A Battery Calculation the Technician Says Is Fine Because the Panel Is New
A 24 V addressable control unit protects a five-story, 60,000 sq ft building. Standby loads are: control unit 0.250 A; 42 addressable detectors at 0.6 mA each; 14 monitor modules at 0.8 mA each; 6 control modules at 1.2 mA each. Alarm loads are: control unit 0.300 A; each detector 5 mA; modules 0.012 A total; 32 horn/strobes at 0.135 A each, split across two notification circuits rated 2.0 A each. Design basis is 24 hours standby plus 5 minutes alarm. The installed battery is two 12 V, 7 Ah units in series. The owner now wants 14 more horn/strobes added to the same two circuits, and the technician says the 7 Ah battery is "fine because the panel is only three years old."
1. Perform the standby battery calculation for the existing system and determine whether the installed battery is adequate.
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Standby current:
- Control unit 0.250 A
- Detectors: 42 x 0.0006 A = 0.0252 A
- Monitor modules: 14 x 0.0008 A = 0.0112 A
- Control modules: 6 x 0.0012 A = 0.0072 A
- Total standby = 0.294 A
Alarm current:
- Control unit 0.300 A + detectors (42 x 0.005 = 0.210 A) + modules 0.012 A + NAC load (32 x 0.135 A = 4.32 A) = 4.842 A
Capacity required:
- Standby: 0.294 A x 24 h = 7.06 Ah
- Alarm: 4.842 A x (5/60) h = 0.40 Ah
- Subtotal 7.46 Ah x 1.20 derating factor for battery aging and temperature = 8.95 Ah
Conclusion: the 7 Ah battery is undersized by roughly 2 Ah, about 22 percent short of the required capacity. Battery age is irrelevant to the calculation; a new, undersized battery is still undersized, and a three-year-old one has already lost part of its nameplate capacity. Specify the next standard size up with margin (typically 12 Ah for this duty) and confirm it fits the enclosure and that the charger is listed for that capacity.
2. Evaluate the notification circuit loading both today and with the proposed 14 additional appliances.
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Today: 32 x 0.135 A = 4.32 A across two circuits rated 2.0 A each, for a total circuit capacity of 4.0 A. Total draw already exceeds combined capacity, and unless the split is nearly perfect, one circuit is overloaded - likely both. Alarm current must also stay within the control unit's overall power supply limit, not just the per-circuit rating.
With 14 more: 46 x 0.135 A = 6.21 A, which is 55 percent over the 4.0 A combined NAC capacity and well beyond what the panel can source while also carrying detection and module load.
Why this matters beyond the arithmetic: an overloaded circuit trips or current-limits, dropping voltage along the run until the far appliances no longer produce rated output. The system may appear to work at acceptance and fail at the far end of the corridor under real conditions.
Options: add a listed, fire-alarm-rated auxiliary power supply (booster) for the added notification, or re-circuit onto additional NACs if the panel has spare capacity and its supply can carry the total. Recalculate voltage drop on each circuit after redistribution, and recalculate the battery - the added appliances change the alarm load used in the calculation.
3. If a notification booster supply is added, what must be verified about that supply's power and supervision?
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A booster is not a plug-in accessory - it is part of the fire alarm system and inherits the same power requirements.
- It must be listed for fire alarm service and installed per its listing and the manufacturer's instructions.
- It must have its own secondary power sized for 24 hours standby plus the alarm duration for the load it carries, with the same 1.20 derating applied. Its battery calculation is separate from the control unit's and must be produced and kept with the system documentation.
- Its charger must be rated to restore the battery within the recharge period required by NFPA 72, accounting for both the battery capacity and the continuous load it supplies.
- It must be supervised by the control unit: AC failure, battery failure, and charger trouble must annunciate at the panel as trouble conditions, and the control unit must be able to confirm the booster's health. An unsupervised booster is a single point of silent failure.
- Activation must be reliable - the booster must be triggered by the same alarm signal that drives the panel NACs, and its output circuits must be monitored for open and ground faults.
- Verify enclosure, mounting location, and circuit voltage drop after the booster is introduced, and update the record drawings and battery calculations accordingly.