BS 5839-1:2017 battery capacity formula
Under British Standard BS 5839-1:2017 Section 25.4, standby batteries for fire detection and fire alarm systems must be sized using the formula: C_min = 1.25 × (I_q × t_q + 1.75 × I_a × t_a).
I_q is quiescent standby supervisory current (Amperes), t_q is standby duration in hours (standard 24 hours for occupied premises, or 48 to 72 hours for buildings unmanned over weekends), I_a is full alarm current (Amperes), and t_a is alarm duration (0.5 hours = 30 minutes standard).
The 1.25 multiplier compensates for battery aging over its 4-year service life (providing an 80% end-of-life capacity margin), while the 1.75 multiplier accounts for temperature variations and reduced battery efficiency during high-rate discharge.
Worked example: Category L2 commercial system
A fire alarm panel with 200 mA (0.20 A) quiescent load and 1.5 A full alarm load requiring 24 hours standby plus 30 minutes (0.5 h) alarm:
Standby load = 0.20 A × 24 h = 4.80 Ah.
Alarm load = 1.75 × 1.50 A × 0.5 h = 1.31 Ah.
Minimum capacity C_min = 1.25 × (4.80 + 1.31) = 7.64 Ah.
A standard commercial 12 Ah sealed lead-acid (VRLA) battery is recommended.
Inputs for the BS 5839 fire alarm battery calculator
Quiescent current (mA)
Normal non-alarm supervisory current of control panel, detectors, and modules in mA.
Alarm current (mA)
Total sounder, beacon, and relay current active during a fire alarm in mA.
Standby period
24 hours for normally occupied buildings, or 48 to 72 hours for unmanned buildings.
CALCULATOR QUESTIONS
BS 5839 fire alarm standby battery calculator questions
What is the BS 5839-1 fire alarm battery formula?
Cmin = 1.25 × (Iq × tq + 1.75 × Ia × ta), where Iq is quiescent current, tq is standby hours, Ia is alarm current, and ta is alarm hours.
How often must fire alarm batteries be replaced in the UK?
BS 5839-1 recommends replacing sealed lead-acid (VRLA) fire alarm standby batteries every 4 to 5 years, or immediately upon failing an annual battery load discharge test.