FA Battery Calculations

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Keri_WW

Keri_WW

Senior Member
I'm working on a pretty small project that is having a few additions to the existing fire alarm system. The reviewer requested that I show the minimum size of the batteries for backup. I ran through the calculations of finding the standby current (0.0549 A) and the alarm current (2.842 A). I'm using a required standby time of 24 hours and a required alarm time of 5 minutes.

Applying a safety factor of 1.2, I am coming up with a required standby capacity of 1.9 Amp Hours. This seems pretty small, so I was wondering what standard size of battery was for your typical small fire alarm system.

(Devices: 1 FACP, 16 Smoke Detectors, 5 Strobes, 4 Horn/Strobe, 3 Pull Stations)

Thanks for the help!
Keri :D:D
 
What kind of FACP are you using? Most fire alarm mfr has battery calc worksheets (excel on their website.

If you want...PM me your e-mail address and will send you a generic calc in excel.
 
I'm guessing you got a hold of the installation manual and did the calcs by hand, or used an online or downloadable calculator per STHOMAS. For small applications you can get rediculously low battery sizes. Pick the next standard size up; maybe 4.5 Ah. Just be careful that the panel itself doesn't require a minimum size. We had a rash of cooked batteries in Siemens panels because the panel's charging circuits were looking for minimum 15 Ah batteries, and someone in the Service group was trying to save the customer some money by using 7 Ah.
 
I to am learning calcs for FA SYSTEMS, Don't forget your amps will be determined by the candellaras that are set on each NAC device. Your allowed 4.4 voltage drop from your FACP to your end device also unless its a class A system and then its all figured right at your FACP or booster.
 
Keri_WW said:
I'm working on a pretty small project that is having a few additions to the existing fire alarm system. The reviewer requested that I show the minimum size of the batteries for backup. I ran through the calculations of finding the standby current (0.0549 A) and the alarm current (2.842 A). I'm using a required standby time of 24 hours and a required alarm time of 5 minutes.

Applying a safety factor of 1.2, I am coming up with a required standby capacity of 1.9 Amp Hours. This seems pretty small, so I was wondering what standard size of battery was for your typical small fire alarm system.

(Devices: 1 FACP, 16 Smoke Detectors, 5 Strobes, 4 Horn/Strobe, 3 Pull Stations)
Click to expand...


I think your calculations are fine. I usually just plain double things for "power" in DC equipment, and unless you're making a million of these things and trying to save 23 cents a unit, you really have two choices.

[1] Buy the bigger battery.

[2] Test the thing to death with what you propose will work (and based on your math I think it'll do fine) and spend more money than you would on building a better product on screwing around with being cheap.

Oh, I forgot to mention, the cheapest working configuration is probably the least number of "C" cells in series.

Being serious though, your calculation method seems fine but I'd still even double that, just because your time in bothering with this is worth a lot more than the diference in the battery prices.
 
Keri_WW said:
The reviewer requested that I show the minimum size of the batteries for backup.
Click to expand...

Is this for a test or is this reviewer associated with designing something?

How bout the minimum amount of gas to get him home after work.

I wonder if he'd be OK with that? Especially given that the milage decreses every time the vehicle's driven. It might be enough to get him home "this" week.
 
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Keri_WW said:
I'm working on a pretty small project that is having a few additions to the existing fire alarm system. The reviewer requested that I show the minimum size of the batteries for backup. I ran through the calculations of finding the standby current (0.0549 A) and the alarm current (2.842 A). I'm using a required standby time of 24 hours and a required alarm time of 5 minutes.

Applying a safety factor of 1.2, I am coming up with a required standby capacity of 1.9 Amp Hours. This seems pretty small, so I was wondering what standard size of battery was for your typical small fire alarm system.

(Devices: 1 FACP, 16 Smoke Detectors, 5 Strobes, 4 Horn/Strobe, 3 Pull Stations)

Thanks for the help!
Keri :D:D
Click to expand...

What brand FACP is this?

As previously stated many brands require a minimum size. Silent Knight, Fire lite, ESL, Edwards Fireworx all require 7 AH as a minimum.

All FACP manuals I have read have the minimum recommended listed
 
You need about 1.6 Amp hours based on what you've said. So I'd go with 3.5. But nhfire77 says 7 AH and that wouldn't bother me in the least either.
 
Here in the UK 7A/H is widely used in both fire alarm and intruder alarms, even when a lesser capacity would appear to suffice.

7 A/H batteries are actually cheaper than the smaller ones, presumably due to mass production.
I can buy the 7 A/H ones for ?12.50, smaller ones from ?15 upwards.
 
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