I recently mentioned that I had the great misfortune of discerning the life of my UPS batteries, followed by replacing my UPS batteries. I replaced all the batteries, so I was given a chance to test them for run time, etc., without regards to keeping the UPS online.
To begin with, a lead-acid battery is an analog device. It's a device that converts chemical interaction into electrical power, though not at a linear rate. Chemistry is not my forté, so we will limit our discussion to the electrical side of the battery — with one quick side note. Vented batteries give off hydrogen gas. If you have a lot of batteries or UPSs in one area (especially an enclosed area), you need a hydrogen content alarm. If that content goes above 4%, you should turn off the power to all battery chargers. Outside venting should reduce the hydrogen content. The 4% point is when hydrogen in the air can explode. This is a bad thing.
Let's review some other types of battery testing. Capacity test uses a load bank with a constant current capability. Each cell in each battery must be monitored continuously. Problems can include the time needed; each battery could take up to 10 hours to test. You need an external load that matches the impedance of the live load. Reliability of the test is high if you truly test the battery's absolute capacity.
Resistance testing measures the internal resistance of each cell. As the battery ages, the resistance increases. Increased internal resistance indicates degrading or aging and can point to premature failure. This test can be done while the battery is in use. It is non-invasive but does not provide absolute capacity.
Let me offer a few bits of information regarding ohmic testing of batteries. Internal ohmic testing offers no conclusive proof, nor is it an accepted industry technique for determining electrical capacity. Ohmic readings should be used as a trending tool, but not as a substitution for actual capacity testing. Ohmic readings at the factory — AC impedance, AC conductance, resistance, or DC resistance testing — can be misleading and should not to be used as a baseline.
Another parameter worth noting is battery float, or the energy applied to the battery to maintain its charge. Temperature, probe placement, and differences in the impedance measurements all are major reasons factory readings maybe different from in-the-field readings. Ohmic values vary from manufacturer to manufacturer, as well as capacity and actual physical size. Float stabilization can take weeks or even months to establish.
The main three parameters that impact the accuracy of ohmic field testing include the baseline. This is the ohmic value of a perfectly good battery. All further tests are compared to the baseline. If there are errors in the field readings, the ohmic baseline may have no value as a reference. Error rates can be ~20% average or as high as ~37%. Whether a battery is good, OK, or failing, an important determination is the baseline and its accuracy.
Other issues are manufacturers' baseline setups. The battery tester will vary from model to model. The person in the field is heavily involved with the accuracy of the baseline, test results, and test repeatability. Remember that manufacturers want their products to meet their specifications and not have the piece returned for warranty replacement. Each manufacturer — whether making a battery, tester, or meter — needs to meet the published specifications, but how that test is performed may have more meaning. The technical staff in the field does not want to keep changing out batteries and constantly buying test equipment until one unit performs as expected. Maintaining uptime is the primary desired result, not developing a detailed test procedure for the field technicians.
Data analysis is the final variable. It has a substantial impact on ohmic testing reliability and any decisions made as a result of these tests. When these data points are combined, our procedures for ohmic testing start to take shape. But are we testing without technician errors and with proper procedures, correct probe placement, and reliability in measurements?
Factory readings using impedance meters from an ABC company and readings by a technician or engineer using a meter from GHI company will typically offer different results. The manufacturer's data is probably useless to the end user for creating a baseline. If you are concerned with actual battery capacity, stop using the internal life meter. Collect voltage, ohmic readings, and an accurate baseline that you create. Operating and testing temperatures are also key in data analysis. Don't forget to use the same calibrated meter and loads. This is what the state of health of the battery system takes.