I assume from your question that you are calculating the size of needed power backup of the fire alarm system during a 24-hour power blackout, and at the end of 24 hours, either 5 minutes or 15 minutes of sounding the alarm.
The battery is rated in Amps of Current available for 1 Hour. An 8 Amp Hour battery can produce 8 amps
, for 1 hour
. Another way of looking at the 8 Amp Hour battery is that it's good for 1 amp
in a period of 8 hours
. The same battery is good for 1/3 of an amp (0.333 amps)
for 24 hours
Standby Battery Usage
When the fire alarm system isn't in alarm, but just standing by, standby battery usage is the total current used from the batteries from the fire alarm system.
During an electrical blackout, the battery has to keep the fire alarm system working for 24 hours. To figure out the amount of Amp Hours needed to keep the system working for 24 hours, multiply the standby current for the fire alarm system by 24 (Hours). 24 is your standby multiplier factor. This is how many Amp Hours are needed just to keep the fire alarm system standing-by for 24 hours.
Alarm Battery Usage
When the fire alarm system is in full alarm, sounding off all horns, speakers, and strobes, alarm battery usage is the total current used from the batteries from the fire alarm system.
At the end of 24 hours of just standing by, the alarms have to sound for either 5 minutes or 15 minutes.
5 minutes is 1/12 of an hour, or about 0.084 hours. The multiplication factor for 5 minutes is 0.084
15 minutes is 1/4 of an hour, or about 0.25 hours. The multiplication factor for 15 minutes is 0.25
To figure out the amount of Amp Hours needed to keep the system working for 5 minutes or 15 minutes, multiply the alarm current for the fire alarm system by either 0.084 (Hours) or 0.25 (hours), which is your standby multiplier factor. This is how many Amp Hours that are needed just to keep the fire alarm system in alarm for 5 minutes or 15 minutes.
Headroom - Adjusting for Variations
Batteries are never the absolute values shown stamped on the side of the battery. Even the National Fire Protection Association, Inc., or NFPA (a publishing house) recognizes this issue and requires that headroom to battery capacity will be included in the design of a building's Fire Detection and Alarm System (FDAS).
Battery life is, on average, five years from the date of manufacturer, not the installation date. Half of the batteries will be good longer than five years, half of the batteries will not be good for even the full five years.
Absolute Amphour Capacity
The amphour capacity stamped on the side of a battery is more of the name of a battery; it isn't the real, or absolute amphour capacity of a battery.
The manufacturer is trying to be accurate when they stamp an amphour number on the side of their battery. However, the manufacturer has to be realistic; the battery is a chemical-to-electrical conversion device, and it will have a + and - factor in the amphour value. See the battery manufacturer's data sheet on that battery to find out how much the absolute amphours of the battery varies from the value stamped on the side.
Besides the variations from the original manufacture of a battery, the real or absolute amphour capacity will change over the life of a battery.
- For a little while after manufacturing, the absolute amphour capacity of a battery will be a little less than what's stamped on the side
- For most of the useful life, the absolute amphour capacity of a battery will be close to what's stamped on the side
- For the last year or so of useful life, the absolute amphour capacity of a battery will be in decline, and when the absolute amphour capacity of a battery is reduced less than 80% of what's stamped on the side, the battery is not considered to be good anymore.
Rate of Electrical Current Use
The absolute amphour capacity of a battery will vary, depending on the rate of current use.
Example: If a 7-amphour battery is drained at a rate of 0.1 amps, it will probably last 70 hours. If, however, the same battery is drained at a rate of 1.0 amps, it won't last a full 7 hours.
Batteries are designed to be used at a normal room temperature. Batteries that are used in warmer environments will have shorter battery life; batteries that are used in cooler environments will have lower amphour capacity.
Adding Headroom to Battery Calculations
To include the NFPA's required headroom, add the standby battery Amp Hours to the alarm battery Amp Hours to get a rough total number of Amp Hours that are needed for the standby batteries.
This is an electronic system, and it is also a life-safety system. In case something changes over time, there should be a minimum of 25% headroom.
Take the rough total Amp Hours that are needed, and multiply it by 1.25 to get the minimum number of Amp Hours needed for the standby batteries.
The number of Amp Hours you calculated are never the number shown on the size of the batteries. You can't use lower Amp Hour batteries than you calculated, but you can always install larger batteries.
Use the next bigger battery size available for the real batteries that are installed.