Fire alarm engineer inspecting an alarm panel in a commercial building

BS 5839-1:2025 Clause 24 and Annex E

Power & Batteries

Standby duration, alarm duration, battery-capacity variables and supply fundamentals.

Standby capacity method

Cmin = k × (I₁ × T₁ + I₂ × T₂ × D)

I₁ / T₁: Quiescent current in amperes and the required standby period in hours.
I₂ / T₂: Maximum alarm current in amperes and alarm period in hours.
k: An ageing/temperature allowance selected for the battery and design.
D: Alarm-current derating from manufacturer data or the Annex E method.

The result is minimum new capacity at the stated rate and temperature. Select the next suitable available size and verify manufacturer data.

Open related calculator

Typical Category M/L duty

Standby: At least 24 hours in the usual case; some premises require longer.
Alarm after standby: At least 30 minutes of the required evacuation signal.
Supply arrangement: Normal and standby supplies should each sustain the system’s maximum alarm load.

Source context: BS 5839-1:2025 Clause 24 and Annex E. This page paraphrases the app’s quick-reference material and does not reproduce or replace the standard.