Battery Capacity and Voltage Drop on a 24 VDC FACP
A 24 VDC addressable fire alarm control panel protects a four-story office building. The measured standby load is 0.145 A, and the measured alarm load is 0.920 A. The panel uses two 7 Ah sealed lead-acid batteries in series. The charger is rated 0.5 A. The design basis is 24 hours of standby followed by 5 minutes of alarm, with a 20 percent aging derate. The battery terminal voltage at the end of discharge is 20.4 V. The battery wiring has a calculated 0.25 V drop at alarm current. The contractor's battery calculation sheet states that 7 Ah is adequate because 0.145 A times 24 hours equals 3.48 Ah and the alarm period is only 5 minutes. The building has no central station connection and is a local alarm only. The AHJ asks for a revised calculation and asks whether 60 hours of standby is required for this business occupancy.
1. Calculate the required battery capacity and determine whether the 7 Ah batteries are adequate for the stated 24 hour standby plus 5 minute alarm basis.
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Use the stated design basis and apply the aging derate to the total load. Standby energy is 0.145 A times 24 h, which equals 3.48 Ah. Alarm energy is 0.920 A times 5 minutes divided by 60 minutes per hour. Five minutes is 0.0833 h, so the alarm energy is about 0.0767 Ah. The undrated total is 3.5567 Ah. Applying the 20 percent aging derate means multiplying by 1.20, which gives about 4.27 Ah. The 7 Ah battery has more capacity than the calculated 4.27 Ah, so it is adequate for the 24 hour plus 5 minute basis, assuming the battery is listed for the application and the panel's charging and voltage limits are met. The contractor's sheet is incomplete because it omits the alarm load and the derate, even though the conclusion happens to be acceptable for this basis.
2. Is 60 hours of standby required for this business occupancy? Show how the answer changes the battery selection.
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Not automatically. A 60 hour standby basis is typically required only where the adopted code, the occupancy type, or the system design imposes it, such as certain central station or proprietary systems, or where the AHJ adopts a longer requirement. For a local alarm system in an ordinary business occupancy, the common design basis is 24 hours unless the applicable code says otherwise. If 60 hours were required, the calculation would be 0.145 A times 60 h, which equals 8.7 Ah, plus the 0.0767 Ah alarm load, for 8.7767 Ah undrated. Applying the 1.20 derate gives about 10.53 Ah. The 7 Ah battery would then fail, and a larger battery or a different design basis would be required. The AHJ should require the designer to state the code basis for 24 or 60 hours rather than assume it.
3. The battery voltage at end of discharge is 20.4 V and the wiring drop is 0.25 V. What should the reviewer check?
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The reviewer should check the panel's listed operating voltage range at the battery terminals and at the load. If the battery reaches 20.4 V at end of discharge and the wiring drops another 0.25 V, the panel may see only about 20.15 V during alarm, which can be below the minimum operating voltage of some listed control units. That is a separate failure from capacity. The designer should provide a voltage drop calculation for the battery wiring and for notification circuits, use larger conductors or shorten the run if needed, and confirm that the panel remains within its listed range at the end of the standby period. The charger rating of 0.5 A also should be checked against the battery manufacturer's recharge requirements and the panel listing. Capacity alone does not prove compliance.