Fire-Alarm / Security Standby Battery
Required battery amp-hours from standby and alarm loads times the aging correction (1.25 per NFPA 72-2022).
Example
You enter
- Standby (supervisory) current (A) 0.5
- Standby period (h) 24
- Alarm current (A) 2
- Alarm period (min) 5
- Aging correction factor (1.25 per NFPA 72-2022) 1.25
You get
- Required capacity (Ah) 15.21
- Next standard battery 18
Checked against published worked examples from: NFPA 72 §10.6 (by name).
Details, formula, and sources
ScopeWith the next standard size (NFPA 72 §10.6).
Formula or tableAh = [(I_standby * h_standby) + (I_alarm * h_alarm)] * derate, with the alarm minutes converted to hours; the next standard battery size is the smallest at or above the requirement.
Source and editionSecondary (standby) battery sizing for a fire-alarm or security control unit - standby amp-hours plus alarm amp-hours times the aging/derate factor - per NFPA 72 National Fire Alarm and Signaling Code §10.6 (secondary power supply) and the panel manufacturer's battery-calculation worksheet, 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 namesstandby_current_a, standby_hours, alarm_current_a, alarm_minutes, derate, required_ah, next_standard_ah
- Aging correction 1.25 default, the NFPA 72-2022 10.6.7.2.1 battery aging correction (editions through 2019 used a 20% margin, 1.2); user-suppliedNFPA 72-2022 / panel worksheet