Battery Capacity After a Hotel System Expansion
A 24 VDC addressable fire alarm control unit with voice evacuation serves a 14-story hotel. During the annual inspection, you review the secondary power supply. The panel is equipped with two 7 Ah sealed lead-acid batteries. The contractor's battery calculation sheet, dated at installation five years ago, lists 0.120 A standby and 1.10 A alarm. However, the system now includes 48 additional addressable monitor modules and 12 remote annunciators added during a renovation. You measure the standby current at 0.34 A and the alarm current at 2.9 A, including the voice evacuation load. The panel charger is rated at 1.5 A. The contractor says the batteries are fine because they are the same size as originally specified and the charger keeps them charged. The system requires 24-hour standby followed by 15 minutes of alarm.
1. Perform a battery capacity check and determine whether the existing batteries are adequate.
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Calculate the required capacity: standby 0.34 A times 24 h equals 8.16 Ah. Alarm 2.9 A times 0.25 h equals 0.725 Ah. Total equals 8.885 Ah. Apply the 20 percent reserve: 8.885 times 1.20 equals 10.66 Ah, or 8.885 divided by 0.80 equals 11.1 Ah depending on the method used. The existing 7 Ah batteries cannot supply this load. They are undersized even before aging. Replace them with batteries rated at least 12 Ah, verify the charger can recharge them, and update the battery calculation to reflect the added modules and annunciators.
2. What other checks are required for the secondary power supply during the inspection?
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Verify the charger output voltage and current, battery terminal condition, battery date and physical condition, and that the batteries are the correct type. Test the system on battery under standby load and then under alarm load; confirm the panel does not drop out. Check that the charger can recharge the batteries within the required time after a discharge. Also verify that the battery calculation includes all loads, including voice evacuation, and that the charger is listed for the battery size. Record the measured currents and voltages.
3. The contractor argues the batteries have lasted five years and the system has never failed. How do you respond?
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Past performance is not proof of capacity. Lead-acid batteries lose capacity with age, and a five-year-old battery may have significantly less than its nameplate rating. The measured load already exceeds the 7 Ah battery's required capacity, so a 24-hour outage plus a 15-minute alarm could exhaust the battery even if it has not failed yet. The correct approach is to replace the batteries, recalculate, and load-test. The inspection should not pass based on age or on the charger maintaining the batteries.