Sizing Secondary Power for a Mercantile Panel
An FAS-I technician is commissioning a 24 VDC addressable panel in a new mercantile building. The panel standby load is 0.35 A with the communicator connected; the alarm load is 2.6 A including ten horn/strobes and the panel itself. The specification and the AHJ require 24 hours of standby followed by 5 minutes of alarm. The installer has furnished two 12 V, 7 Ah batteries in series and says, 'the standby load is 0.35 A, so 24 hours is 8.4 amp-hours, and the 7 Ah batteries are close enough.' The panel's charger is rated for 18 Ah maximum. The building has no voice evacuation system. The batteries were manufactured 14 months ago and have never been charged. The final inspection is in two days. The panel is not configured for a 60-hour standby.
1. Calculate the required battery capacity and evaluate the installed batteries.
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Standby: 0.35 A × 24 h = 8.4 Ah. Alarm: 2.6 A × (5 min ÷ 60 min/h) = 0.22 Ah. Subtotal 8.62 Ah. Apply the 20 percent derating for battery aging and end-of-life capacity: 8.62 × 1.2 = 10.34 Ah, so the smallest standard pair that meets the requirement is 12 V, 12 Ah batteries in series (12 Ah). The installed 7 Ah pair provides roughly 5.8 Ah after the same derating, which is less than the 8.62 Ah the load needs even before aging, so the panel will not complete the required standby period near end of life. The installer's comparison omits the alarm load and the derate, and compares a battery rating directly to a load calculation as if they were the same quantity. I would reject the batteries.
2. Besides capacity, what would you check before accepting these batteries?
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First, they must be listed for fire alarm use and be of the type and size the panel manufacturer allows. The charger is rated for 18 Ah maximum, so 12 Ah is within range, but the charger must also be able to recharge them within the time the manufacturer and NFPA 72 require. Second, the batteries are 14 months old and have never been charged; sealed lead-acid batteries left that long can sulfate and lose capacity, so check date codes and either load-test them per the manufacturer or replace them with fresh stock. Third, verify the series connection, polarity, fuse, and terminal torque, and confirm the panel annunciates battery disconnect, low battery (typically about 20.4 V on a 24 V system), and charger trouble. Fourth, record the battery make, model, date code, and calculated capacity on the record of completion.
3. Describe the test you would perform to show the secondary supply works.
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With the system in normal standby and the batteries charged, open the AC circuit breaker and confirm the panel transfers to battery without any trouble other than the expected AC-loss indication, and that the AC-loss trouble is annunciated and transmitted. Then put the system in alarm - activate a notification circuit or use the panel's alarm test - and measure battery voltage at the battery terminals while the full alarm load is applied. Confirm the voltage stays above the panel's minimum operating voltage for the duration and that no appliance drops out. Restore AC, confirm the charger returns to a normal charge rate and the AC trouble clears, and record the readings and times. This verifies transfer and load capability; the 24-hour standby capability is proven by the calculation plus a periodic discharge test, not by a short functional check.