Q&A: Battery testing
Although recommended procedures for battery testing are well documented, test engineers are sometimes unclear about the details. This results in a significant number of calls to our helpline so we’re devoting this item to providing answers for some of the questions we’re most frequently asked.
Q: The IEEE standards that cover maintenance and testing of batteries include this recommendation for the six-month maintenance interval: “Verify that the station battery can perform as manufactured by evaluating cell/unit measurements indicative of battery performance against the battery baseline (e.g., ohmic measurements).” But what exactly does “battery baseline” mean and how do you establish this baseline?
A: Baseline is one of the most misunderstood terms relating to ohmic testing. The baseline is a reference value against which you make comparisons. Although manufacturers sometimes provide baseline values and may even include them on the battery data sheet, the IEEE does not recommend using these values. This is because different ohmic testers use different signals and test frequencies, which means they will give different results for the same battery, and you have no way of knowing which tester the battery manufacturer used to establish the published baseline values. A further complication comes from the fact that the manufacturer usually performs tests on “green” batteries – those that have not yet completed formation. The IEEE recommends that users perform baseline tests on batteries after installation and after formation has been completed. For a VLA battery, this process typically takes around a week during which time the battery is fully charged, equalised, cooled down and capacity-tested to confirm that it has reached 100% capacity. New VRLA batteries need time for trapped gases to escape, so they often need to stay on charge for between three and six months before baseline values can be established. Gel cells can take up to a year to reach full formation. Overall, however, the most important thing to bear in mind is that ohmic testing is not an absolute test, it’s a relative test where you compare the results of new tests with the baseline value and look for changes.
Q: When comparing an ohmic measurement to the baseline value, how much change is considered acceptable?
A: The IEEE recommendations say that a 50% – 100% change from the baseline on a cell is serious and warrants further investigation, but it’s important to take into account the criticality of the application and battery type. VLA and VRLA batteries fail in different ways. The typical failure mode for a VLA cell is positive grid corrosion. When a VLA cell fails, it fails in shorted mode, which means that current can still pass through it. This means that series strings can be used even in critical applications. VRLA cells, however, most often fail due to drying out and they fail in open mode, which means that they may not be able to pass current. In critical applications therefore, they should be used in parallel. With these differences in mind, a 50% – 100% change from baseline is a good screening criterion for VLA batteries but with VRLA batteries you may want to consider being a little more cautious and use 20% – 30%.
Q: On a battery discharge test, what should I do when one cell has reached the end voltage earlier than the others?
A: You’re actually doing an overall discharge test of all the cells and it’s inevitable that some will reach the end voltage – let’s say this is 1.75 V – earlier than others. You shouldn’t stop the test when one cell reaches 1.75 V; you should stop it when the average cell voltage is 1.75 V. At this point, some cells could be at 1.8 V and others at 1.6 V. Monitor the overall battery voltage during the test and, if you have for example 60 cells, stop the test when the voltage reaches 60 x 1.75 V = 105 V.
Q: Does discharge testing reduce the life of a battery? I’ve heard it called ‘destructive testing’ as weaker cells may fail during the test. Is this correct?
A: Discharge testing, as the name suggests, involves discharging the battery and, after the test is finished, recharging it so that it can be returned to service. Now every battery is good for a certain number of charge/discharge cycles – often around 1,000. If you follow IEEE guidelines, during the lifetime of a battery with a nominal life of 20 years, you will perform a discharge test just four or maybe five times. In other words, testing will reduce the number of cycles the battery is theoretically good for from 1,000 to 995. In practice, this reduction is insignificant and it’s well worth making this tiny sacrifice to be sure that the battery is definitely capable of supplying the required load when it’s called upon to do so.
