Battery Calculator

Fire Alarm Panel — AS7240.2

Calculate the required battery capacity for your fire alarm panel based on quiescent and alarm loads. Generate a branded report for your records.

Battery Calculation

Quiescent Load

Alarm Load

Standby Time

Hours

Alarm Time

Hours

Installed Battery Capacity

Used battery (in service)Uses L = 1.1 for an existing battery required to perform for at least another 12 months. Leave unticked for new batteries (L = 1.25).

Battery Charger Optional

Check the panel's charger can recharge a flat battery in time while still running the system.

Based on DFES requirements in Western Australia.

Current Charging Capacity (optional)

The panel / power supply's charging current from its data sheet. Leave blank to just work out the required charging current.

Uses the quiescent and alarm loads above. Recharge figures use — (required capacity — enter the installed battery to use that instead)

Report Details

Site Name

Test Reference

Tested By

Required Capacity

—

Amp-hours (Ah)

C₂₀ — capacity at the 20 h discharge rate

Formula: C₂₀ = 1.25 × ((Standby hrs × Quiescent) + (2 × (Alarm load × Alarm hrs))). C₂₀ is the battery's rated capacity at a 20-hour discharge — the Ah figure printed on the battery.

Required Charging Current

—

Amps (A)

Formula: 1.25 × ((Quiescent × 5) + (2 × (Alarm load × 0.5))) ÷ 24

AS7240.2 — Battery Capacity Requirements

In the event of primary power failure, batteries must maintain the system in quiescent mode for the required standby period, then power two worst-case alarm zones for the required alarm duration. New batteries must provide at least 125% (L=1.25) of calculated capacity to account for 20% capacity loss over the battery's useful life; used batteries required to perform for at least 12 months use L=1.1. A capacity de-rating factor (F_C) of 2 is applied to the alarm load to account for discharge rate characteristics.

Where: C₂₀ = capacity at 20h discharge rate (Ah), I_Q = quiescent current, T_Q = standby time, F_C = de-rating factor (2), I_A = alarm current, T_A = alarm time, L = deterioration factor (1.25 new / 1.1 used)

Battery Charger Requirements — DFES (Western Australia)

Battery capacity sizes the “bucket” (Ah) — how long the system survives a power outage. The charger sizes the “tap” (A) — how quickly it recovers afterwards. Once mains power returns, the charger must return enough charge to cover 5 hours of quiescent load plus 30 minutes of alarm load within 24 hours, while the power supply also carries the alarm load. An undersized charger can take days to recharge large batteries, leaving the system unprotected if a second outage occurs. This charger check is based on DFES requirements in Western Australia and may not apply in other states.

Where: I_C = L × [(I_Q × 5) + F_C × (I_A × 0.5)] ÷ 24 (required charging current, A), Minimum PSU = I_C + I_A, Ah returned in 24 h = charge current × 24 × 0.8 (80% charge efficiency), time to 80% = battery Ah ÷ charge current

Generated using the Essential Fire Services Battery Calculator

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All calculations should be verified by a qualified fire protection technician.