’s Blog.

This month I will discuss a subject that can be confusing for those performing maintenance of vented lead-acid (VLA) stationary batteries. I’ll be discussing electrolyte, specific gravity, and its measurement.

First, a few definitions.

Electrolyte - as defined in IEEE 1881-2016, IEEE Standard Glossary of Stationary Battery Terminology is “An aqueous or nonaqueous medium that provides the ion- transport mechanism between the positive and negative electrodes of a cell.”

Specific Gravity - as defined in IEEE 1881-2016, IEEE Standard Glossary of Stationary Battery Terminology is “The ratio of the mass of a given volume of electrolyte to the mass of an equal volume of water at a specified temperature.”

Hydrometer - an instrument, mechanical or electronic, for determining the specific gravity of a liquid.

To expand on these definitions and put them in context, by starting with the electrolyte. The specific gravity of water is 1.000. Therefore, battery electrolyte specific gravity in a normally operating lead-acid battery will be higher than that of water. If you’re operating calcium alloy, standard gravity vented lead-acid batteries, the specific gravity is likely to be about 1.205 to 1.225. The nominal specific gravity for these cells is generally 1.215, allowing for an acceptable range of +/- .010 points. Expressing values in verbally results in dropping the decimal. We know it’s there, but it’s spoken as “twelve fifteen, plus or minus ten points”, for example. This isn’t the only specific gravity used for batteries. “High gravity” cells are 1.250 (1.240-1.260) and lead selenium cells are generally nominally rated at 1.240.

Measurement of electrolyte specific gravity historically employed the use of a float hydrometer. The cost is about $30-$40 US. See Figure 1. The scale on the float inside a vessel is graduated in points along the length of the float. Calibration is achieved by the addition of weight in the bottom of the float.

Figure 1. Float Type Hydrometer

An electrolyte sample is drawn into the vessel by squeezing the bulb at the top of the vessel. Assuming the sample quantity is adequate to result in buoyancy of the float, the level is read against the float scale to measure specific gravity.

A specific gravity measurement made with a float hydrometer requires temperature and level correction before it can be considered truly accurate. The reference temperature for lead-acid batteries in the U.S. is 77°F. One point (.001) of gravity correction is needed for each 3°F above or below this temperature. Points are added above 77 and subtracted below. Level correction refers to the location of the electrolyte level in the cell, indicated between the level lines on the side of the container. Approximately 6 points (.006) are added to the temperature corrected reading for each ¼” the level is below the high-level line. Lowering of the level is the result of loss of water in the electrolyte partly due to evaporation with the reminder due to electrolysis. The latter of these is caused by float current passing through the battery during normal float charging.

Over the years, more accurate means of measuring specific gravity came about. Today, electronic digital hydrometers are commonplace for users and service companies. See Figure 2. The cost for these instruments ranges widely from $500 to $5000 with various features and capabilities. Shop carefully before buying. Advantages include high accuracy, digital displays the virtually eliminate reading errors. The displayed value generally indicates a temperature corrected reading as well as electrolyte temperature. Readings can be stored in many of these instruments and later transferred to a computer for detailed analysis, archiving and merging with related maintenance data.

Figure 2. Digital Type Hydrometer

So, what does a specific gravity reading indicate about a battery? Generally, the measurement indicates the state of charge of a cell. That’s a broad statement and requires explanation as the measurement is influenced by temperature, electrolyte level, type of cell, i.e., lead calcium, lead antimony, and lead selenium.

A low specific gravity reading means a cell is at a low state of charge. Corrective action may be required. There are times when readings are not accurate, such as after water additions and discharge events. In both cases, the time required to recharge, get the sulfuric acid out of the plates and back into solution with the water can take from a few weeks to a couple of months, depending on available charge current, the ampere-hour size of the cell, quantity of water added and depth of discharge. Until that time, specific gravity readings are not going to provide much useful information as to state of charge. The specific gravity lags the ampere-hours returned by the charger after a discharge.

The higher gassing rates of lead-antimony cells make specific gravity measurements a better

indicator of the condition of the cells than for non-lead-antimony designs. The lower rates of gassing in lead calcium and pure lead cells mean the electrolyte is slow to diffuse after charging or water additions, and an accurate indication of the cells’ condition may not be available for several months. Therefore, for cells with lead-antimony plates, it is recommended to measure the float voltage and the specific gravity of the pilot cells to characterize the state of charge.

For additional information relating to specific gravity measurements and maintenance of vented lead-acid batteries, consult the operating and maintenance instructions for your battery make and model. Another excellent source of information is IEEE 450-2010, IEEE Recommended Practice for

Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications, available at https://www.techstreet.com/publishers/ieee.

Rick Tressler provides battery training and education services to technicians, electricians, installers, and maintainers and others working with stationary battery systems. For information on classes, seminars and technical support, check out the website at www.ricktressler.com or contact Rick at rick@ricktressler.com, 614.632.7521.

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