Recently I had the great fortune (and misfortune) of replacing my UPS (uninterruptable power supply) batteries. My two UPS units are familiar brands. One is used for a server and the other is for my video editing equipment. One is rated for 30A at 120V and the other is 20A at 120V. The rated operating time is where the dirty details come into play. My 20A unit has a rated run-time of 15 minutes with a load drawing 1000 watts. I was getting nowhere near that amount of time. I wanted to test the old batteries and then check out the new, replacement ones. I could not test the old batteries in my 30A unit, because its batteries were open circuit.
There are two standard types of battery tests, a performance or State of Charge (SoC) and the State of Health (SoH). The SoC compares published data in terms of A-h (ampere-hour) of capacity to the actual capacity. This test starts with isolating a single cell or a group of cells (i.e., a battery). You then connect the cell or battery to a load-box (a bank of calibrated resistors) that can dissipate the expected power. The intent here is to discharge the cell or battery to a specific level and measure the elapsed time. Specifically, this discharge test drops the charge to a Depth of Discharge (DoD) level of greater than 90%. This provides a reasonable and reliable check of performance, but only at the time of test.
An alternative to the full DoD is a partial discharge using the system as a load, not an external load resistor. This system load is a more accurate run-time test. This allows the batteries to support the entire actual load, so we'll call this a real-life test. Performance is measured by the published run-time chart or graph. Capacity can be estimated by extrapolation if you know the size of the actual load. The highest accuracy of the test is when the DoD discharge is close to 100%.
A second type of test is the State of Health (SoH). Fault modes such as sulfation, chemistry dry-out, operating temperatures outside of permissible range, and incorrect float charge will affect batteries negatively. Electrical capacity of SoH will have a baseline from ohmic measurements. We will go over ohmic testing in the next article. For now, you just need to know that an ohmic test is an impedance (or conductance) derived from an AC signal driven into the battery; or the internal resistance derived from a DC technique.
This capacity test will detect when the SoH has declined to the point of near-end of battery life. It does not show how much life is remaining until enough testing is done to show a trend. What we need is a test that changes over the life of a battery. Ohmic might be the answer. The problem is that a battery is a non-linear device that is elegant in one manner and clumsy in others. Mixing chemistry with electricity can be most difficult to predict.
Resolution of the testing meters and impedance of test load (if used) will all affect test outcomes. A method of testing may require several readings. Even with this care in the techniques used, test readings may not match up with one another; or from battery to battery; or with just one battery but on different days. Your baseline results may vary.
What I have learned about batteries is that testing for SoC, SoH, and DoD are the present methods for determining battery condition. All tests need to be performed and a lot of time dedicated to meter reading, both before and after discharges. You must pay careful attention to the loads and to equilibrium of battery cells.
Of course, another approach is to look at the manufacturer's specifications. Print out the run-time graph or chart. See if the manufacturer provides a half-life spec. Test your batteries based on this information. My UPS has a hot-swap battery compartment so I can test individual batteries or replace the entire battery compartment without powering down. If I have less than half of the run-time available, it is time to change out. Batteries are cheap compared to test equipment, servers, and the expense of having a long-duration test crash before you get your results.
What testing methods have you used to measure your batteries? Do you have any horror stories related to battery tests?