Battery Sizing for a 24 V Panel After Adding Notification Circuits
A 24 V addressable panel has a 4 A main power supply. Existing standby load is 1.1 A; alarm load is 2.0 A. The owner adds two notification appliance circuits drawing 0.9 A each in alarm, and adds 12 smoke detectors drawing 0.02 A each in standby. The panel's listed battery charger provides 0.5 A maximum charging current. Required standby is 24 hours, followed by 5 minutes of alarm. The installer proposes two 12 V, 18 Ah batteries in series. The panel's low-battery cutoff is 20.4 V, and the battery must be derated for aging and end-of-life capacity.
1. Compute the required battery capacity and determine whether the proposed 18 Ah set is adequate.
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New standby load: 1.1 A + (12 x 0.02 A) = 1.34 A. New alarm load: 2.0 A + (2 x 0.9 A) = 3.8 A, which is under the 4 A supply, so no load-shedding is needed.
Required capacity = standby load x standby hours + alarm load x alarm hours. Using the 5-minute alarm: 3.8 A x (5/60 h) = 0.317 Ah. Standby: 1.34 A x 24 h = 32.16 Ah. Subtotal = 32.48 Ah.
Apply a derating factor (commonly 1.20 for aging/end-of-life, per the manufacturer's instructions). 32.48 x 1.20 = 38.98 Ah. The proposed 18 Ah set is grossly inadequate; roughly 40 Ah is needed. Two 12 V batteries in series give 24 V but do not increase Ah, so the installer must select larger batteries or more parallel strings as permitted by the listing.
Also verify recharge: the charger must restore the battery within the code-required recharge time. With a 0.5 A charger, a 40 Ah bank would take far longer than the typical 24-hour recharge requirement, so the charger and battery pair must be evaluated together; the design may need a larger charger or a smaller verified load.
2. What if the actual alarm duration required by the AHJ is 15 minutes rather than 5?
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Recalculate the alarm portion: 3.8 A x (15/60 h) = 0.95 Ah. Adding to 32.16 Ah standby gives 33.11 Ah; with the 1.20 factor, 39.73 Ah. The change is small relative to standby, which is why standby dominates in 24-hour systems. The key lesson: the alarm duration is a project-specific requirement and must be confirmed in writing, not assumed. The battery selection should be documented with the exact standby and alarm durations, the derating factor, and the charger's ability to recharge within the required time.