More Than Runtime: Battery Sizing for UPS Systems

The Mistakes That Compromise Critical Power Reliability

By Walter Soares, LATAM Sales Manager and Critical Power Solutions Specialist – C&D Technologies

When discussing UPS systems, battery bank sizing is often treated as nothing more than a mathematical exercise: define the required runtime, calculate the discharge current, and select the necessary battery capacity. While these are fundamental steps, experience shows that designing a reliable system requires much more than simply following a formula.

Over the years, I have observed that many failures in critical power systems are not related to battery quality itself, but rather to the assumptions made during the design, installation, and maintenance of the battery bank. This is precisely where engineers, consultants, and facility operators have the opportunity to make a meaningful difference.

During a recent webinar hosted by C&D Technologies on the application of IEEE 485, we received dozens of questions from professionals working in data centers, financial institutions, industrial facilities, and other mission-critical environments. One aspect stood out to me: most of the questions were not about the sizing calculations themselves, but about the batteries' actual operating conditions.

That reflects an important shift in our industry.

The Biggest Mistake Is Assuming Every Battery Operates Under Ideal Conditions

The battery life specified by the manufacturer—commonly referred to as Design Life—represents an expected service life under carefully controlled operating conditions.

In reality, however, very few battery banks operate under those ideal conditions.

High temperatures, frequent discharges, inadequate charging practices, imbalances between parallel strings, and insufficient maintenance often result in a Service Life that is significantly shorter than the battery's design life.

In other words, two installations using exactly the same battery can deliver completely different levels of performance simply because they operate under different environmental and operating conditions.

That is why battery sizing should consider not only the runtime required by the UPS, but also the expected operating profile of the application.

Temperature Remains the Most Overlooked Factor

Among all the questions we received, few were as revealing as those related to temperature.

It is still common to find installations where the battery bank shares the same environment as the UPS without a proper evaluation of the battery's actual operating temperature.

Although this may seem like a minor detail, its impact is substantial.

As a general rule for VRLA batteries, an average increase of approximately 10°C (18°F) above the reference operating temperature can reduce battery service life by nearly half.

At the same time, low temperatures reduce the battery's available discharge capacity and may require a larger battery bank than originally anticipated.

Even more importantly, relying on generic correction factors is not always the best approach. Whenever possible, engineers should use the battery manufacturer's performance curves and temperature correction factors for the most accurate sizing results.

Capacity Testing Remains the Most Reliable Assessment Tool

Another frequently discussed topic is capacity testing.

There is a natural reluctance to perform discharge tests on batteries approaching the end of their service life, particularly in mission-critical applications such as data centers.

That concern is understandable. A capacity test can place additional stress on an already degraded battery bank.

On the other hand, it remains the only method capable of confirming the battery's actual available capacity.

For this reason, rather than following a fixed testing schedule, organizations should adopt a risk-based approach. As batteries age, visual inspections, internal resistance measurements, conductance testing, temperature monitoring, and operational history become increasingly important when deciding whether to perform another capacity test or proactively replace the battery bank.

Maintenance Should Be Trend-Based, Not Event-Based

Another practice that deserves reconsideration is reactive maintenance.

Many organizations still perform battery maintenance only after an alarm is triggered or a failure occurs. In practice, this reactive approach increases costs while reducing the availability of critical systems.

Today's best practices point in a different direction: monitoring trends rather than reacting to events.

Gradual increases in internal resistance, voltage imbalances between battery blocks, rising float current, and small temperature deviations typically appear long before an actual failure occurs.

By recording and analyzing these trends over time, organizations can plan battery replacements proactively, minimize unexpected downtime, and significantly reduce operational risk.

Reliability Begins During System Design

It is also important to remember that many issues attributed to batteries actually originate during the system design phase.

Undersized cables, unequal cable lengths between parallel strings, poor electrical connections, improperly specified protective devices, and incomplete maintenance documentation can all compromise system performance throughout its service life.

Proper battery sizing means evaluating the system as a whole.

The battery is only one of several components responsible for maintaining the availability of mission-critical infrastructure.

The Role of Industry Standards

Industry standards such as IEEE 485, IEEE 1187, and IEEE 1188 continue to serve as essential references for engineers and system designers.

Beyond providing calculation methodologies, these standards establish best practices for installation, inspection, testing, and maintenance.

However, no standard can replace a thorough evaluation of actual operating conditions or the manufacturer's specific recommendations. Every application has unique characteristics and should be evaluated accordingly.

Conclusion

As data centers, healthcare facilities, telecommunications networks, and industrial operations become increasingly dependent on uninterrupted power availability, batteries are no longer viewed simply as a source of backup runtime during an outage.

They have become a strategic asset for business continuity.

Properly sizing a battery bank is about much more than providing a few minutes of backup power. It means understanding how the system will operate over the coming years, how it will be maintained, and which factors may compromise its performance long before it reaches its expected end of life.

The most reliable critical power systems begin with accurate engineering calculations, but they are sustained only when engineering, operations, and maintenance work together as an integrated team.

That is the mindset that will enable our industry to move beyond reactive maintenance toward a truly predictive approach to managing critical power infrastructure.