Battery Self Test: Can I Trust It?

Many UPS systems today come with a UPS battery self-test feature. While this function can often aid in understanding a UPS battery’s health it may not always provide the full scope of the battery’s health.  This article will go on to discuss how the UPS self-test function works, and how to accurately test a UPS battery.

What is the self-test function?

Most modern, or smart, UPS units can complete a ‘self-test’ operation on the batteries within the unit.  Often, these self-tests take place automatically when the UPS is turned on or they can be programmed to take place at preferred intervals.  One of the most common ways the self-test function is completed by temporarily discharging the batteries to determine how long they would run. 

When a UPS is performing a self-test using the discharging method, UPS systems are designed to measure the voltage that the string of batteries is providing. During the duration of the test, the UPS will measure the starting voltage and ending voltage of the string to determine if the DC voltage remains within an acceptable tolerance. Should the DC voltage dip below a threshold limit during this time, it will cause the UPS to end the self test, and generate an alarm.   The unit does not determine how many volts are being produced by a single battery. When a battery within the string is starting to fail, it may continue to pass voltage along even if the individual cell is not within the recommended voltage range. Essentially, this could lead to a battery string to pass a self test, even if there is a failing cell. Should the the battery completely fail, the cell could fail to an “open” state and may no longer pass voltage at all.  When this happens, the unit often will fail the self-test and trigger an alarm.  At this point, if the facility experienced a power loss or other power quality issues, the system may not be capable of supporting loads, and the critical systems could be left vulnerable.

 Ideally, it’s best to catch a failing battery right at the start of its deterioration rather than when it is entirely failed.

What is the best practice with battery testing?

So, what are best practices when it comes to accurately examining a UPS battery’s health? Quality Power Solutions and many OEMs recommend having UPS batteries assessed bi-annually using a combination of tests. There is a range of UPS Battery testing methods, the most common methods may include: Electro-Chemical Testing, Impedance Testing, Load Bank Testing, and Battery Monitoring.

Types of Battery Testing:

Electro-Chemical Testing

The electro-chemical testing method involves comparing data from batteries to algorithms of common battery conditions. This test is accomplished by placing probes on the battery’s terminals to measure the frequency response to voltage and current signals passed into the battery. These results are then cross-referenced with the data of a healthy battery. This method tests a battery’s sulphation, electrolyte dry-out, and impedance, which can provide enough data to determine the overall health of the battery.

Impedance Testing

A battery’s impedance is a combination of a battery’s internal resistance and its reactance.

Impedance = resistance + reactance

The impedance test is performed by applying an AC current to each battery by attaching probes to each block terminal. A battery’s impedance is measured in milliohms. The impedance test is typically completed annually to compile and track the performance history of each battery cell over time. This test provides data that can assist in identifying signs of deterioration or cells with a high internal impedance that might require replacing. A benefit of this test is that it does not put a lot of stress on a battery, and it does not require that they be taken offline.  

Load Bank Testing

Load bank testing, also referred to as discharge testing, is considered the most comprehensive UPS battery test, because it provides the actual capacity of the battery string. This approach involves reviewing the batteries under both normal and peak load conditions. From there it can be identified which battery cells still hold a charge and which might be approaching the end of service life. While the load bank test provides quality data, it does require the batteries to be taken out of service when it is performed, and could limit the systems reliability while waiting for batteries to recharge.

So, Should I Trust my Self Test Feature?

Altogether, the self-test function should be used as a tool to help you monitor your batteries.  However, it should not be the only form of battery health measurement that you complete. Considering that once a unit has received a failed self-test, the UPS system and facility at large are in danger of experiencing potential power issues. For preventative measures, it is recommended by power experts that one of the UPS battery tests exclaimed above is performed every six months. Regular UPS tests and maintenance inspections are key to reducing downtime. Critical power backup equipment, like a UPS System, will fail without proper preventative maintenance, especially when they are located in harsh, dusty, or corrosive environments.

Is your critical power system in need of preventative maintenance? QPS builds custom preventative maintenance programs for UPS systems and generators to ensure you are prepared for anything. Contact our experts today by filling out the form below.