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[ The PC Guide | Systems and Components Reference Guide | Power | External Power | Uninterruptible Power Supplies | Parts of the Uninterruptible Power Supply ]


Other than the core circuitry of the UPS, the other main component is the battery, which of course holds the energy that is used by the UPS to run your equipment. It is the size of the battery more than anything else that dictates the size of the UPS unit as a whole. The size of the battery is also proportional to the amount of energy that is stored in the UPS, and therefore, the length of time that the UPS will run for a given load.

The batteries used  in most UPSes are in many ways the same as those used in most cars: both are lead-acid, 12V batteries. However, there are some important differences. Car batteries generate electricity by the reaction of sulfuric acid on lead plates that are suspended within the liquid. They are called flooded cell batteries for this reason. These types of batteries are not suitable for use in a UPS because there is the potential for the acid to spill from the battery, and because during charging the batteries can produce explosive hydrogen gas, which can be very dangerous in a closed environment. Therefore, most UPS batteries are a special type: sealed, valve-regulated lead-acid. The batteries are closed to avoid any possibility of hydrogen escaping, or acid spilling. To further reduce the chances of an acid spill, the acid is contained within mats of felt or fiberglass to hold it in place. These batteries have many advantages over conventional flooded cells, including the fact that they can be shipped by parcel carrier since they are protected against spills, while conventional flooded cells are considered hazardous materials. Their main disadvantage is that they cost two to three times as much for a battery of the same capacity.

All batteries are rated in terms of their nominal voltage (in Volts), and their capacity (in Amp-hours or "Ah"). 1 Amp-hour represents energy sufficient to provide 1A of current for one hour at the rated voltage. The larger the Ah rating, the more capacity the battery has. (Most UPS brochures don't discuss the Ah capacity of the battery directly, so you may have to look at a specification sheet to get the details.) So for example, in theory a 17 Ah 12V battery can run a 17A, 12V load for one hour, or a 1A, 12V load for 17 hours. In practice, the capacity of the battery depends on how fast you are drawing current from it, so saying a battery has a particular Ah capacity implies measuring it at a particular discharge rate. A battery that is 20 Ah if it is discharged at the rate of 1 Amp per hour may only have a capacity of 18 Ah if discharged at a rate of 5 Amps per hour. This means a 1A load would run for 20 hours, but a 5A load would run not for 4 hours, but for only 3 hours. This has an important influence on the run time of a UPS.

A sealed lead-acid battery mounted in a metal frame in a
UPS. The writing on the side of the battery tells you the
vitals of this particular battery unit: 12V, 7.2 Amp-hours.

Tip: Some UPSes have expandable battery packs, helpful if you add loads to your system. You can add additional battery capacity using a special connector on the UPS and a pack available from the manufacturer.

A family of UPSes. All are the same basic design,
but vary by their battery size and therefore their
capacity and run time. The units on the left have
expansion battery packs underneath them.
(APC's SmartUPS line.)

Image American Power Conversion Corp.
Image used with permission.

Since they are based on a chemical reaction, all batteries fail eventually. Many better UPSes will detect this condition by examining the voltage of the battery as it is charging. Others may detect it during a self-test. Over time, the voltage of a battery when it is full will degrade, and its capacity will diminish, leaving it less able to provide protection for your equipment. Fortunately, most newer UPSes have user-replaceable batteries. Following the simple procedure described in your user manual, you should be able to remove the existing battery and install a new one. Replacement batteries are normally available from the UPS's manufacturer or authorized dealer.

As this diagram shows, it is pretty simple to install a new battery in a
consumer-grade UPS designed with user-replaceable batteries.

Image American Power Conversion Corp.
Image used with permission.

Battery life is a function of how well you take care of it. Lead-acid batteries prefer to be left full, and "cycled" as infrequently as possible. Under normal use of a UPS, the battery is being kept "topped up" by the internal charger, and the battery itself rarely runs down; this is nearly ideal for extending battery life. It is important if you do end up running the UPS on battery, that when you begin to get warning indicators that the battery has almost depleted, you heed them, and get the battery recharged again as soon as possible. Generally, a UPS will not discharge its battery 100%, because taking a battery down to the fully-discharged state dramatically reduces its life.

Have you ever noticed that if your car fails to start, it's usually on a cold winter morning? The reason for this, aside from the car's mechanisms perhaps being sluggish due to the cold, is that battery capacity and voltage both diminish in cold temperatures. Your UPS should be stored and used at room temperature. If the UPS is stored somewhere cold, be sure to acclimate it to room temperature for 24 hours before using it.

Warning: Respect your UPS's specifications. Do not store a UPS or battery at a temperature colder than the UPS manufacturer specifies, especially if it is discharged, or it may be damaged.

As a final note, remember that when you first plug in a new UPS, it will take several hours before the battery is charged. Until the battery is full, the UPS will probably function fine on line power, but you will be susceptible to problems if the power goes out.

Warning: Lead-acid batteries are potentially hazardous, even sealed ones. Respect these cautions when handling batteries:

  • Never subject a battery to an avoidable physical shock. They should not be banged or dropped, as damage and leakage may result.
  • Lead-acid batteries must be properly recycled due to the hazardous chemicals they contain. Do not dispose of batteries in the household trash.
  • Never dispose of any battery in a fire; it could explode.
  • Short-circuiting a battery by connecting its two terminals directly together with no load, can cause it to overheat and explode. Always use rubber-handled tools when working with batteries. Watch out for loose jewelry or anything else that could come into contact with both terminals of the battery simultaneously. If metal contacts both terminals the heat produced may melt the item to the terminals and make it impossible to remove, creating a very dangerous situation.

Tip: Lots and lots of interesting information on batteries can be found at the Arizona Wind & Sun Battery FAQ.

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