Batteries are one of the most important components of an Uninterruptible Power Supply (UPS) because they provide backup power whenever the main electrical supply fails. The performance of a UPS largely depends on the type, capacity, condition, and maintenance of its batteries. A battery stores electrical energy during normal operation and supplies that energy to the UPS inverter during a power outage, allowing connected equipment to continue operating.
There are three main types of batteries commonly used in UPS systems:
- Valve-Regulated Lead-Acid (VRLA) Batteries
- Flooded Cell or Vented Lead-Acid (VLA) Batteries
- Lithium-Ion Batteries
Each battery type offers different advantages in terms of maintenance, lifespan, cost, and performance.
Types of UPS Batteries
| Battery Type | Description |
|---|---|
| Valve-Regulated Lead-Acid (VRLA) | A sealed lead-acid battery that requires very little maintenance and is widely used in commercial UPS systems. |
| Flooded Cell (VLA) | A vented lead-acid battery that contains liquid electrolyte and requires regular maintenance, such as checking electrolyte levels. |
| Lithium-Ion | A modern battery technology known for its longer lifespan, higher efficiency, lower weight, and faster charging compared to lead-acid batteries. |
UPS Battery Runtime
The backup time provided by a battery-powered UPS depends on several factors rather than battery capacity alone. Important factors include:
- The type of battery used.
- The size and total capacity of the battery bank.
- The rate at which the battery is discharged.
- The efficiency of the UPS inverter.
For lead-acid batteries, the available battery capacity changes depending on how quickly the battery is discharged. This relationship is described by Peukert’s Law, which explains that higher discharge rates reduce the effective capacity of the battery.
UPS manufacturers usually specify battery backup time in minutes for packaged UPS systems. However, in large installations such as data centers, accurately estimating backup time requires detailed calculations involving the electrical load, battery characteristics, and inverter efficiency to ensure the required runtime is achieved.
Common Battery Characteristics and Load Testing
When a lead-acid battery is charged or discharged, the chemical reaction initially occurs only at the surface where the battery electrodes come into contact with the electrolyte. This temporary stored energy is often referred to as interface charge.
Over time, the charged chemicals gradually spread throughout the active material of the battery by a process known as diffusion. As this process continues, the stored energy becomes evenly distributed throughout the battery.
For example, if a battery has been completely discharged, such as when a car’s headlights remain switched on overnight, and is then charged for only a few minutes, the battery develops charge mainly at the electrode surface. Although the battery voltage quickly rises close to the charger voltage, the charge remains concentrated only near the interface. After several hours, this surface charge may not spread sufficiently throughout the battery, resulting in inadequate stored energy even though the battery appears to be fully charged. In such a situation, the battery may still be unable to start the vehicle.
UPS Battery Self-Testing
Many UPS systems perform automatic self-tests that last only a few seconds. These short tests primarily verify whether the battery is connected and functioning but may not accurately determine the actual backup capacity of the battery.
Because of the temporary interface charge, a battery may appear healthy during a short self-test while still being unable to provide its expected runtime during an actual power failure.
To determine the true battery capacity, a recalibration or rundown test is often required. During this test, the battery is discharged much more deeply so that its real backup capability can be measured accurately.
Effects of Deep Discharge Testing
Although deep discharge testing provides a more accurate measurement of battery performance, it also has disadvantages.
When batteries are deeply discharged, the chemical compounds inside the battery begin forming stable crystalline structures. These crystals may not completely dissolve when the battery is recharged, permanently reducing the battery’s ability to store energy.
For lead-acid batteries, this process is known as sulfation. Similar damage can also occur in Nickel-Cadmium (NiCd) and Lithium-Ion batteries after repeated deep discharges.
Because deep discharge gradually reduces battery life, it is generally recommended that rundown tests be performed only occasionally, typically once every six months to one year.
Battery Testing Summary
| Testing Method | Purpose | Limitation |
|---|---|---|
| Short Self-Test | Checks basic battery operation. | Does not accurately measure actual backup capacity. |
| Deep Discharge (Rundown Test) | Measures true battery runtime and capacity. | Causes gradual battery wear and should be performed infrequently. |
Testing Battery Strings and Individual Cells
Large commercial UPS systems often contain battery banks consisting of many batteries connected together in a battery string. These systems allow technicians to isolate and test individual batteries or individual cells without affecting the operation of the remaining battery bank.
In some UPS installations, batteries are made up of combined battery units such as 12 V lead-acid batteries, while others use individual electrochemical cells connected in series.
During maintenance, one battery or cell can be temporarily isolated and replaced with a jumper connection. This allows technicians to perform a discharge test on the isolated battery while the remaining batteries continue providing backup protection.
Many modern UPS systems also include sensor wires connected between individual battery cells. These sensors continuously measure the electrical characteristics of each cell separately as well as the overall battery string, making it easier to identify weak or failing batteries.
Monitoring Series-Parallel Battery Strings
Some UPS systems use series-parallel battery arrangements, where multiple battery strings operate in parallel to increase total capacity.
In such configurations, it is important to monitor the current flowing between the parallel strings because electricity may naturally flow from stronger battery strings into weaker ones. This balancing current can occur when one string contains weak cells, dead cells with high internal resistance, or short-circuited cells.
These interactions must be considered when evaluating the performance of individual battery cells and strings.
Series-Parallel Battery Interactions
Battery strings connected in series-parallel can develop several unusual failure modes because the parallel strings influence one another.
If one battery within a series string becomes short-circuited or completely fails, the maximum voltage produced by that entire series string decreases.
Since other healthy strings are connected in parallel, they automatically discharge into the weakened string until all string voltages become equal. This balancing process may overcharge the remaining healthy batteries within the damaged string, causing excessive heating, electrolyte boiling, and gas generation.
Charging systems usually estimate battery condition by measuring the total battery string voltage. Because the damaged string produces a lower voltage, the charging system may incorrectly assume that the entire battery bank requires additional charging. As a result, it continuously attempts to recharge the batteries, leading to overcharging and accelerated deterioration of the batteries in the faulty string.
When lead-acid batteries are used, batteries in previously healthy parallel strings may also begin to develop sulfation because they can never reach a complete charge. Even if the damaged battery is eventually replaced, the storage capacity of the remaining batteries may already have been permanently reduced.
To avoid these complex interactions, some installations eliminate parallel battery strings entirely and instead use separate charge controllers and inverters for each individual series battery string.
Problems in Series-Parallel Battery Strings
| Problem | Effect |
|---|---|
| Failed Cell | Reduces the voltage of the entire battery string. |
| Current Balancing | Healthy strings discharge into weaker strings. |
| Continuous Charging | Charging system repeatedly overcharges damaged strings. |
| Sulfation | Permanently reduces battery capacity in lead-acid batteries. |
| Recommended Solution | Use separate charge controllers and inverters instead of parallel battery strings. |
Series New and Old Battery Interactions
Even when batteries are connected only in series, mixing new batteries with older batteries can create performance problems.
Older batteries generally have a lower storage capacity because of natural aging. As a result, they discharge more quickly than new batteries and also become fully charged much sooner during recharging.
During discharge, the voltage of the entire battery string falls as the older batteries become exhausted, even though the newer batteries may still contain usable energy. This remaining energy may continue flowing through the older batteries but provides little practical benefit and is largely lost as heat.
If the batteries are designed to operate within a specific discharge range, the greater capacity of the new batteries may force the older batteries to discharge below their safe operating limit, causing permanent damage.
Recharge cycles also create problems. Since older batteries charge more rapidly, the total battery string voltage rises quickly. The charger interprets this high voltage as a fully charged battery bank and reduces the charging current. Consequently, the newer batteries receive only a slow charge and may never become fully charged. Over repeated charging cycles, chemical crystallization can occur inside the newer batteries, gradually reducing their capacity until it approaches that of the older batteries.
Because of these interactions, many industrial UPS manufacturers recommend replacing the entire battery bank at the same time instead of replacing only individual batteries. Although this approach is more expensive, it helps maintain balanced battery performance and maximizes the service life of the UPS battery system.
Problems Caused by Mixing New and Old Batteries
| Issue | Result |
|---|---|
| Different Battery Capacities | Older batteries discharge faster than newer batteries. |
| Uneven Charging | Older batteries reach full charge earlier than newer batteries. |
| Energy Loss | Remaining energy from newer batteries is dissipated as heat. |
| Reduced Battery Life | New batteries gradually lose capacity to match older batteries. |
| Recommended Practice | Replace the complete battery bank instead of mixing old and new batteries. |
UPS Standards
To ensure the safety, performance, electromagnetic compatibility, and environmental compliance of UPS systems, several international standards have been developed by the International Electrotechnical Commission (IEC).
| Standard | Description |
|---|---|
| IEC 62040-1:2017 | Specifies the general and safety requirements for Uninterruptible Power Supply (UPS) systems. |
| IEC 62040-2:2016 | Defines the electromagnetic compatibility (EMC) requirements for UPS systems. |
| IEC 62040-3:2021 | Describes methods for specifying UPS performance and testing requirements. |
| IEC 62040-4:2013 | Covers the environmental aspects of UPS systems, including environmental requirements and reporting. |