Battery Sizing After Adding Notification Appliances
An existing 24 VDC conventional FACP in a small retail building was sized years ago with a 7 Ah battery. The owner now adds a rear storage addition with six new horn/strobes on the same NAC. The installer measures standby current at 0.35 A and alarm current at 2.8 A after the addition. The panel's listed maximum NAC output is not exceeded, and the panel's charger is rated 0.5 A. The installer wants to keep the existing 7 Ah battery because 'the panel still runs and the battery holds.' The battery is three years old and has never been load-tested. The AHJ is concerned about the required 24-hour standby plus 5-minute alarm duration.
1. Using the measured currents, evaluate whether the 7 Ah battery is adequate for 24 hours standby plus 5 minutes of alarm. Show your calculation and state assumptions.
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Required capacity = standby load for 24 h plus alarm load for 5 min, with a derating/aging factor. Standby: 0.35 A × 24 h = 8.4 Ah. Alarm: 2.8 A × (5/60 h) = 0.233 Ah. Sum = 8.633 Ah. Applying a 1.25 aging/derating factor gives about 10.8 Ah required. A 7 Ah battery is therefore inadequate even before derating (7 Ah < 8.633 Ah), and clearly inadequate with derating. The installer's 'it still runs' test is not a capacity test. The battery must be replaced with one sized to at least the calculated requirement, and the charger must be capable of restoring it. The calculation assumes the measured currents are the true worst-case standby and alarm loads and that the panel's listed battery charging capability is respected.
2. The installer says the charger is fine because it is rated 0.5 A. What charger and battery concerns should the AHJ raise?
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The charger must be able to recharge the battery within the time allowed by the manufacturer's listing and maintain the standby load while doing so. A 0.5 A charger may be marginal or insufficient for a larger replacement battery, especially if the standby load (0.35 A) consumes most of the charger output, leaving little for recharge. The AHJ should require: (1) a battery sized per the calculation with manufacturer-listed maximum; (2) verification that the charger's rated output can both carry standby load and recharge the battery per listing; (3) that the replacement battery does not exceed the panel's listed maximum battery size; (4) a documented load test of the new battery; and (5) that the installation remains within the panel's listed NAC and battery limits. Simply having a '0.5 A charger' does not establish adequacy.