Power Supplies, Batteries, and Secondary Power Calculations
Primary and secondary power
The control unit needs a dedicated, labeled branch circuit (the primary supply) and a secondary supply - sealed lead-acid or nickel-cadmium batteries, or another listed source. Batteries must carry the standby load for 24 hours and then the alarm load for 5 minutes; some applications, notably emergency voice/alarm communications, require a longer alarm period, so verify the applicable table in the adopted edition. A listed charger sized for the load plus a recharge margin maintains the battery, and the system must annunciate charger failure, battery depletion, and disconnect.
The calculation
Ah = [(standby current x 24 h) + (alarm current x alarm minutes / 60)] x 1.20
The 1.20 factor is the required capacity margin. Use current draws at the end of the standby period, when voltage is lowest and current highest - not the 24 V nominal value. Include every device at its alarm setting plus the panel's own alarm draw.
Worked example
Standby 0.45 A for 24 h = 10.8 Ah. Alarm 2.6 A for 5 min = 0.217 Ah. Subtotal 11.02 Ah x 1.2 = 13.2 Ah, so specify a 14 Ah or larger listed battery. If the same panel drove voice evacuation for 15 minutes, the alarm term becomes 0.65 Ah for a 13.7 Ah result - still 14 Ah, but the margin narrows.
Traps
- Forgetting the 20 percent margin or using nominal-voltage currents.
- Sizing on quiescent current only.
- Omitting notification appliance current from the alarm load.
- Specifying a battery the charger cannot recharge in the required time, or ignoring battery age criteria.
Rule of thumb
Any added device changes the alarm load, the voltage drop, and the required battery - recalculate all three.