Worked problem

Fire Alarm Control Panel Battery Calculation

Size the standby battery for a fire alarm panel with 0.5 A standby and 2.5 A alarm current, using a 24-hour standby period and 5 minutes of alarm.

Fire Protection · Last reviewed

Given

A fire alarm control panel has a total standby current of 0.5 A and a total alarm current of 2.5 A (all notification appliances on). The system needs 24 hours of standby followed by 5 minutes of alarm, as is common for a system without voice evacuation. A 20% design margin is applied for battery ageing and temperature.

Find

The minimum battery capacity in amp-hours and a standard battery size.

Solution

  1. Standby capacity = 0.5 A × 24 h = 12.0 Ah.
  2. Alarm capacity = 2.5 A × (5 ÷ 60) h = 0.208 Ah.
  3. Total = 12.0 + 0.208 = 12.21 Ah.
  4. With a 20% margin: 12.21 × 1.20 = 14.65 Ah.
  5. Standard sealed lead-acid sizes include 7, 12, 18 and 26 Ah, so choose 18 Ah. A 12 Ah battery is below the requirement.

Answer

14.65 Ah required | use an 18 Ah battery

Common mistakes

  • Forgetting the alarm period or using the alarm current for the full 24 hours.
  • Skipping the margin. Battery capacity falls with age and low temperature, and the manufacturer data governs.
  • Using the wrong standby and alarm periods. The required periods depend on the code edition, the system type (for example, emergency voice) and the authority having jurisdiction.
  • Leaving out the current drawn by auxiliary loads connected to the panel, such as door holders or relays.

Try two variations

  1. Voice evacuation with a 15-minute alarm period: 0.5 × 24 + 2.5 × 0.25 = 12.63 Ah. With a 20% margin that is 15.15 Ah, so an 18 Ah battery still works.
  2. A panel with a higher standby draw of 0.8 A and a 5-minute alarm: 0.8 × 24 + 2.5 × (5 ÷ 60) = 19.41 Ah. With a 20% margin that is 23.3 Ah, so a 26 Ah battery is the first standard size that works.

The standby period is the largest part of the total, which is why a small change in standby current changes the battery size more than a large change in alarm current.

Related tools and labs

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Questions

Which standard sets the 24-hour and 5-minute periods?

NFPA 72 sets the secondary power requirements. Confirm the edition and any local amendments, and use the periods that apply to your system type.

Is the 20% margin required?

It is a common design practice and not a number from the standard. Follow the panel manufacturer's battery calculation sheet and your authority having jurisdiction.

Why does standby current matter more than alarm current?

The standby period is 24 hours and the alarm period is 5 or 15 minutes, so standby draws far more charge. Reducing standby current is the best way to shrink the battery.

Do the batteries need to be replaced on a schedule?

Sealed lead-acid batteries lose capacity with age. Manufacturers and inspection standards set test and replacement intervals, commonly every few years.

Can the panel charge a larger battery?

Only up to the charger rating. If the battery needed is larger than the panel charger can recharge in the required time, add a supplementary power supply with its own charger.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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