How do I calculate the standby battery for a fire alarm panel?

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  • November 10, 2026 at 9:55 am #5536
    MEP Center EngineeringMEP Center Engineering
    Keymaster

    Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

    A fire alarm panel draws 0.45 A in quiescent condition (panel, detectors and modules) and 2.8 A in full alarm (including sounders). The project needs 24 h standby followed by alarm.

    • What formula should I use under BS 5839-1 / EN 54-4, and what changes under NFPA 72?
    • Which safety factors are applied?
    • How do I size the charger, and what is often overlooked?
    November 10, 2026 at 9:55 am #5537
    MEP Center EngineeringMEP Center Engineering
    Keymaster

    Under BS 5839-1: C = 1.25 × (T1 × I1 + D × I2 × T2), with T2 = 0.5 h and D = 1.75. For this panel that gives 16.6 Ah, so use two 12 V 18 Ah batteries. Under NFPA 72 (24 h plus 5 min alarm plus a 20 % margin) the result is 13.2 Ah, and the same 18 Ah battery is a sensible choice.

    BS 5839-1 method

    • T1: standby time, 24 h (longer, often 72 h, where a fault may go unnoticed, for example in buildings not continuously occupied and without remote monitoring).
    • I1: quiescent current, 0.45 A.
    • I2: alarm current, 2.8 A. T2: 0.5 h.
    • D = 1.75: derating for the high discharge rate in alarm. 1.25: ageing factor.

    C = 1.25 × (24 × 0.45 + 1.75 × 2.8 × 0.5) = 1.25 × (10.8 + 2.45) = 16.6 Ah. Next standard size: 17 or 18 Ah. Choose 18 Ah for a small reserve.

    NFPA 72 method

    • 24 h standby, then 5 min in alarm (15 min for an emergency voice/alarm communication system).
    • Capacity: 24 × 0.45 + 2.8 × 5/60 = 10.8 + 0.23 = 11.0 Ah.
    • Add the 20 % safety margin: 11.0 × 1.2 = 13.2 Ah. Next standard size above 12 Ah: 17 or 18 Ah.

    Charger and recharge

    EN 54-4 and BS 5839-1 require recharge to 80 % within 24 h. NFPA 72 requires recharge within 48 h. Rough charger current: 0.8 × 18 / 24 = 0.6 A for recharge, plus the 0.45 A quiescent load, about 1.05 A. Check that the panel’s power supply is rated for this and for the largest battery it allows.

    Where the current figures come from

    • Quiescent: panel, each loop device (sum from datasheets), remote displays, interface modules and relays held energised.
    • Alarm: the above plus every sounder, beacon and voice-alarm load, and door-holder relays if they draw current in alarm. Many beacons draw more than the sounders.
    • Loads powered by separate power supplies need their own battery calculations. Do not forget them.

    Common mistakes

    • Leaving out the 1.25 and 1.75 factors, which halves the battery size.
    • Using typical device currents instead of the maximum alarm currents.
    • Choosing a battery larger than the panel’s charger is rated for.
    • Battery enclosures in hot locations: capacity and life fall quickly above 25 °C.

    Step-by-step article: Fire Alarm Standby Battery Calculation. Voice-alarm loads: Voice Alarm Speaker Spacing and Intelligibility.

    For deeper study: Fire Detection and Alarm System Design course.

    Check the edition of the code that applies to your project.

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