Calculate minimum battery capacity for fire alarm systems (BS 5839)
i
mA
mA
Enter values and click 'Calculate'
Calculation Parameters:This calculation uses the BS 5839 formula with the following parameters: 24-hour standby, 30-minute alarm duration, 1.75 derating factor, and 1.25 aging factor.
⚠️ Disclaimer: This calculator is provided as a guidance tool only. Always verify calculations independently and consult current BS 5839 standards. Professional fire alarm system design should be undertaken by qualified engineers.
BS 5839-1 systems are typically designed so that, if the mains fails, the batteries can sustain the system for 24 hours standby followed by 30 minutes in full alarm.
Where a building is not continuously manned, guidance commonly increases the standby expectation to 72 hours before the alarm period — reflecting the likelihood that a mains failure may go unnoticed for longer in an unoccupied building.