Free tool
Fire alarm battery calculator
Work out the minimum standby battery capacity for a fire alarm panel, using the BS 5839-1 method.
How it works
The calculator uses the formula from BS 5839-1:
Cmin = 1.25 × (T1 × I1 + D × T2 × I2)
- T1: standby period in hours (usually 24 or 72)
- I1: total quiescent current in amps (panel plus every device)
- T2: alarm period in hours (usually 0.5)
- I2: total alarm current in amps
- D: de-rating factor of 1.75 for the high discharge rate in alarm
- 1.25: allowance for battery ageing
Take the quiescent and alarm currents from the panel and device data sheets. The result is the minimum capacity in amp-hours; always round up to the next standard battery size.
This tool is a guide for competent persons. Always check the panel manufacturer's maximum battery size and charger capability, and confirm the standby period against your fire risk assessment and BS 5839-1.