Specific gravity is the ratio of the weight of a solution to the weight of an equal volume of water at a specified temperature. The measurement of specific gravity as applied to battery maintenance is used as an indicator of the state of charge (SOC) of a cell or battery. The specific gravity decreases during the discharging of a battery to a value near that of pure water and it increases during a recharge. The battery is considered fully charged when specific gravity reaches its highest possible value.
Specific gravity tests are performed on vented/wet cells. As mentioned earlier, this measurement is temperature dependent and all measured values must be compensated for electrolyte temperatures above or below 25 °C (77 °F). Electrolyte level can affect the specific gravity measurement due to the fact that water is lost during the recharge cycle. So if there is less water and the same amount of acid due to a full recharge, then the specific gravity measurement will be higher than it should. Ideally specific gravity should be measured when the electrolyte level is correct and when the battery is fully charged and has been at float conditions for three days. Each time the cell is recharged the sulphate that was present on the plates is recombined into the electrolyte solution in the form of sulphuric acid. Because sulphuric acid has a higher density than water the sulphuric acid tends to migrate to the bottom of the jar. This will lead to a lower specific gravity measurement at the top of the jar than at the bottom. An equalization charge will normally mix the electrolyte so that the specific gravity measurement will be accurate at any level measured.
A sample of electrolyte is drawn for a specific gravity test
There are three types of load tests, the Acceptance Test, the Performance Test, and the Service Test. The acceptance test is made at the beginning of the life of the battery and is usually performed at the factory or at the location of installation and is based on the design capacity of the battery. The performance test, which is also based on the battery capacity, is typically performed two to three years after installation and then every five years, based on IEEE recommendation. The service test is based on the load and is used to determine if the battery will correctly supply the load and is performed as needed.
Load testing or discharge testing is the single absolute test when it comes to the ability to determine whether a battery is going to be capable of supplying the amount of current for the designed amount of time. There are some drawbacks however. First, every battery has a certain number of deep discharges it can tolerate before failure, so each time a discharge test is performed some of the life is taken away. The second drawback is the complexity, cost and time to correctly perform a discharge test. If the battery system being tested is in service, a backup system will need to be installed while the main system is tested in the event of a power outage. A temporary external load comparable to the normal load being served will need to be connected to the main system under test. The time of the test can last as long as the rating on the battery.
Battery load/discharge testing
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