UPS Battery Maintenance 101

Oct. 21, 2010
How to properly maintain and manage UPS batteries

It’s a well-known fact in the industry that the battery is the most vulnerable part of a UPS. In fact, battery failure is a leading cause of load loss. Understanding how to properly maintain and manage UPS batteries can not only extend battery service life, but it can also help prevent costly downtime.

The most common type of battery used in today’s UPSs is the valve-regulated lead acid (VRLA) battery, also known as a sealed or maintenance-free battery. VRLA batteries are sealed, usually within polypropylene plastic, which offers the advantage of not containing any sloshing liquid that might leak or drip. Because water cannot be added to VRLA batteries, recombination of water is critical to their life and health. In addition, any factor that increases the rate of evaporation or water loss, such as temperature or heat from the charging current, reduces the life of the battery.

Let’s take a look at some of the most frequently asked questions when it comes to UPS battery maintenance.

1. What is the “end of useful life?”

The IEEE defines “end of useful life” for a UPS battery as being the point when it can no longer supply 80% of its rated capacity in ampere-hours. When your battery reaches 80% of its rated capacity, the aging process accelerates, and you should replace it.

2. Is there any difference between the batteries used by smaller UPSs (250VA to 3kVA range) and the ones used by larger UPSs?

While basic battery technology — and the risks to battery life — remain the same regardless of UPS size, there are some inherent differences between large and small applications. First, smaller UPSs typically have only one VRLA battery that supports the load and requires maintenance. As systems get larger, increasing battery capacity to support the load gets more complicated. Larger systems may require multiple strings of batteries, introducing complexity to battery maintenance and support. Individual batteries must be monitored to prevent a single bad battery from taking down an entire string and putting the load at risk. In addition, as systems get larger, the use of wet-cell batteries become much more common.

3. My UPS has been in storage for more than a year. Are the batteries still good?

As batteries sit unused, with no charging regimen, their life will decrease. Due to the self-discharge characteristics of lead-acid batteries, it is recommended you charge them every three to four months of storage. Otherwise, permanent loss of capacity will occur between 18 and 30 months. To prolong shelf life without charging, store batteries at 10°C (50°F) or less.

4. What is the difference between hot-swappable and user-replaceable batteries?

Hot-swappable batteries allow the batteries to be changed out while the UPS is running. User-replaceable batteries are usually found in smaller UPSs and require no special tools or training to replace. Batteries can be both hot-swappable and user-replaceable.

5. How is battery runtime affected if I reduce the load on the UPS?

The battery runtime will increase if the load is reduced. As a general rule, if you reduce the load by half, you triple the runtime.

6. If I add more batteries to a UPS, can I add more load?

Adding more batteries to a UPS can increase the battery runtime to support the load. However, adding more batteries to the UPS does not increase the capacity of the UPS. Be sure your UPS is adequately sized for your load, then add batteries to fit your runtime needs.

7. What is the average lifespan of UPS batteries?

The standard lifespan for VRLA batteries is three to five years. However, expected life can vary greatly due to environmental conditions, number of discharge cycles, and adequate maintenance. Have a regular schedule of battery maintenance and monitoring to ensure you know when your batteries are reaching the end of their life.

8. Does the UPS need to have a load on it to charge its batteries?

Generally, the UPS does not require a minimum load to charge its batteries. Once connected to a standard supply of electricity (via input plug or hardwiring), your UPS should charge its batteries regardless of how much load, if any, is attached to it.

9. How can you ensure UPS batteries are in good condition and have maximum holdover in the event of a power failure?

You should have the UPS and its batteries thoroughly inspected and tested by a trained field technician. On smaller UPS models, you can often initiate a self test of the unit by pressing a button on the front panel.

10.What is thermal runaway?

Thermal runaway occurs when the heat generated in a lead-acid cell exceeds its ability to dissipate that heat, which can lead to an explosion, especially in sealed cells. The heat generated in the cell may occur without any warning signs and may be caused by overcharging, excessive charging, internal physical damage, internal short circuit, or a hot environment.

11.Why do batteries fail?

Batteries can fail for a multitude of reasons, but the most common causes include:

  • High or uneven temperatures
  • Inaccurate float charge voltage
  • Loose inter-cell links or connections
  • Loss of electrolyte due to drying out or a damaged case
  • Lack of maintenance/aging.

12.How is battery performance typically measured?

Batteries are generally rated for 100+ discharges and recharges. However, many batteries show a marked decline in charging capacity after as few as 10 discharges. The lower the charge the battery can accept, the less runtime it can deliver. Look for batteries with a high-rate design that sustains stable performance for a long service term.

13. What should a UPS service plan cover?

When evaluating service plans, pay special attention to what is and isn’t covered. Support agreements and warranties for large UPS models usually cover internal electronics only, with battery coverage available as an optional extra. A strong, comprehensive service plan should cover all of the following:

UPS electronics parts and labor coverage: This covers a UPS’s basic electrical components, excluding the battery.

UPS battery parts and labor coverage: To prevent UPS failure, batteries should be replaced at least every 5 yr. Batteries that are discharged frequently or used in a warm environment should be replaced more often.

Preventive maintenance: Preventive maintenance visits allow field technicians to annually inspect, test, calibrate, and upgrade UPS and/or battery components, ensuring factory-specified performance. You might also consider adding remote monitoring services to your maintenance plan.

Sember is a UPS product line manager for Eaton based in Raleigh, N.C. He can be reached at [email protected].

SIDEBAR: Factors Affecting Battery Life

All UPS batteries have a limited service life, regardless of how or where the UPS is deployed. Although determining battery life can be tricky, there are four primary factors that contribute to a battery’s overall lifespan.

Ambient temperature

Because the rated capacity of a battery is based on an ambient temperature of 25°C (77°F), any variation can affect performance and reduce battery life. For every 8.3°C (15°F) average annual temperature above 25°C (77°F), the life of the battery is reduced by 50%.

Battery chemistry

UPS batteries are electro-chemical devices whose ability to store and deliver power slowly decreases over time. Even if all guidelines for storage, maintenance, and usage are followed, batteries will still require replacement after a certain period of time.


After a UPS operates on battery power during a power failure, the battery is recharged for future use, which is called the discharge cycle. At installation, the battery is at 100% of its rated capacity, but each discharge and subsequent recharge slightly reduces the relative capacity of the battery. Once the chemistry is depleted, the cells fail and the battery must be replaced.


For larger UPS models, service and maintenance of batteries are critical to the reliability of the UPS. Periodic preventive maintenance not only extends battery string life by preventing loose connections and removing corrosion, but it can help identify ailing batteries before they fail. Even though sealed batteries are sometimes referred to as “maintenance-free,” they still require scheduled service, as “maintenance-free” refers only to the fact that they do not require replacement fluid.

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