[IMAGE]11935[/IMAGE]Figuring Out Standby Capacity
Now let’s probe a little deeper and fill in some of the unknowns for our calculations.
Begin by listing all of the loads to be placed on battery power during standby operation. Be sure to include the current drawn by the motherboard in the fire alarm panel as well as relays, internal modules, smoke detectors, addressable input modules, the digital alarm communications transmitter (DACT) or City Tie module, and any other devices. In addition, list all the load currents that will be experienced during alarm. Be sure to list them separately and insert their values into the appropriate locations of the formula.
As an example, let’s say that the standby current is 0.5A and the alarm current is 12.0A.
- First, calculate standby capacity: 24H X 0.5A = 12AH
- Then, alarm capacity: 0.083H (5 minutes) X 12A = 0.996AH
- Then, total raw capacity: 12AH + 0.9961AH = 12.996AH
- And finally, headroom: 12.996AH X 1.2 (+20%) = 15.5952AH
Most likely the installer will use two 18AH, 12VDC batteries in series for this application. Not only does this assure that the batteries used in this fire alarm control panel meet the minimum requirement with an additional 20-percent safety margin, but it also addresses to some extent the normal degradation of battery power during the batteries’ lifetime.
Although this will vary from manufacturer to manufacturer, as a general rule battery capacity will reach the halfway point after about three years of operation. It’s good to have some additional headroom just in case.
Observing Code Requirements
In actuality, it’s a combination of NFPA 72 and NFPA 70, National Electric Code (NEC) that determines how and where we install our batteries. NEC’s Article 480, 2005 Edition, for example, offers helpful information on battery use.
For instance, assure that the batteries you install are well protected from physical harm. Of course, we do that by placing them in a metal box. We then install that box where it is not likely to come in contact with tow motors and other potential sources of harm.
When batteries are installed on racks, those racks, according to NEC, must be treated or painted with a substance that will retard deterioration due to potential contact with the battery electrolytes.
If the location of the batteries is different than the fire alarm panel, the exact location must be noted inside the panel for reference. We must also protect open battery contacts so they do not come into contact with metal boxes or another battery contact. Each battery must also be marked with the date when it is installed, or a date code must be available on the battery for reference at a later time.
Special Storage Considerations
Venting of gases in battery compartments is an important issue that you should be aware of even though it’s the manufacturer of the fire alarm panel that usually takes care of this. Due to chemical processes that occur during recharging, gases can occur in the vicinity of these batteries. Thus, proper ventilation must be assured in order to avoid a potential explosion.
Because these batteries are usually confined inside a metal box, heat can become a problem in some cases. This is especially true when placing the metal box in an environment with a relatively high ambient temperature. For this reason, look ahead at the time of installation and be sure to consider temperature.
Freezing can be just as detrimental to the proper operation of a battery supply as overheating. If it’s likely to experience high or low temperatures during the life of the system, then it may be expedient to install the battery box in a location someplace remote from the alarm control panel.
For more information on battery calculations and installation, refer to the manufacturer’s installation instructions. I also recommend you consult Chapter 10 of NFPA 72 and Article 480 of NFPA 70, 2005, for code references and other information.
Al Colombo is an award-winning writer who has covered electronic security and life safety since 1986. Visit his Web site at www.firenetonline.com, and check out his Security Sense blog.
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