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) 14.6
- Standby (supervisory) current (A) 0.5
- Alarm current (A) 2
- Alarm period (min) 5
- Derate / aging factor 1.2
You get
- Standby time supported 24 h
Details, formula, and sources
Hs = (battery_Ah/derate - alarm_Ah) / standby_current (NFPA 72 §10.6). A 14.6 Ah battery at 0.5 A standby, 2 A / 5 min alarm, and 1.2 derate holds 24 h; an 18 Ah battery holds about 30 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.
Hs = (battery_Ah / derate - I_alarm * alarm_min/60) / I_standby; the inverse of required_Ah = [(I_standby * Hs) + (I_alarm * alarm_h)] * derate.
Secondary (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.
First-principles amp-hour accounting. The AHJ-adopted NFPA 72 edition, the listed panel, and the battery manufacturer's derating govern.
Estimate. AHJ and licensed fire protection engineer govern.
Field names used by the API: battery_ah, standby_current_a, alarm_current_a, alarm_minutes, derate, standby_hours
- Derate on capacity the aging/derate factor divides the usable capacity (not credited); 1.2 default, >= 1.0 expectedNFPA 72 / panel worksheet
- Alarm reserve first the alarm amp-hour reserve is subtracted before dividing by the standby currentNFPA 72 secondary-power method