Standby Battery Runtime from Capacity
The standby (supervisory) hours an installed fire-alarm or security battery supports before the alarm load.
Example
You enter
- Installed battery capacity (Ah) 18
- Standby (supervisory) current (A) 0.5
- Alarm current (A) 2
- Alarm period (min) 5
- Aging correction factor (1.25 per NFPA 72-2022) 1.25
You get
- Standby time supported 28.5 h
Checked against published worked examples from: DSC (Digital Security Controls, Johnson Controls).
Details, formula, and sources
ScopeHs = (battery_Ah/derate - alarm_Ah) / standby_current (NFPA 72 §10.6). At 0.5 A standby and 2 A / 5 min alarm, an 18 Ah battery holds about 28.5 h under the NFPA 72-2022 1.25 aging correction; a 14.6 Ah battery that held 24 h at the older 1.2 margin now holds 23 h. Answers 'how long will this battery last' instead of the size for a required time. The derate divides the usable capacity; a battery too small for even the alarm reserve is flagged. The AHJ and the panel worksheet govern.
Formula or tableHs = (battery_Ah / derate - I_alarm * alarm_min/60) / I_standby; the inverse of required_Ah = [(I_standby * Hs) + (I_alarm * alarm_h)] * derate.
Source and editionSecondary (standby) battery runtime for a fire-alarm or security control unit - the sizing worksheet solved for the standby period - per NFPA 72 National Fire Alarm and Signaling Code §10.6 (secondary power supply), by name.
Free accessFirst-principles amp-hour accounting. The AHJ-adopted NFPA 72 edition, the listed panel, and the battery manufacturer's derating govern.
What governsEstimate. AHJ and licensed fire protection engineer govern.
API field namesbattery_ah, standby_current_a, alarm_current_a, alarm_minutes, derate, standby_hours
- Derate on capacity the aging correction divides the usable capacity (not credited); 1.25 default per NFPA 72-2022 (1.2 through 2019)NFPA 72-2022 / panel worksheet
- Alarm reserve first the alarm amp-hour reserve is subtracted before dividing by the standby currentNFPA 72 secondary-power method