UPS stands for Uninterruptible Power Supply. Its purpose is to supply power to certain critical or important electronic devices whenever main electric power is interrupted. Depending on the specific design, a UPS contains one or more batteries, components to clean up the AC power provided by the utility company and a relay to switch from main utility power to the battery power in the case of a "stand-by" UPS.
Because the electric power provided by your local utility can suddenly be interrupted due to a black out, thunder storm or some mechanical failure. A UPS can maintain operation of critical equipment until utility power is restored or until the equipment can be properly shut down.
The most typical application is to maintain power on a computer so that, should main electric power be suddenly lost (because of a black out, a thunder storm or some other reason), recent work can be saved before the computer shuts down. Besides computers, other important data processor driven electronics can benefit from battery support:
- Telephone and telecommunications equipment
- Digital signage player and display
- Digital video recording systems
- Digital mixing consoles
- Access control and security systems
- Emergency evacuation public address systems
The right UPS for your application is determined by several factors. To select the right UPS you will need to know:
- The power requirement of your equipment load. How much electric current does your equipment need? This is usually measured in amps (current) or watts (power).
- How long does the UPS need to support operation of the equipment in case of a black-out? This will be UPSs required "run time".
- How critical is the application? Of course, any application is important if you are considering a UPS. However, since the most reliable on-line UPS models are more expensive than the simpler stand-by models it is best to match the features of the UPS with the value of the application.
The UPS models are described based on their power rating in terms of their wattage (W) rating. Match this with the power requirement of your application. The run-time of the UPS is stated for full capacity current draw and one-half capacity. The most critical applications (security or network communications) should probably be supported by an on-line model. The less critical applications (cash registers or work stations) are usually matched with a stand-by device.
VA stands for "volt-amp" which equals volts x amps. In the United States standard residential and commercial electric power is 120 volts AC . Current flows to electric powered equipment at different rates, depending on how much power the equipment uses. This measure of current is amperes, or "amps".
From Alkaline to Zinc, there are many types of batteries. Some have practical application. Some are only experimental. All are used as a means to store electricity.
Since the beginning of the Space Program and, now, with development of electric cars and solar power, battery technology has evolved in general and in certain specialized applications.
SEALED LEAD ACID
Most UPSs in support of computers and other digital processing equipment rely on lead acid batteries. Those use the same technology as the battery in a gasoline powered car. Invented in 1859, the lead acid battery is reliable and inexpensive. Unfortunately, it is heavy and large for the amount of power it can produce compared to several other battery types. Some lead acid batteries require maintenance to keep liquid electrolyte levels up. Batteries in UPSs do not require maintenance.
LITHIUM IRON
Recent rapid growth in the use of lithium iron batteries began in 1991 when Sony commercialized that technology. By many comparisons to advancing technology, this is a well developed and mature battery design. Lithium iron is a low maintenance, high energy density type of battery technology. Most of today's mobile phones run on a single LiFe cell.
The most economical lithium iron battery in terms of cost-to-energy ratio is the cylindrical 18650 (used in the Juice Goose SL-350 UPS). The name derives from its dimension,18mm x 65mm (0.7" x 2.5"). Picture a larger AA battery. This is a very adaptable and reliable battery design. It's a truly amazing fact that the Tesla "battery" is made from strings and bricks and layers of over 6,800 of these individual 18650 cells.
Like all battery technologies lithium iron batteries call for proper application and management. They require a protection circuit to limit the peak voltage during charge and to control voltage and current on discharge. In addition, the cell temperature should be managed to avoid extremes. Storage and/or use in a temperature controlled environment will increase the life of the battery. But, LiFe battery technology certainly provides a small, economical and powerful alternative to lead acid and other existing options.
A "stand-by" or "off-line" UPS directs main utility power to your equipment as long as it's available. When the utility power fails it switches to the battery as a source of temporary power. "Line interactive" designs also power connected equipment using available utility power, except they also provide voltage regulation within a limited range in order to reduce the occurrence of battery use. An "on-line" type of UPS powers connected equipment from battery power 100% of the time. Available utility power is only used to charge the battery.
SL350 is a stand-by UPS.
KIN and SBN models are line interactive.
SCV models are on-line.
"Run time” refers to the amount of time a UPS will provide power once the main utility power is interrupted. Standard battery run time for a common UPS is around 5 to 10 minutes under full load and about twice that under half load. (Run times are often quoted as minutes under full and half load.)
The battery run time during a power outage is a function of the current load placed on the battery and the size of the battery. The demand for current by the equipment being powered is expressed in "amps”. The amount of "juice” stored in a battery is expressed in terms of "amp-hours”. (One amp-hour is a current flow of one amp for one hour.) If you want to know how long a given set of equipment will operate off a UPS battery, add up the current in amps that the equipment will pull and divide the amp hours of the UPS by that number. (e.g. A ½ amp load on a 2 amp-hour battery will run for 4 hours.)
So, it makes sense that the smaller the load relative to the size of the battery the longer the run time. Over-sizing a UPS relative to the load is one way to get extended run time. Some UPS models allow special external battery packs to be connected to the UPS to extend run times. Check with your supplier or the product specifications.
A battery will not provide full voltage through its entire rated amp-hour life. Some reduction in voltage will occur at the end of the functional operation of the battery, just as a flashlight becomes dim but still works. Some UPSs will provide automatic shutdown or alarm when this voltage decline is imminent. So, if you have a specific required battery run time in case of a power outage, it’s best to have a bit of amp-hour head room on top of what you calculate.
"Middle market” and top of the line UPSs have replaceable batteries. Since even continually charged batteries have a limited service life of three to five years, being able to replace them is a nice feature. UPS batteries pose no danger with normal operation. But, they should be disposed of properly.
Recent advances in battery technology provide choices in battery designs for various applications and price points. Lead acid batteries are heavy and are the most common type used. A 600 watt UPS weighs around 30 pounds. A 2200 watt unit weighs around 80 pounds.
Simple and sophisticated UPSs alike have warning indicators for loss of main utility power and for low battery power. A warning will also indicate when the battery is reaching the end of its useful life and needs to be replaced. In some UPS models the battery status can be remotely monitored over RS232 or Ethernet communication links.
PF = W / VA
The power factor is the ratio of real power (W) to the apparent power (VA) in the circuit. The higher the number, the more efficient the circuit. In most practical applications the power factor figure is between 0.50 and 0.90.
Real power is the capacity of the circuit to perform work (watts). Apparent power is simply the product of the voltage and current (volt amps) of the circuit. Due to energy stored in the load or due to a non-linear load, the apparent power (volt amps) will be greater than the real power (watts).
Why is this important? A load with a low power factor needs more current to do the same amount of work than a load with a high power factor. This will lead to higher operating costs and may require higher capital costs for equipment with higher current ratings.
When researching UPS battery backup for a specific application, look at the specifications of the models being considered. Compare units based on the cost of work capacity per dollar (watts / $).