Ametek

Common Questions
[pic]How Do I Size Batteries for a UPS application?
[pic]What Are The Advantages/Disadvantages of Different Battery Technologies?
[pic]Why Fuses Instead of Breakers For Distribution Panels?
[pic]What Is Ferroresonant?
[pic]What Is Pulse-Width-Modulation (PWM)?
[pic]What Is A UPS?
[pic]What Is On-Line?
[pic]What Is Off-Line/Stand-By?
[pic]What Is The CBEMA Curve and WhatDoes It Mean?
[pic]How Do I Size My UPS?
[pic]What Is A Crest Factor, and How Do I Factor This Into My UPS Sizing?
[pic]Why Single Phase Instead of Three Phase?

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How Do I Size Batteries for a UPS application?     Back To Top
UPS batteries are sized to provide emergency back-up power to the UPS in the event of a total AC blackout. While it is often a matter of convenience or personalpreference, the length of back-up time required is primarily a function of the process being protected. It is also governed by the cost of the battery.

Since a UPS is utilized to provide continuous power to a process of one kind or another, the batteries should be sized to carry the load for as long as it takes to either complete the process, perform an orderly shut-down of the process, or bringa generator on-line to power the load. A UPS inverter converts DC power to AC power. The battery must be sized large enough to provide the DC input power (in kilowatts) for a fully loaded inverter for the specified length of time. The DC input power is determined by applying the load power factor, and inverter conversion efficiency to the inverter's AC power rating.

Next, you must determinethe DC bus voltage, and the corresponding number of battery cells. Then, you can determine the battery capacity per cell required for the load. (Note that a battery has a nominal voltage of 2 volts per cell. So, on a nominal 120 VDC bus, you will need 60 cells.) Then, using discharge data supplied by a battery manufacturer, you can determine the battery type and size required.

Following is atypical battery sizing example: 20 kVA UPS (inverter) with a DC to AC conversion efficiency of 87%; a load power factor of 0.8, and a nominal 120 VDC bus.
20 kVA x 0.8
87%
60 cells
=
16 kW
87%
60 cells
=
18.39 kW
60 cells
=
0.307 kW/cell

Using discharge tables supplied by the battery manufacturer(s), select the type of battery that meets the above requirements of 0.307 kilowatts percell.

Please note that complex load duty cycle profiles, commonly referred to as step load profiles, should be referred to your UPS supplier for proper sizing.

FOR MORE INFORMATION, GO TO SELECTION AND SIZING OF BATTERIES FOR UPS BACKUP FOUND IN THE TECHNICAL PAPERS SECTION OF THIS SITE.
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What Are The Advantages/Disadvantages of Different Battery Technologies?     Back To Top
There areseveral different types of batteries available for UPS applications. However, they basically breakdown into two technologies: Lead Acid and Nickel Cadmium. Lead acid batteries are further divided into two types: Lead Calcium and Lead Antimony. Lead Calcium batteries can be broken down into two categories: traditional wet cell (or flooded) and valve regulated lead acid, (or VRLA-sometimesmistakenly referred to as maintenance free). The proper battery for a given application is dependent upon a number of factors. (Refer to Table 1 below for a quick comparison between battery technologies.)
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1. Ambient temperature. The optimum average ambient temperature for batteries is 77ºF. Ambient temperatures above that will significantly reduce battery life. The rule of thumb is that anincrease in ambient temperature of 15ºF above 77ºF will reduce the battery life by 50%.

Certain battery types are more susceptible to the effects of temperature than others. For example, a VRLA battery with a 5 or 10 year design life is much more prone to premature failure due to temperature extremes than a wet cell with a 20 or 30 year design life. The least susceptible is the Nickel...