Author: Brian Ribeiro

An uninterruptible power supply must do two things exceptionally well: protect the load and minimize the energy consumed while providing that protection.

In a double-conversion UPS, incoming AC power is converted to DC and then reconstructed as regulated AC. This architecture isolates critical equipment from many utility disturbances, but every conversion stage produces some loss. In a large data center or industrial facility, even a small improvement in UPS efficiency can reduce electricity use, cooling demand and operating cost.

Silicon carbide devices and advanced insulated-gate bipolar transistors are helping UPS manufacturers reduce those losses without giving up the power quality and availability required by critical loads.

Where does UPS power loss come from?

A conventional online UPS contains several power-handling components:

• Input rectifier or converter
• DC link
• Battery interface
• Output inverter
• Static bypass
• Magnetic components
• Filters
• Cooling fans
• Control and auxiliary power supplies

Power is lost in semiconductor conduction, switching transitions, inductors, transformers, capacitors, conductors and cooling systems.

The semiconductor devices in the rectifier and inverter are particularly important because they must carry the UPS load continuously while switching thousands of times per second.

Total semiconductor loss can be divided into two broad categories.

Conduction loss

Conduction loss occurs while a semiconductor is carrying current. It depends on the device’s on-state voltage or resistance, load current, duty cycle and junction temperature.

A device with lower on-state voltage reduces the power converted to heat during each conduction interval.

Switching loss

Switching loss occurs while a device changes between its off and on states. Voltage and current overlap briefly during each transition, producing an energy loss.

Total switching loss increases with:

• Switching frequency
• Bus voltage
• Load current
• Turn-on and turn-off energy
• Diode reverse recovery
• Parasitic inductance
• Gate-drive behavior

Reducing either form of loss can improve UPS efficiency, but semiconductor design involves tradeoffs. A device optimized for very low conduction loss may not provide the lowest switching loss, and vice versa. The best device depends on the UPS topology, voltage, switching frequency and load profile.

Why UPS efficiency matters

UPS losses become heat inside the electrical room. That creates two operating costs:

1. The electricity lost in the UPS
2. The additional electricity needed to remove the heat

Consider a UPS delivering a constant 1 MW load. At 96% efficiency, its approximate input is 1.042 MW and its internal loss is about 41.7 kW. At 97.5% efficiency, the input is approximately 1.026 MW and the loss falls to about 25.6 kW.

That 1.5-percentage-point improvement reduces UPS loss by roughly 16 kW at this operating point—before accounting for the associated reduction in cooling energy.

Actual savings depend on the load profile, operating mode, redundancy configuration, electricity rate and cooling-system efficiency. This is why an efficiency curve is more useful than a single peak-efficiency number.

The role of IGBTs in modern UPS systems

The insulated-gate bipolar transistor, or IGBT, combines a voltage-controlled gate with strong high-voltage and high-current capability. IGBTs have been widely adopted in medium- and high-capacity UPS converters and inverters.

Fuji Electric’s UPS history illustrates the effect of this transition. Compared with earlier thyristor-based systems, its adoption of IGBTs for both the converter and inverter increased conversion efficiency and substantially reduced equipment size.

IGBT technology has continued to advance. Improvements include:

• Thinner semiconductor structures
• Optimized trench-gate designs
• Improved field-stop layers
• Lower on-state voltage
• Reduced switching energy
• Improved diode performance
• Higher allowable junction temperature
• Lower-inductance packaging
• Reverse-conducting and reverse-blocking functions

These developments allow UPS designers to reduce losses without simply increasing semiconductor die area.

How advanced IGBTs improve efficiency

Better conduction and switching tradeoffs

IGBT designers generally seek to reduce on-state voltage without creating an unacceptable increase in switching loss. Newer chip structures provide greater control over the carrier distribution inside the device, improving this tradeoff.

Fuji Electric’s 7th-generation X-Series IGBT technology uses thinner and optimized chip structures to reduce inverter loss. In one published comparison involving 1,200 V, 75 A devices operating at 8 kHz, the X-Series reduced inverter loss by 10% relative to the preceding V-Series under the stated test conditions.

This is a device and test-condition comparison, not a guarantee that every complete UPS will gain 10% efficiency. System performance also depends on topology, controls, magnetics, cooling and the operating point.

Improved freewheeling diode performance

The switching device does not operate alone. In many converter topologies, current commutates between the IGBT and a freewheeling diode.

Diode forward-voltage and reverse-recovery behavior affect:

• Conduction loss
• Turn-on loss
• Voltage overshoot
• Electromagnetic interference
• Device stress

An improved diode can therefore increase the benefit obtained from an advanced IGBT.

Reverse-conducting IGBTs

A reverse-conducting IGBT integrates IGBT and freewheeling-diode functions into one chip. This can increase power density and simplify chip placement inside the module.

Fuji Electric has developed X-Series RC-IGBT modules that combine 7th-generation IGBT and diode functionality, supporting higher power density and improved performance in applicable industrial power converters.

Reverse-blocking IGBTs

Most conventional IGBTs are designed primarily to block voltage in one direction. A reverse-blocking IGBT, or RB-IGBT, is designed to block voltage in both directions.

In suitable three-level converter circuits, an RB-IGBT can reduce the number of series-connected semiconductor elements in a current path. Fewer conducting junctions can mean lower conduction loss.

Fuji Electric has applied proprietary RB-IGBT technology in advanced T-type neutral-point-clamped conversion circuits used in high-efficiency UPS systems.

What makes silicon carbide different?

Conventional power semiconductors are made from silicon. Silicon carbide, or SiC, is a wide-bandgap semiconductor material with physical properties that support:

• Higher electric-field strength
• Higher-voltage operation with thinner active regions
• Faster switching
• Lower switching energy
• Lower reverse-recovery loss
• Higher-temperature capability
• Higher switching frequency

The two SiC devices most relevant to UPS designs are SiC Schottky barrier diodes and SiC MOSFETs.

SiC Schottky barrier diodes

A SiC Schottky barrier diode, or SiC-SBD, has very little reverse-recovery charge compared with a conventional silicon bipolar diode.

When a silicon diode changes from conducting to blocking, stored charge must be removed. The resulting reverse-recovery current increases loss in both the diode and the switching transistor. It can also contribute to current spikes, voltage overshoot and electromagnetic noise.

Replacing the diode with a suitable SiC-SBD can:

• Reduce diode recovery loss
• Reduce IGBT turn-on loss
• Lower current overshoot
• Reduce thermal stress
• Support higher switching frequency
• Simplify management of switching transients

A combination of silicon IGBTs and SiC diodes is often described as a hybrid SiC solution. It captures an important portion of SiC’s efficiency benefit while retaining the established high-current capability and cost structure of IGBTs.

Fuji Electric has developed 1,200 V SiC-SBD products for systems including data-center and telecommunications power supplies, targeting lower forward loss and improved surge-current capability.

SiC MOSFETs

A SiC MOSFET replaces the silicon IGBT as the actively controlled switching device. It can switch more rapidly and with lower switching loss, particularly at light and medium loads where an IGBT’s characteristic voltage drop can represent a significant penalty.

Potential UPS benefits include:

• Lower turn-on and turn-off loss
• Reduced part-load loss
• Higher switching frequency
• Smaller magnetic components
• Smaller filters
• Reduced heat-sink requirements
• Higher power density

SiC MOSFETs also conduct reverse current through their channel when controlled appropriately. Their behavior differs from both silicon MOSFETs and IGBTs, so the gate drive, dead time and protection system must be designed specifically for the device.

Fuji Electric notes that faster switching can reduce power loss and that higher switching frequencies can contribute to smaller power-conversion equipment.

Why faster switching can make a UPS smaller

Higher switching frequency moves the converter’s switching ripple to a higher frequency. This can reduce the inductance and capacitance needed to filter it, allowing smaller reactors and filter components.

A smaller magnetic component can provide several system-level benefits:

• Reduced copper loss
• Reduced core material
• Lower weight
• Smaller enclosure
• Shorter power paths
• Greater power density

However, frequency cannot be increased without limit. Higher frequency increases the number of switching events per second and may increase losses in semiconductors, magnetic cores, capacitors and conductors.

SiC expands the useful design range by reducing the energy lost in each switching transition. The optimum switching frequency remains a system-level decision.

Three-level topologies reduce semiconductor stress

A traditional two-level inverter switches each phase output between the positive and negative DC rails. A three-level topology adds an intermediate voltage state.

Depending on the circuit, this can:

• Reduce the voltage switched by an individual device
• Lower switching loss
• Reduce output-voltage steps
• Reduce filter requirements
• Improve waveform quality
• Support a higher DC bus with available device ratings

The semiconductor benefit depends on the specific topology and current path. A poorly optimized three-level circuit can add conduction devices and control complexity. Advanced devices such as RB-IGBTs can help reduce that penalty.

Fuji Electric’s advanced T-type neutral-point-clamped architecture combines RB-IGBT and SiC technology to reduce conversion loss in applicable UPS systems. Its 7400WX-T3U achieved a published efficiency of 97.4% and at least 96% efficiency at a 25% load factor under the product’s stated conditions.

Part-load efficiency is essential

UPS systems rarely operate at their nameplate rating continuously.

Common reasons include:

• N+1 or 2N redundancy
• Future capacity reserved for growth
• Variable IT or production loads
• Conservative initial sizing
• Modular systems operating with spare capacity

A system designed for redundancy may place each UPS module at 25%, 40% or 50% load during normal operation. Peak efficiency at 100% load may therefore say little about annual energy consumption.

SiC and advanced IGBT technologies can improve part-load efficiency by reducing switching, conduction and auxiliary losses. System controls can add another layer of improvement by operating the appropriate number of modules near a more efficient load point.

Fuji Electric’s 7400WX-T3U includes an optimum-load operating mode that adjusts the number or loading of operating UPS units to improve overall system efficiency.

When evaluating a UPS, request efficiency at:

• 10% load, if available
• 25% load
• 50% load
• 75% load
• 100% load

Apply those values to the facility’s expected annual load distribution.

Reduced loss lowers cooling requirements

Every kilowatt not lost in the UPS is a kilowatt that does not have to be removed as heat.

Lower semiconductor loss can reduce:

• Heat-sink size
• Cooling airflow
• Fan power
• Electrical-room cooling demand
• Local hot spots
• Component temperature
• Acoustic noise

Lower temperature can also support reliability because many power-electronic component aging mechanisms accelerate as temperature increases.

This benefit is not automatic. Higher-temperature-capable SiC devices should not be used as a reason to expose capacitors, fans, control boards and batteries to excessive heat. The entire UPS contains components with different thermal limits.

The better design objective is often to use the semiconductor’s lower loss and temperature capability to create thermal margin—not simply to operate every component hotter.

Higher power density can reduce facility footprint

When lower semiconductor loss is combined with smaller magnetics and reduced cooling requirements, a UPS can deliver more power from a smaller footprint.

This is valuable in:

• Data centers
• Semiconductor plants
• Hospitals
• Financial facilities
• Telecommunications sites
• Industrial control rooms

Floor space carries a significant opportunity cost in these facilities. A more compact UPS can leave additional space for production equipment, IT racks, batteries or future capacity.

Fuji Electric has demonstrated a SiC-based research power unit with 81 kW/L power density and 99.5% efficiency at a 37 kW rated output under the prototype’s test conditions. [6] Although this is not a complete UPS specification, it illustrates how low-loss SiC devices can support compact power-conversion hardware.

Efficiency mode and semiconductor efficiency are different

Many UPS products offer an economy, high-efficiency or bypass-based operating mode. Under acceptable utility conditions, such a mode supplies the load through a bypass path rather than continuously processing all power through the rectifier and inverter.

This can produce efficiency higher than normal double-conversion operation. For example, Fuji Electric’s UPS7500WX lists 98.5% efficiency in its HE mode.

It is important to distinguish:

• Efficiency gained from lower-loss semiconductors in the active conversion path
• Efficiency gained by bypassing some or all of that conversion path

The operating modes provide different levels of power conditioning and may have different transfer behavior. The appropriate mode depends on:

• Load sensitivity
• Utility power quality
• Transfer-time tolerance
• Harmonic and power-factor requirements
• Facility risk policy
• Applicable standards

Semiconductor improvements increase the efficiency of online operation, helping reduce the energy penalty when the application requires continuous power conversion.

Efficiency improvements can support reliability

Lower internal loss may reduce thermal stress on:

• Power semiconductor junctions
• DC-link capacitors
• Magnetic components
• Busbars
• Cooling fans
• Nearby control electronics

Reduced fan demand may also extend fan life or allow redundant cooling arrangements to operate with greater margin.

However, efficiency and reliability must be validated together. Faster switching can create:

• Higher (dv/dt)
• Higher (di/dt)
• Voltage overshoot
• Common-mode current
• Electromagnetic interference
• Insulation stress
• More demanding gate-drive requirements

Successful SiC application depends on low-inductance packaging, compact busbars, suitable gate drivers, effective protection and careful EMC design. A fast semiconductor installed in a layout developed for slower devices may not deliver the expected reliability or efficiency.

Packaging is part of the efficiency equation

A semiconductor chip cannot perform independently of its package.

Package features affect:

• Electrical resistance
• Parasitic inductance
• Switching overshoot
• Heat transfer
• Current sharing
• Maximum junction temperature
• Power-cycling capability

Lower-inductance packages allow faster switching with less voltage overshoot. Improved insulating substrates and thermal interfaces move heat away from the semiconductor more effectively.

Fuji Electric’s 7th-generation X-Series power modules combine optimized chips with lower-inductance packaging and improved heat dissipation.

This coordinated chip-and-package approach is essential because reducing device switching loss is of limited value if parasitic inductance forces the designer to slow the device substantially.

Choosing between SiC and advanced IGBT solutions

SiC is not automatically the most economical option for every UPS.

Advanced IGBTs may remain attractive when:

• Power levels are high
• Switching frequency is moderate
• Conduction loss dominates
• Cost sensitivity is significant
• Proven silicon platforms meet the efficiency target
• Existing manufacturing and service infrastructure is important

Hybrid SiC solutions may be effective when:

• Diode reverse recovery is a major loss mechanism
• Improved switching performance is needed
• A full SiC design is not required
• The designer wants to balance performance and cost

All-SiC solutions may be attractive when:

• Very low switching loss is important
• Part-load efficiency is a priority
• High switching frequency enables smaller magnetics
• Power density is valuable
• Cooling capacity or footprint is constrained

The comparison should be made at the system level. Semiconductor cost should be weighed against savings in magnetics, heat sinks, fans, enclosure volume, cooling infrastructure and lifetime electricity use.

How to compare high-efficiency UPS systems

When evaluating a UPS that uses SiC or advanced IGBT technology, ask for:

1. Efficiency in normal online mode
2. Efficiency at 25%, 50%, 75% and 100% load
3. Efficiency in optional economy or high-efficiency modes
4. Whether auxiliary and fan power are included
5. Test voltage, power factor and environmental conditions
6. Heat rejection at representative loads
7. Input power factor and harmonic performance
8. Output waveform performance
9. Overload and short-circuit capability
10. Semiconductor topology
11. Redundancy and module-loading strategy
12. Transfer behavior between operating modes
13. Cooling-system redundancy
14. Expected capacitor and fan service life
15. Applicable efficiency-test standards

Avoid comparing values measured under different modes or test conditions. A bypass-mode efficiency from one product should not be compared directly with the online double-conversion efficiency of another.

Calculate annual loss—not just nominal efficiency

The most useful metric is the annual energy lost under the expected operating profile.

For each load interval:

[
P_{\text{loss}} = P_{\text{out}}\left(\frac{1}{\eta}-1\right)
]

Annual loss energy can then be estimated by multiplying the loss at each operating point by the number of hours spent there.

Include:

• Normal load variation
• Redundancy configuration
• Planned future growth
• Battery charging
• Economy-mode operating hours
• Cooling energy
• Module staging
• Maintenance bypass periods

This lifecycle analysis may show that a product with a small peak-efficiency advantage offers less annual benefit than one with a flatter part-load curve.

Power semiconductors and UPS design are evolving together

The efficiency of a UPS is not determined by one device alone. It results from the interaction of semiconductor technology, circuit topology, gate control, packaging, magnetics, cooling and system operation.

Advanced IGBTs continue to improve the balance between conduction loss, switching loss, power density and cost. SiC-SBDs reduce reverse-recovery loss, while SiC MOSFETs enable faster switching and smaller passive components. Three-level circuits and intelligent module controls turn those device improvements into system-level savings.

Fuji Electric develops both UPS systems and the power semiconductors used in power-conversion equipment. Its portfolio includes advanced IGBT, hybrid SiC and SiC technologies intended for high-efficiency converters and uninterruptible power supplies. 

By evaluating performance across the actual load profile—and separating online conversion efficiency from bypass-mode efficiency—facility designers can choose a UPS that protects critical loads while reducing energy use, cooling demand and total operating cost.

A commercial uninterruptible power supply is only as dependable as its stored-energy system. When utility power fails, the UPS battery must immediately support the inverter and keep the critical load operating until power returns, a generator starts, or equipment can shut down safely.

In medium and large UPS installations, that energy is commonly supplied by one or more battery strings. The way those strings are configured affects DC-bus voltage, runtime, fault current, maintenance flexibility, battery life, and system availability.

Getting the configuration right requires more than connecting enough batteries to achieve a desired voltage. The batteries, conductors, protective devices, charger, monitoring system, and UPS must be designed as one coordinated system.

What Is a Battery String?

A battery string is a group of battery cells or monoblocs connected in series.

In a series connection, the positive terminal of one battery is connected to the negative terminal of the next. The individual battery voltages add together to create the DC voltage required by the UPS.

For example, connecting forty 12-volt batteries in series creates a nominal string voltage of:

40 batteries × 12 volts = 480 VDC nominal

The actual string voltage will vary with battery chemistry, state of charge, charging mode, temperature, load, and battery condition.

The string’s ampere-hour capacity does not add in a series connection. If each battery is rated at 100 Ah, the complete series string is nominally a 480-volt, 100-Ah battery string, not a 4,000-Ah string.

Commercial UPS systems may use long strings operating at several hundred volts DC. Vertiv documentation, for example, describes UPS strings made from series-connected batteries and warns that complete string voltage can reach potentially lethal levels. Installation and maintenance must therefore be performed by qualified personnel using the applicable electrical-safety procedures. 

Series Determines Voltage

The number of cells or battery blocks in series is selected to match the DC input range of the UPS.

That number is not arbitrary. The UPS manufacturer establishes limits for:

• Nominal battery voltage
• Minimum discharge voltage
• Maximum charging voltage
• Permitted number of cells or blocks
• Battery chemistry
• Charger capacity
• Battery-test functions
• DC overcurrent protection
• Runtime configuration

Too few batteries may allow the DC voltage to fall below the inverter’s minimum operating threshold before the batteries have delivered their intended capacity.

Too many batteries may exceed the UPS, charger, capacitor, switching-device, or insulation rating, especially during charging, when string voltage is higher than nominal.

The correct series count must therefore come from the approved UPS and battery documentation. It should not be changed merely to obtain a different runtime.

Parallel Strings Determine Capacity and Runtime

When one series string cannot provide the required runtime or current, complete strings may be connected in parallel.

In a parallel arrangement:

• Each string has the same nominal voltage.
• The available ampere-hour capacity increases.
• Current is shared among the strings.
• Potential runtime increases.
• Available DC fault current also increases.

Two identical 480-volt, 100-Ah strings connected in parallel form a nominal 480-volt, 200-Ah battery bank. The voltage remains 480 volts because the strings are paralleled, while the capacity increases.

This relationship can be summarized simply:

• Batteries in series increase voltage.
• Complete strings in parallel increase capacity and current capability.

Actual runtime will not necessarily double when the number of strings doubles. Battery discharge rate, age, temperature, UPS efficiency, cell cutoff voltage, load level, and conductor losses all influence usable runtime.

Build Every Parallel String the Same Way

Parallel strings should be electrically equivalent.

Each string should normally have the same:

• Number of cells or monoblocs
• Battery manufacturer and model
• Chemistry
• Nominal capacity
• Age and service history
• Temperature exposure
• Connection method
• Conductor size
• Approximate cable length
• Protective-device arrangement

If one string has lower resistance, it may supply more discharge current and accept more charging current than the others. That string can run hotter and age faster, increasing the imbalance further.

Equal-length, equal-size cabling helps keep the resistance of the parallel branches similar. EnerSys specifically recommends using the same cable size and approximately the same cable length for each parallel string so that string resistance remains comparable. EnerSys UPS Battery Quick Start Guide

A balanced bus arrangement is also important. Connecting the UPS positive and negative leads in a way that favors the physically closest string can create unequal current paths. Commercial battery systems commonly use a properly designed common bus or coordinated connection geometry to achieve better sharing.

Do Not Mix Unlike Batteries

Installing new batteries into an aged string may look economical, but it can create an electrically mismatched system.

An older battery may have:

• Lower capacity
• Higher internal resistance
• Greater self-discharge
• Different charge acceptance
• Different terminal voltage under load

In a series string, every battery carries the same current. A weak battery can therefore reach its discharge limit before the rest of the string, restricting the capacity of the entire string.

During charging, voltage can also divide unevenly. A deteriorated unit may become overcharged or undercharged even when total string voltage appears normal.

Mixing different capacities, models, chemistries, or ages can make these problems worse. Unless the battery and UPS manufacturers provide a specific approved procedure, replacement should generally be performed at the complete-string or complete-bank level rather than by casually mixing dissimilar units.

Why the Weakest Battery Matters

Series-connected batteries carry the same current, but they do not necessarily have the same capacity or terminal voltage.

Suppose one battery in a long string has lost substantial capacity. During a discharge, that battery may reach a dangerously low voltage before the others. The UPS sees only total string voltage unless battery-level monitoring is installed, so the problem may remain hidden until runtime is needed.

A weak unit can cause:

• Reduced backup time
• Premature low-battery shutdown
• Excessive voltage imbalance
• Localized heating
• Reversal of a severely depleted cell
• Failed battery tests
• Unexpected loss of the entire string

This is why total string voltage alone is not a sufficient measure of battery health.

Protect Each Parallel String Individually

When several strings share a DC bus, each string should have appropriately engineered isolation and overcurrent protection.

Depending on the UPS design, this may include a:

• Battery circuit breaker
• Fused disconnect
• Battery disconnect cabinet
• Manufacturer-designed internal breaker
• Contactor coordinated with the UPS protection system

Individual protection serves several purposes:

• Isolates a faulted string
• Protects string conductors
• Allows maintenance on one string
• Prevents healthy strings from feeding excessive current into a failed parallel branch
• Provides a visible or controlled means of disconnection

Vertiv guidance states that parallel strings should be equipped with disconnecting means so maintenance on one string does not interfere with the others. It also recommends locating the battery circuit breaker close to the battery terminals and minimizing connection distance. Vertiv Liebert EXM UPS user manual

Protection must be rated for DC operation at the maximum possible system voltage and fault current. An AC-only breaker is not automatically suitable because interrupting DC presents different arc-extinguishing requirements.

Protective-device sizing and coordination must follow the UPS manufacturer’s instructions, the battery manufacturer’s limits, the engineered design, and applicable codes.

More Parallel Strings Are Not Always Better

Adding strings can increase runtime, but it also changes the electrical system.

Potential consequences include:

• Greater prospective fault current
• More complex protection coordination
• Increased charger demand
• Longer recharge time
• More connections that can loosen or corrode
• Greater monitoring and maintenance burden
• More difficult current sharing
• Increased floor-space and structural requirements

The UPS may also impose a maximum supported number of strings or external battery modules. Eaton, for example, publishes product-specific limits and requires equal external-battery-module quantities for UPS units in certain parallel arrangements to maintain equivalent runtimes. Eaton 9155 Parallel UPS user guide

The permissible number of strings must be verified for the exact UPS model rather than estimated from charger voltage alone.

Charger Capacity Must Match the Battery Bank

A larger battery bank requires more energy to recharge.

If parallel strings are added without considering charger capacity, the UPS may still operate but require an unacceptably long time to restore full charge after a discharge. This creates a period in which another outage could occur before the system has recovered its intended runtime.

The charger must also maintain the batteries at the correct voltage for their chemistry and temperature. Excessive charge voltage can accelerate corrosion, water loss, gas generation, or thermal problems. Insufficient voltage can leave the battery undercharged and promote capacity loss.

Configuration should account for:

• Battery chemistry
• Total bank capacity
• Desired recharge time
• Maximum charger current
• UPS auxiliary loads
• Generator compatibility
• Temperature compensation
• Manufacturer charging limits

Recharge performance should be evaluated following the design discharge, not just after a brief self-test.

Runtime Is Not Simply Ampere-Hours Divided by Amps

A rough energy estimate can be useful:

Nominal energy = nominal string voltage × ampere-hour capacity

However, this does not equal the energy available to the load.

Usable runtime is affected by:

• Discharge rate
• Battery terminal-voltage curve
• UPS low-voltage cutoff
• Battery age
• Temperature
• Inverter efficiency
• DC-cable voltage drop
• Battery imbalance
• Required end-of-life margin
• Actual critical load

Lead-acid capacity is particularly dependent on discharge rate. A battery rated over a long discharge period may deliver less than its nominal ampere-hour rating during a high-rate UPS discharge.

The correct approach is to use manufacturer discharge data or approved sizing software at the required load, end voltage, temperature, aging factor, and runtime.

Configure for End-of-Life Performance

A new battery bank should normally provide more than the bare minimum required runtime. Batteries lose capacity as they age, so a system sized to meet the requirement only on its first day may fall short well before its planned replacement date.

A sound design considers:

• Required runtime at end of life
• Battery aging factor
• Lowest expected battery-room temperature
• Future load growth
• UPS conversion losses
• Design margin
• Generator start and stabilization time
• Shutdown requirements if generation fails

Excessive oversizing has drawbacks, including cost, footprint, fault current, and recharge time. The goal is a justified margin, not unlimited capacity.

Temperature Strongly Affects Battery Performance

Battery-room temperature influences both available capacity and service life.

Low temperature generally reduces immediate discharge capacity. High temperature may temporarily improve available capacity but accelerates aging and can shorten service life significantly.

Strings connected in parallel should experience similar temperatures. A string near an air-conditioning outlet and another beside a warm UPS cabinet may age differently even if they were installed at the same time.

Good thermal design includes:

• Uniform airflow
• Avoidance of local hot spots
• Temperature monitoring
• Suitable ventilation
• Separation from heat-producing equipment where practical
• Charger temperature compensation when specified
• Alarm thresholds for abnormal conditions

Temperature sensors should be positioned according to the manufacturer’s guidance rather than simply placed at the coolest point in the room.

VRLA, Flooded Lead-Acid, and Lithium-Ion Strings

The meaning of a string remains similar across battery chemistries, but configuration and protection requirements differ.

Valve-regulated lead-acid

VRLA batteries are common in commercial UPS systems because they are compact and require less routine electrolyte maintenance than flooded cells. They remain sensitive to temperature, charging conditions, connection quality, and aging.

Flooded lead-acid

Flooded batteries may offer long service life and detailed cell-level inspection opportunities, but they require appropriate rooms, ventilation, spill management, maintenance access, and electrolyte procedures.

Lithium-ion

Lithium-ion UPS systems typically use modules combined into controlled strings with a battery-management system. The BMS monitors conditions such as cell voltage, temperature, current, and contactor state.

Lithium-ion modules should not be treated as interchangeable substitutes for lead-acid blocks. Charger behavior, communication, protection, fault response, and UPS compatibility must be specifically approved.

Battery Monitoring Should Go Below the String Level

Useful battery monitoring may include:

• Total string voltage
• Individual cell or monobloc voltage
• String current
• Float current
• Internal resistance or conductance trends
• Battery temperature
• Ambient temperature
• Ground-fault or insulation status
• Breaker position
• Connection resistance
• State of charge
• Discharge history

The most useful diagnostic information often comes from comparison and trending. One battery that gradually diverges from the rest may be more significant than an isolated reading that remains within a broad limit.

Monitoring does not replace inspection, testing, or maintenance. It helps identify where closer investigation is needed.

Common Configuration Mistakes

Incorrect number of batteries in series

This can place the DC bus outside the UPS charging or operating range.

Unequal parallel strings

Different battery models, capacities, ages, or conductor resistance can produce uneven current sharing.

No individual string isolation

A faulted string may be fed by the remaining strings, and maintenance may require taking the entire battery bank out of service.

Adding strings without checking the charger

Runtime may increase, but recharge time may become unacceptable.

Sizing from nominal ampere-hours alone

This overlooks discharge rate, cutoff voltage, temperature, aging, and UPS efficiency.

Assuming equal voltage means equal health

Parallel strings can show the same terminal voltage while having very different capacity or internal resistance.

Replacing only visibly failed units

The remaining batteries may be near the same wear-out condition, leading to repeated failures and continued imbalance.

Ignoring cable resistance

Unequal cable lengths, conductor sizes, or termination quality can cause unequal loading.

Mixing battery chemistries

Different chemistries require different charging, monitoring, and protective behavior.

Closing a battery breaker without checking voltage and polarity

A voltage mismatch or reversed connection can produce extreme current and catastrophic damage.

Commissioning a Commercial UPS Battery Bank

A formal commissioning process should verify both the design and the installation.

Typical checks include:

1. Confirm the approved battery model, chemistry, quantity, and series count.
2. Verify that every parallel string has the same configuration.
3. Inspect batteries for shipping or installation damage.
4. Confirm polarity at the battery, string, cabinet, and UPS connections.
5. Measure individual battery and complete-string voltages.
6. Compare open-circuit voltage spread before final connection.
7. Verify cable size, routing, identification, and approximate branch equality.
8. Confirm terminal torque using the battery manufacturer’s specified values.
9. Check barriers, terminal covers, grounding, ventilation, and working clearances.
10. Verify each string’s protective device and DC interrupting rating.
11. Confirm UPS battery settings, capacity, cell count, and charging parameters.
12. Check monitoring and temperature sensors.
13. Verify breaker-status and alarm signals.
14. Perform manufacturer-approved functional and runtime testing.
15. Record baseline voltage, temperature, resistance, and conductance data.

The EnerSys commissioning guidance emphasizes checking voltage spread, using clean contact surfaces, applying specified terminal torque, confirming string voltage, and establishing the correct charge conditions before service. EnerSys UPS Battery Quick Start Guide

Safe Maintenance Requires More Than Opening the UPS Input

Turning off the AC supply does not necessarily remove the battery hazard. The battery bank remains an energized DC source capable of delivering very high current.

Even when a battery breaker is open, hazardous voltage may remain inside individual strings or across sections of the bank.

Maintenance planning should address:

• Lockout and isolation
• Verification of voltage
• DC-rated test equipment
• Insulated tools
• Appropriate personal protective equipment
• Removal of conductive jewelry
• Protection against accidental short circuits
• Correct lifting and handling
• Chemical and fire hazards
• Manufacturer-prescribed procedures
• Qualified-person requirements

Battery strings must never be treated as safe solely because the UPS display is off.

A Practical Configuration Example

Assume a UPS requires a nominal 480 VDC battery input and the approved design uses 12-volt, 100-Ah battery blocks.

One string would contain:

480 V ÷ 12 V per battery = 40 batteries in series

That creates one nominal:

480 V, 100 Ah string

If the approved runtime calculation requires 300 Ah, the design might use three identical strings in parallel:

3 strings × 100 Ah = 300 Ah nominal bank capacity

The resulting bank would contain:

• 40 batteries per string
• 3 parallel strings
• 120 batteries total
• 480 VDC nominal bank voltage
• 300 Ah nominal capacity

Each string would require coordinated conductors and suitable isolation and overcurrent protection. The charger, UPS configuration, bus, cabinet, monitoring, ventilation, and fault-current rating would all need to support the three-string arrangement.

This example illustrates the electrical relationships; it is not a substitute for manufacturer-approved sizing.

The Bottom Line

A battery string is a series-connected group of batteries that supplies the DC voltage required by a commercial UPS. Connecting complete, identical strings in parallel increases capacity and potential runtime without increasing nominal bank voltage.

A reliable configuration follows several core principles:

• Use the UPS manufacturer’s approved battery type and series count.
• Keep parallel strings electrically and thermally equivalent.
• Avoid mixing different models, capacities, ages, or chemistries.
• Give each string suitable DC-rated protection and isolation.
• Balance conductor resistance among parallel branches.
• Verify charger capacity and recharge time.
• Size runtime using real discharge data and end-of-life conditions.
• Monitor individual batteries as well as the complete string.
• Treat the battery bank as an energized high-voltage source at all times.

The best battery-bank design is not simply the arrangement with the most batteries. It is the configuration that delivers the required end-of-life runtime, shares current predictably, isolates faults safely, can be maintained without unnecessary risk, and remains fully compatible with the UPS that depends on it.