It’s well understood that the battery in a UPS is the most vulnerable part of the system. In fact, battery failure is a leading cause of load loss. Knowing how to maintain and manage UPS batteries will extend their life and save data center managers time and trouble in the future.

Improvements in battery technology have been evolutionary rather than revolutionary. Capabilities such as advanced charging regimens, software management for accurate remaining life information and firmware adding intelligence to batteries have reduced, but not eliminated, the risks inherent in any battery. As a result, it’s prudent if not essential to take a close look at what may be increasing the risk of unexpected load loss from a failing UPS battery. After all, even large installations with many batteries are vulnerable to the failure of a single battery.

UPS Battery Overview

There are primarily two kinds of batteries in UPSs: valve-regulated lead acid (VRLA), also known as sealed or maintenance free, and wet cell (also called flooded cell or vented cell). VRLA batteries usually have lower up-front costs but a shorter lifetime than wet cell—usually around five years. Wet-cell batteries require more-advanced maintenance but have a longer lifetime—up to 20 years.


VRLA batteries are sealed, usually in polypropylene plastic. They were developed because they have the advantage of containing no sloshing liquid that might leak or drip out when inverted or handled roughly. The term valve regulated refers to the method of gas release. If the gas pressure becomes too great inside the battery, the valve will vent when it reaches a certain pressure.

During the charging of a lead-acid battery, hydrogen is normally liberated. In a vented battery, the hydrogen escapes into the atmosphere. In a VRLA battery, the hydrogen recombines with oxygen, so water loss is minimized. Under normal float conditions, virtually all the hydrogen and oxygen is approximately 98% recombined. Resealable valves vent nonrecombined gases only when pressure exceeds a safety threshold.

A VRLA battery is distinguishable from a flooded-cell battery by the rate at which oxygen is evolved from the positive plate and diffused to the negative plate, ultimately forming water. This rate is several orders of magnitude faster than in a flooded-cell battery. Because water can’t be added, its recombination is critical to the life and health of a VRLA battery. Any factor that increases the evaporation rate or water loss—such as ambient temperature and heat from the charging current—reduces the battery life.

Wet Cell/Flooded Cell

Wet-cell/flooded-cell batteries have thick lead-based plates that are flooded with an acid electrolyte. This design is highly reliable: failures normally don’t occur until halfway through their 20-year prorated life, at which time the failure mode is most often a short circuit. This situation is not an extreme emergency because any one shorted cell only affects overall reserve time by a very small percentage. Although they’re very reliable and have a long life, however, wet-cell batteries exhibit downsides. They require more safety measures and a space-consuming separate battery room.

Regardless of the differences in UPS battery types, both require monitoring and maintenance to ensure maximum life and system availability.

Battery Arrangement and Power

In most UPSs, you don’t use just one cell at a time. They’re normally grouped together serially to create higher voltages or in parallel to deliver higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up.

But batteries are less linear, as the graphic depicts. For example, all batteries have a maximum current they can produce; a 500 milliamp-hour battery can’t produce 30,000 milliamps for one second, because there’s no way for its chemical reactions to happen that quickly. It is also important to realize that at higher currents, batteries can produce a lot of heat, wasting some of their power.

Like all batteries, UPS batteries are electrochemical devices. A UPS employs a lead-acid storage battery in which the electrodes are grids of lead containing lead oxides that change in composition during charging and discharging, and the electrolyte is dilute sulfuric acid. In other words, they contain components that react with each other to create DC electrical current. These components are the following:

  • Electrolyte—The medium (comprising purified water and sulfuric acid) that provides the ion-transport mechanism between the positive and negative electrodes of a cell, immobilized in VRLA batteries, and in liquid form in flooded-cell batteries
  • Grid—A perforated or corrugated lead or lead-alloy plate used as a conductor and support for the active material
  • Anode—The terminal where the current flows in
  • Cathode—The terminal where the current flows out
  • Valve (used in VRLA batteries)—A means to vent the buildup of gas that goes beyond predetermined levels
  • Separator—A device for the physical separation and electrical isolation of electrodes with opposing polarities
  • Jar—The container holding the battery components

Shelf Life and Storage

To improve service-life expectations and reliability, it’s important to ensure that the batteries are properly stored before being installed and placed into service. Storage facilities should be climate controlled with proper ventilation capabilities so batteries can remain cool and dry. Failure to comply with proper storage leads to shortened run times and reduced capacity.

A rule of thumb in terms of time is no more than six months of storage in a properly designed storage facility. When the battery system is commissioned, an acceptance test should be performed to identify any flaws in the manufacturing process, improper storage or perhaps even hidden damage. Environmentally controlled storage facilities are recommended.

Ambient Temperature

The rated capacity of a lead-acid battery is based on an ambient temperature of 77°F. It’s important to realize that any variation from this operating temperature can alter the battery’s performance and shorten its expected life.

For every 15°F average annual temperature above 77° F, the life of the battery drops by 50 percent. Ambient temperatures below 77°F may reduce the battery backup time, similar to a car battery on a cold morning.

Battery Chemistry

UPS batteries are electrochemical devices whose ability to store and deliver power slowly decreases over time. Even if you follow all the guidelines for proper storage, usage and maintenance, batteries still require replacement after a certain period.

Positive-grid corrosion has been the most common end-of-life factor for UPS batteries. This problem is a result of the normal aging process owing to UPS battery chemistry and involves the gradual breakdown of the inner wires of the positive grid inside the battery.


During brownouts and blackouts caused by utility failure, the UPS operates on battery power. Once the utility power is restored, the battery is recharged for future use. This entire loop is called a discharge cycle. Each discharge and subsequent recharge reduces the relative capacity of the battery by a small amount. The length of the discharge cycle will determine the rate of reduction in battery capacity.

A good analogy is a loaf of bread. It can be sliced into many thin slices, or a few thicker slices. You still have the same amount of bread either way. Similarly, a UPS battery’s capacity can be used up over a large number of short cycles or fewer cycles of longer duration.

Lead-acid chemistry, like others used in rechargeable batteries, can only undergo a maximum number of discharge/recharge cycles before the chemistry is depleted. Once the chemistry is depleted, the cells fail and the battery must be replaced.


The most important thing is to maximize your UPS uptime. With proper checks and maintenance, the end of battery life can be estimated and replacements can be scheduled without any interruption or loss of backup power. Without regular maintenance or checkups, your UPS battery may experience heat-generating resistance at the terminals, improper (unbalanced) loading, reduced power protection and premature failure.

The most important thing is to maximize your UPS uptime. With proper checks and maintenance, the end of battery life can be estimated and replacements can be scheduled without any interruption or loss of backup power. Without regular maintenance or checkups, your UPS battery may experience heat-generating resistance at the terminals, improper (unbalanced) loading, reduced power protection and premature failure.

Monitoring and Management

In a perfect world, you’d be able to monitor your batteries and IT infrastructure equipment constantly, making sure they’re protecting critical power and running efficiently. That’s not reality, though, as your team has more to tend to than just the IT environment. That’s where a comprehensive monitoring and management platform can make a big difference, helping to qualify the factors listed above and act as a second set of eyes on your equipment.

Using a next-level monitoring and management service can be helpful for collecting and analyzing data from batteries and other power-infrastructure devices, providing the insight needed to make recommendations and act on your behalf. It means continuous monitoring of your batteries, time savings and peace of mind that your batteries are covered.

Batteries are a critical part of the UPS, and determining battery life can be tricky. It’s a specification that’s often promoted on the basis design life, defined as how long the battery can be expected to perform under ideal conditions.