The main objective of a battery system is to provide standby and emergency power to critical equipment during an AC outage. Photo: Paul Chernlkhowsky (Flickr CC).jpg
Batteries are complex chemical mechanisms. They are comprised of a number of cells connected in a series path. If any one component fails the entire series connection fails.
The main objective of a battery system is to provide standby and emergency power to critical equipment during an AC outage. A good battery maintenance program may prevent, or at least, reduce the costs and damage to critical equipment.
With so many applications and options for testing batteries it's hard know what the best testing scheme is. In this article we will briefly discuss three quick, yet effective tests that will help identify abnormalities in the battery system using simple testing and measurement equipment.
1.) Specific Gravity
The most accurate and direct way to test the state of charge of a battery cell is to determine the specific gravity of the battery electrolyte. The higher the specific gravity of the electrolyte, the higher the state of charge. A low specific gravity may mean that the charger voltage is set too low, causing plate sulfation to occur.
Specific gravity is measured using a hydrometer, an instrument that measures the ratio of the density of the electrolyte to the density of water. Readings are obtained by carefully drawing a sample of the electrolyte from the battery into the hydrometer.
Specific gravity is measured using a hydrometer. Digital hydrometers like the one pictured above are the easiest way to obtain readings. Photo: BAE Canada.
Measurements are temperature dependent, meaning the specific gravity will vary depending on the temperature inside the batteries. Some testers will correct the values automatically, while others may require calculating values manually through an equation. Either way it is important to use temperature corrected values for accurate trending data.
Specific gravity is not traditionally known to provide much value in determining impending battery failure, as it only changes slightly after the first 3 to 6 months of a battery's life. The initial change is caused by the completion of the formation process, where inactive paste material is activated by a complex bundle of chemical reactions.
WARNING: Battery electrolyte fluid contains sulfuric acid. Use extreme caution when working with these fluids, always wearing the appropriate PPE. Work should only be performed by qualified personnel.
2.) Float Voltage Test
Incorrect float voltage will indicate problems with the cell. Photo: BAE Canada.
Measuring float voltage is one of the more traditional methods of testing a battery, but it's important to understand that a correct cell voltage doesn't indicate anything except that the cell is fully charged. Incorrect cell voltages do indicate, however, something about the condition of the cell.
Because the sum of all cell voltages must equal the battery charger setting, the lower voltage cells must be counteracted by overcharging the cells in better condition, which leads to higher operating temperatures.
Note: It is possible that one cell floats much higher to compensate for several cells floating a little low.
A low float voltage shows a cell that is not fully charging and cannot supply full capacity, which may be an indication of plate sulfation. A high float voltage is an indication of overcharging. This can lead to premature grid corrosion and higher temperatures in the battery, the worst culprit of shortened battery life.
Measurements should be made with the battery disconnected from the load and charging system in order to obtain most accurate readings. Use a multimeter set on DC voltage when taking your reading. Compare the values to the limits specified by the manufacturer.
Remember that abnormal float voltage indicates something is wrong. However, if the float voltage is normal, it says nothing about the state-of-cell, capacity, or state-of-health.
3.) Intercell Resistance
Simple maintenance procedures will prevent intercell connections from ending up like theses. Photo: Batteries Unlimited
Many times batteries fail, not because of weak cells, but due to weak intercell connections.
Generally, hardware should be tightened to the low end of the torque scale that is recommended by the battery manufacturer. But it is far better to validate intercell connection resistance using a low-resistance ohmmeter.
By following IEEE Recommended Practices, electrical measurements can even find errors often overlooked, such as a washer stuck between the post and an intercell connection, whereas torquing will not.
IEEE specifies that the variation of intercell connection resistance be less than ten percent. For example, a 70-micro-ohm intercell connection resistance should not deviate by more than 7 micro-ohms. It's also recommend that ten percent of the intercell connectors be measured quarterly, and all intercell connectors measured annually.
On multiple-post cells, measurements should be made straight across both connections, and then diagonally to check for balance in the cell and connections. Measuring only straight across does not adequately test the cell due to the parallel circuits for the current.
- Megger Battery Testing Guide
- Trojan Battery Company Battery Maintenance
- Battery Testing and Maintenance
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