Fire Alarm Battery Calculator
How the formula works
A fire alarm battery must run the system during a power cut (standby time) and then still power all alarms (alarm time). The calculator adds both and applies an ageing factor.
Formulas
Standby current = Panel + (Detectors × each) + (Sounders × each)
Alarm current = Panel + (Detectors × each) + (Sounders × each)
Standby capacity (Ah) = Standby current (A) × Standby time (h)
Alarm capacity (Ah) = Alarm current (A) × Alarm time (min) / 60
Required capacity = (Standby Ah + Alarm Ah) × Ageing and safety factor
Where:
- Currents are entered in mA and converted to A by dividing by 1,000
- The ageing factor allows for battery capacity loss over its life, low temperature and discharge losses
- Batteries are 12 V units connected in series to reach the system voltage
Typical values
Standby time: 24 hours (NFPA 72), 72 hours (EN 54-4 and BS 5839-1 for systems without remote monitoring). Alarm time: 5 minutes (NFPA 72 general alarm), 15 minutes (voice evacuation), 30 minutes (EN 54 and BS 5839-1). Ageing and safety factor: 1.20 to 1.30. Standard battery sizes used: 1.2, 2.3, 4.5, 7, 9, 12, 18, 24, 26, 33, 40, 55, 65, 75, 100, 120, 150 and 200 Ah. Local code and the manufacturer data are final. Use the Custom option for your own values.
How to use the calculator: enter the panel current, then the detector and sounder quantities with their currents from the data sheets. Select the times and factor, then click Calculate.
Important notes: battery capacity is normally given at the 20 hour rate. For a high alarm current over a short time, check the maker’s discharge curve.
Worked example
Example 1: Small building, 24 hours + 30 minutes
24 V system. Panel 120 mA standby and 300 mA alarm. 80 detectors at 0.3 mA standby and 0.5 mA alarm. 12 sounders at 30 mA alarm. Factor 1.25.
Standby current = 144 mA. Alarm current = 700 mA. Standby capacity = 3.46 Ah. Alarm capacity = 0.35 Ah. Total = 3.81 Ah. Required = 4.76 Ah. Standard battery: 2 × 12 V, 7 Ah.
Example 2: Medium building, 72 hours + 30 minutes
24 V system. Panel 150 mA and 400 mA. 200 detectors at 0.3 and 0.5 mA. 30 sounders at 25 mA alarm. Factor 1.25.
Standby current = 210 mA. Alarm current = 1,250 mA. Standby capacity = 15.12 Ah. Alarm capacity = 0.63 Ah. Required = 19.68 Ah. Standard battery: 2 × 12 V, 24 Ah.
Example 3: NFPA 72, 24 hours + 5 minutes
24 V system. Panel 300 mA and 900 mA. 400 detectors at 0.4 and 0.6 mA. 60 strobe sounders at 80 mA alarm. Factor 1.20.
Standby current = 460 mA. Alarm current = 5,940 mA. Standby capacity = 11.04 Ah. Alarm capacity = 0.50 Ah. Required = 13.84 Ah. Standard battery: 2 × 12 V, 18 Ah.
Example 4: Large site, 48 V
48 V system. Panel 500 mA and 2,000 mA. 1,000 detectors at 0.3 and 0.5 mA. 200 sounders at 40 mA alarm. 24 hours + 30 minutes, factor 1.25.
Standby current = 800 mA. Alarm current = 10,500 mA. Standby capacity = 19.20 Ah. Alarm capacity = 5.25 Ah. Required = 30.56 Ah. Standard battery: 4 × 12 V, 33 Ah.
Common mistakes
- Using only the standby current and forgetting the alarm load.
- Using the panel’s mains current instead of its battery (standby) current.
- Forgetting the ageing factor, so the battery fails after a few years.
- Mixing up mA and A when entering device currents.
- Using the wrong standby time. NFPA 72 and EN 54 give different durations.
- Leaving out relay modules, isolators, door holders and remote displays.
- Counting sounders in standby when they draw current only in alarm.
- Choosing a battery smaller than the calculated Ah instead of the next standard size.
- Ignoring low room temperature, which reduces battery capacity.
Frequently asked questions
How do I calculate fire alarm battery capacity?
Multiply the standby current by the standby hours, add the alarm current times the alarm hours, then multiply by an ageing factor.
What standby time is required?
NFPA 72 commonly uses 24 hours. EN 54-4 and BS 5839-1 may need 24 or 72 hours depending on the system. Check your local code.
What alarm time should I use?
5 minutes is common under NFPA 72, 15 minutes for voice evacuation, and 30 minutes under EN 54 and BS 5839-1.
Why add an ageing factor?
Batteries lose capacity with age and cold. A factor of 1.20 to 1.30 keeps the system safe near the end of battery life.
How do I find the device current?
Take the standby and alarm current of each device from the manufacturer data sheet.
Why do sounders have zero standby current?
Most sounders draw current only when sounding. Enter a standby value if the data sheet shows one.
How are two 12 V batteries connected for 24 V?
In series. The voltage adds and the Ah stays the same.
What if I need more than 200 Ah?
Connect batteries or strings in parallel. This calculator shows the number of parallel strings.
What is Ah?
Ah (ampere-hour) is the battery capacity. A 7 Ah battery can supply 1 A for about 7 hours at the rated discharge rate.
Does this size the battery charger?
No. The charger must be sized from the battery capacity and the maker’s recharge time, usually within 24 to 48 hours.
Can I use this for emergency lighting or other systems?
The same method works for other battery backups. For the loads and duration, follow the standard for that system.
Is this a final design?
No. It is a preliminary size. Final design should follow the local code and the manufacturer data.