Fire Alarm Battery and Notification Circuit After Added Horn/Strobes
A 24 VDC fire alarm control panel in a Middletown, Delaware, medical office building uses two 12 V, 7 Ah sealed lead-acid batteries in series. The system has 24-hour standby and 5-minute alarm. Calculated standby load is 0.22 A; alarm load is 3.4 A after six horn/strobes were added. The batteries are three years old. The panel charger is rated 1.5 A. On the longest notification appliance circuit, 16 AWG conductors run 220 ft one-way to the last appliance. The panel terminals measure 23.8 V in alarm; the last horn/strobe is rated minimum 16 V and draws 1.8 A. During the monthly test the panel went into low-battery alarm after about four minutes of alarm. The owner asks whether replacing the 7 Ah batteries with the same size will solve the problem and whether the new horn/strobes are causing a voltage drop issue.
1. Calculate the required battery capacity with a 20 percent margin and evaluate the 7 Ah batteries.
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Standby energy = 0.22 A x 24 h = 5.28 Ah. Alarm energy = 3.4 A x (5 min / 60) = 0.283 Ah. Total = 5.563 Ah. With a 20 percent margin: 5.563 x 1.2 = 6.68 Ah. Two 12 V, 7 Ah batteries in series provide 24 V but still only 7 Ah capacity. The calculated requirement is 6.68 Ah, so a new 7 Ah battery is only slightly above the calculated number. At a 3.4 A discharge rate, the effective capacity of a small 7 Ah battery is reduced by high-rate discharge, and these batteries are three years old. Replacing with the same size may restore operation temporarily but leaves almost no margin. I would specify 12 Ah batteries, verify they fit the enclosure, and confirm the charger is listed for the larger capacity.
2. Calculate the steady-state voltage drop on the 220 ft 16 AWG notification circuit and determine whether the last appliance receives its minimum voltage. What else must be checked?
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16 AWG copper is about 4.0 ohms per 1,000 ft. The loop is 440 ft, so R = 4.0 x 0.440 = 1.76 ohms. At 1.8 A, drop = 1.8 x 1.76 = 3.17 V. Voltage at the last appliance = 23.8 - 3.17 = 20.63 V, which is above the 16 V minimum in steady state. The circuit is acceptable for running current. However, strobes and horns have inrush current and synchronization loads; check the manufacturer's inrush and minimum operating voltage. If inrush causes the last appliance to fall below 16 V, use 14 AWG or split the circuit. For comparison, 14 AWG at 440 ft has about 1.11 ohms, so a 5 A inrush would drop 5.55 V, leaving 18.25 V. The low-battery alarm is not explained by steady-state NAC voltage drop.
3. What final recommendation and testing/record-keeping steps would you give the owner?
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Replace the batteries with 12 Ah sealed lead-acid units listed for fire alarm use, and do not mix old and new batteries. Verify the panel charger can recharge the larger batteries while carrying the standby load, and confirm the battery enclosure is rated for the size. Perform the manufacturer's battery calculation using the actual alarm load, including the new horn/strobes. Test 24-hour standby followed by 5 minutes of alarm, or the test method required by the AHJ and NFPA 72. Record battery replacement date, calculated load, charger voltage, and test results in the inspection log. If the NAC inrush calculation shows insufficient voltage, correct the circuit before final acceptance.