Battery monitoring improves UPS reliability

We already rely on electronic devices in many aspects of our work and life. From mobile communications to data centers, a variety of devices must operate with minimal downtime, which makes the reliability of power supply a critical issue. Uninterruptible power supplies (UPS) can provide backup power during power outages, so they are widely used to ensure the normal operation of critical electronic equipment during power outages. Although other scientific technologies can be applied (such as flywheel energy storage), most UPS still use various batteries to store energy. Batteries can provide a considerable amount of energy and can provide power almost instantly. If the UPS needs to operate reliably, it is most important that the batteries are not only fully charged, but also in good condition.

Batteries have a limited lifespan. If environmental conditions (especially temperature) are outside the optimal range, the lifespan of the battery will be greatly reduced. The batteries in most devices are generally replaced at regular intervals specified in the warranty (usually every 5 years). However, this approach also has the disadvantage that batteries will lose power quickly when used outside the expected environmental conditions, while well-maintained batteries will have a longer lifespan.

Modern UPSs are required to provide higher power output, so many batteries are needed. In large battery packs, a single battery failure will cause the entire battery pack to fail. Large and medium UPSs are redundant to ensure that a battery pack failure does not cause the entire UPS to fail. At the same time, the UPS will continue to operate, the peak current output will be reduced, and the time the system can run with the UPS will be shortened. In addition, a failed battery will also affect other battery blocks in the battery pack, thereby shortening the service life of these battery blocks.

Battery monitoring and maintenance represents a significant cost associated with operating a UPS. Typically, engineers will visit the site regularly (perhaps monthly) to measure the electrical characteristics of the batteries within the unit. Engineers will often measure the battery voltage to identify if the battery is out of range and replace it if it is. Output voltage is not necessarily a good indicator of battery failure, so a battery may fail between routine inspections, requiring an additional inspection by an engineer.

On the one hand, online monitoring of batteries reduces the time engineers actually spend on-site to check the status of each battery, improves the efficiency of on-site inspections, and thus reduces costs; on the other hand, online monitoring also enables preventive maintenance. By identifying possible faults, engineers can replace faulty batteries during routine inspections, thereby ensuring more reliable operation of the device and eliminating the need for emergency inspections.

LEM uses the SenTInel battery monitoring system to measure batteries in broadcast equipment equipped with an 800 kVA UPS. Figure 1 shows the output voltage of several cells in a battery pack. In this example, each battery pack has 200 cells and can provide a maximum voltage of 440V. The voltage varies greatly, mainly due to incorrect battery conditioning, which will be discussed later.

The graph clearly shows that one battery is outputting 2 V instead of the normal 2.2 V. Although one battery is producing a lower voltage than expected, the difference is quite small and stable. This behavior is common and makes using output voltage as a predictor of failure unreliable, as the voltage can remain within the threshold range and therefore not trigger the alarm.

Figure 1 Battery output voltage

In this example, the Sentinel battery monitoring system was used to evaluate the effectiveness of the scheduled maintenance program, not to indicate potential problems. Because no action was taken, Figure 2 shows that the battery failed completely on October 9. Note that the voltage of the failed battery remained constant and showed no signs of possible failure until the battery voltage dropped to 0.7 V. After the battery was replaced on November 19, the battery voltage returned to normal.

Figure 2 Battery failure

Since the output voltage did not change before the battery failed, it is not a good indicator of a possible failure. Another characteristic parameter of the battery, impedance, is a better indicator, as shown in Figure 3. The graph illustrates that the impedance, which rose in June, increased by more than 20% in early July. The trend is easy to detect: Measuring the impedance can detect problems three months before the battery fails. If the customer uses the impedance data, the battery can be replaced during routine preventive maintenance before the battery deteriorates and fails.

Figure 3 Battery impedance prediction failure

Long-term monitoring of the batteries provides additional useful information for improving UPS reliability. For example, in Figure 1, we can clearly see a large number of charge/discharge events (indicated by the multiple spikes on the voltage trace). Although all batteries need conditioning, the batteries are being discharged too frequently, 4 to 5 times per month. While some battery conditioning extends the life, too many discharge events shorten it: a normal configuration will only cycle two to three times per year. Typically, batteries are warrantied for 20 to 50 cycles. In this case, we are considering that the battery may exceed this warranty in just a few months, and a battery replacement plan every 5 years may mean that the battery will experience several times more discharge events than it is designed to withstand.

The frequent charge/discharge cycles at this site were caused by the installer leaving the UPS in commissioning mode, which caused the battery to cycle through charges for testing purposes. This surprisingly common mistake can significantly shorten the life of the battery. The misconfiguration may not be apparent during an engineer's walk-through of a site that is continuously and automatically monitored, but the resulting problems are obvious.

Another reason for shortened battery life is high temperatures. Even a small increase in temperature can increase the probability of unwanted chemical reactions within the battery, ultimately leading to battery failure. Typically, battery manufacturers indicate that the battery should be used at 20 °C. Figure 4 shows the ambient temperature over time in this system, at one point the temperature reached 22 °C. The air conditioning system failed to maintain the temperature within an acceptable range, which resulted in shortened battery life. Also, elevated temperatures can void the battery manufacturer’s warranty.

Figure 4 Temperature monitoring

We have found that long-term monitoring of batteries using devices such as the Sentinel produced by LEM has many advantages in addition to reducing costs by making field visits by engineers more efficient. In this example, automated monitoring of battery impedance can identify batteries that are about to fail three months before they become ineffective.

Continuous monitoring also makes it easy to identify UPS configuration problems: in particular, incorrect charge/discharge frequencies that can significantly shorten battery life. Monitoring also measures environmental conditions, ensuring that battery life is not shortened due to the effects of high temperatures.

Long-term monitoring of batteries can maximize the life of the batteries, ensure that the battery pack does not need to be replaced prematurely, and ensure that deteriorating batteries are detected early so that they can be replaced before the battery pack is exhausted, thereby reducing the risk of battery failure and saving money. Although critical devices such as UPS are not usually the first target for cost savings, it is important for users to convert their devices to long-term online monitoring because this will not only cut costs but also increase the reliability of the device.