Understanding UPS Batteries

Understanding your UPS battery can extend battery life, prevent costly downtime and save time and money.

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 your UPS batteries will extend battery life, and save you 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 the batteries have reduced, but not eliminated, the risks inherent in depending on any battery. Therefore it is prudent, if not essential; to take a close look at what may be increasing your 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.

There are primarily two kinds of batteries used in UPSs — valve regulated; lead acid batteries (VRLA), also known as sealed or maintenance-free, and wet-cell (also called flooded-cell). VRLA batteries usually have lower up-front costs but have 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 within polypropylene plastic, which in turn is sealed in a metal casing. VRLA batteries 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 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 of the hydrogen and oxygen is recombined. Re-sealable valves vent non-recombined gases only when pressure exceeds a safety threshold.

UPS Battery Overview

Internal and external components of a valve-regulated lead acid (VRLA) battery. A VRLA battery is distinguished 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 a flooded-cell battery. Because water cannot be added, recombination of water is critical to the life and health of a VRLA battery. Any factor that increases the rate of evaporation or water loss reduces the life of the battery. Such factors can include ambient temperature and heat from the charging current.


Wet-cell/flooded-cell batteries have thick lead-based plates that are flooded with an acid electrolyte. This is a highly reliable design, failures normally don’t occur until halfway through their 20-year pro-rated 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. However, while they are very reliable with a long life, there are downsides to wet-cell batteries as well. They require more safety measures, and a space-consuming separate battery room to use this technology. Regardless of the differences in UPS battery types, they both require monitoring and maintenance to ensure maximum life and system availability.

Battery arrangement and power

In most UPSs, you do not use just one cell at a time. They are normally grouped together serially to form higher voltages, or in parallel to form higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up. However, batteries are not quite as linear. For example, all batteries have a maximum current they can produce–a 500 milliamp-hour battery cannot produce 30,000 milliamps for one second, because there is no way for the battery’s chemical reactions to happen that quickly. And at higher current levels, batteries can produce a lot of heat, which wastes some of their power.

Like all batteries, UPS batteries are electro-chemical devices. A UPS uses 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 diluted sulfuric acid. In other words, they contain components that react with each other to create DC electrical current.

These Components Include The Follow:
  • Electrolyte – The medium that provides the ion transport mechanism between the positive and negative electrodes of a cell; in VRLA batteries the electrolyte is immobilized, while in flooded-cell batteries it is in liquid form
  • 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) – Used to vent the build-up of gas that goes beyond pre-determined levels
  • Separator – A device used for the physical separation and electrical isolation of electrodes of opposing polarities
  • Jar – The container holding the battery components