When it comes to electrical systems in industrial and commercial facilities and power plants, total system reliability is the top priority. An approach to electrical system service that encompasses both ac and dc power systems is required.
AC and dc systems play an interconnected and equally critical role in ensuring overall reliability, and daily production and equipment protection require seamless interaction between the two systems. DC power enables the ac uninterruptible power supply (UPS) to bridge short power interruptions that could disrupt production, while dc-powered protective relays provide critical protection to transformers, turbo generators, motors, and other ac systems to prevent costly damage and downtime.
While both ac and dc systems are critical to continuous operation, the dc power system is more vulnerable to failure because it is comprised of batteries that have a shorter lifespan and age more unpredictably than ac system components. Failure of a single battery cell can cause systems supported by that battery string to fail.
As a result, batteries represent the weak link in the power system chain. Strengthening this weak link through a well-designed and disciplined approach to battery maintenance is one of the most cost-effective measures that can be taken to ensure reliability and prevent costly downtime in industrial and commercial facilities and power plants.
Design Life Versus Service Life
It’s well known that batteries are subject to wear and faster aging than other electrical system components. Still, misunderstanding battery life expectancy is common and stems from confusing battery design life with battery service life.
Battery design life, which is specified by the manufacturer, takes into account cell design and battery aging under controlled conditions in the manufacturer’s laboratory. This rarely, if ever, matches real-world operating conditions.
Battery service life considers how application, installation design, changing operating conditions, and maintenance practices impact battery aging. Service life is almost always shorter than design life — often by a significant amount. How batteries are handled, the environment in which they operate, and inadequate maintenance and replacement practices can all lead to premature aging and failure.
Proper battery maintenance to prevent failures and extend service life can reduce the number of replacements. A regular, effective maintenance program includes:
- Visual and mechanical inspection
- Testing resistance
- Recording float and supply voltage measurements
- Electrolyte inspection
- Recording gravity readings
- Testing battery bank capacity
- Proactive replacement
- Checking charger and rack integrity
- Monitoring
Inspection and Testing
For industrial and commercial applications, battery manufacturers typically cite adherence to standards from the Institute of Electrical and Electronics Engineers (IEEE) as a requirement to maintain a valid product warranty. Similarly, power plants are expected to meet requirements from the North American Electric Reliability Corporation (NERC). These standards serve as the foundation for consistent preventive battery maintenance.
IEEE standards, which vary based on battery type, provide recommended practices for maintenance, testing, and replacement of batteries for stationary applications. They specify the frequency and type of inspection or measurements that must be made and recorded to validate battery condition.
Vented lead-acid (VLA) and valve-regulated lead-acid (VRLA) batteries require monthly, quarterly, and annual inspections. The standards specify the measurements to be taken at each interval, such as string/cell voltage and battery float charging current. Capacity testing requirements for VLA and VRLA batteries are very similar. They should be tested at installation, during periodic intervals (no longer than 25 percent of the expected service life), and annually when the battery shows signs of degradation or has reached 85 percent of the expected service life. The IEEE standard also suggests testing a VRLA battery when internal ohmic values have changed significantly between readings or physical changes have occurred. Degradation is indicated when battery capacity drops more than 10 percent from its capacity on the previous test or is below 90 percent of the manufacturer’s rating.
A battery kept in service beyond expected service life is at increased risk for failure. Performing regular inspections and testing allows potential issues such as high resistance or corrosive inter-cell connections to be identified and corrected before they impact battery life. In general, the more frequent the inspection or testing intervals, the better the program. Gathering data on a more regular basis supports the ability to trend battery performance, which can be a valuable tool in predicting failure and extending battery service life.
Continuous Monitoring
While following IEEE recommendations is the best practice and is required to maintain warranty protection, everyday pressures and resource limitations can make it difficult to accomplish this on a consistent basis. It’s easy to put off battery inspections because the problems created by missed inspections aren’t immediate. However, problems inevitably arise that extend beyond the battery. For example, one company relied on their UPS to operate an emergency relief valve, but when the supporting dc power system failed, the valve did not close; this caused a dangerous chemical release. In another case, failure of dc system batteries supporting a call center knocked out some emergency services including the ability to field 911 calls. Timely inspection — or better yet, remote monitoring — would likely have discovered the pending battery failure in each of these cases.
Remote monitoring is a cost-effective solution that can reduce the need for manual battery inspection and help ensure ongoing battery health. Instead of waiting for an inevitable failure or replacing a battery prematurely to prevent problems, a monitoring system provides continuous visibility into cell voltage, internal resistance, cycle history, overall string voltage, current, and temperature.
Monitoring systems provide an accurate picture of battery health without discharging batteries by measuring the internal resistance of all cells in the battery string. As a battery ages and loses capacity, cell resistance increases. In an aged string, a substantial increase in the resistance of one cell should be considered end of life for the battery string.
In addition to identifying batteries approaching failure, the information gained from battery monitoring can help maximize battery life. VRLA batteries are sensitive to temperature and float voltage settings. A battery monitor can provide ambient temperature, cell voltage, internal resistance, and data logging, allowing these conditions to be better controlled to minimize degradation.
Remote Service
A battery monitoring system offers an additional benefit: It enables remote service, removing the burden of battery maintenance from in-house staff and expanding the value of the monitoring system.
Remote services organizations provide the dedicated resources and specialized expertise to perform real-time diagnosis of potential problems and nearly instant notification when a problem occurs.
When performance data falls outside established parameters, the system transmits an alert to remote power system engineers and/or to product experts who can analyze the data to assess the situation. For example, they could recognize that an emergency generator has started and is providing power, but at the wrong time and for the wrong reason. The data would identify the issue and also pinpoint the probable cause. In turn, this would generate a work order to inspect, repair, or replace the part that caused the alert and/or notify key personnel. In either case, problems with the potential to result in lost production or damage are avoided.
A dedicated service organization can also often identify larger infrastructure issues based on battery type or application. Because they have access to data from multiple locations, potential problems caused by a specific part or component can be spotted and maintenance activities can be tailored based on the specific performance of certain batteries across a wide range of sites.
Battery Replacement
IEEE standards recommend replacing a battery if its capacity is below 80 percent of the manufacturer’s rating. The replacement should be made as soon as these conditions are observed during testing or when identified by the monitoring system.
Depending on in-house skill level, it may be wise for specialists to perform replacement to avoid any problem. If a battery is connected improperly following replacement, it will affect the entire string. In addition, physical characteristics such as plate condition or abnormally high cell temperatures should be observed. These conditions can guide decisions on whether to replace a complete string or an individual cell.
The baseline resistance (Figure 1) of a new battery or string should be recorded. While these measurements are commonly made immediately following installation, research shows that delaying baseline measurement provides a more accurate measurement. After installation, resistance tends to drop and then level off, and a baseline measurement taken too soon can be off by as much as 25 percent. Therefore, on average, baseline values should be established after a battery has been in service between six and nine months. Comparing the changing features of battery data against an accurate baseline helps identify performance patterns for better end-of-life forecasting.
Figure 1: Baseline Resistance for Battery Change-Out
Mobile DC Power Unit
Performing battery maintenance can pose a risk to operations. If the utility source drops off line or suffers a power dip while batteries are being serviced, protection and other business-critical systems may fail to operate as intended, or the entire load may be dropped, with costly and potentially disastrous results.
In critical facilities such as chemical plants, refineries, and power plants, disabling the battery system to perform required inspections, tests, and component replacements is not an option. Access to a temporary power solution is necessary in these situations. Mobile power systems provide dependable, temporary power with the added convenience of timely on-site services. By working with a service provider with a mobile dc power option, a facility can support and back up critical loads, ensuring no business interruption during battery maintenance or replacement.
Integrating DC Power into a Comprehensive Maintenance Strategy
As more and more emphasis is placed on availability and reliability of critical power systems, the importance of protection and backup systems — and the batteries they depend on — cannot be overemphasized. Integrating dc power maintenance into a total systems approach is essential to maximizing uptime for industrial and commercial facilities and power plants. The first step to ensure availability is to implement a testing and maintenance program that is compliant with IEEE and/or NERC guidelines. In most cases, remote battery monitoring proves to be the most cost-effective approach to ensure the necessary visibility to identify potential failure conditions and prolong battery life.
Conclusion
Regular maintenance and monitoring enable the replacement of batteries and other components based on performance trending instead of by age. Using a mobile dc power solution during system maintenance helps ensure the backup power system continues to function during maintenance and testing.
Brandon Schuler joined Vertiv’s Electrical Reliability Services team in 2014 and has 20 years of experience in performing and managing electrical/electronics testing and maintenance. Brandon served for eight years in the United States Air Force, trained in advanced electronics and electrical principles, earned several factory-level certifications, and supported several units as a military technician at multiple duty stations. Additionally, he earned an associate’s degree in electronics engineering technology from Community College, Airforce. As an expert electrical technician and an experienced team leader, Brandon helps customers get the most out of the electrical assets that support their critical processes and systems.