Battery Sizing After a Panel Load Grew Over Ten Years
You are inspecting a 24 V addressable fire alarm control unit serving a non-voice, protected-premises commercial building. The unit is equipped with two 12 V, 18 Ah sealed lead-acid batteries dated six years ago. The original calculation sheet, still in the panel, lists 0.180 A standby and 1.10 A alarm. Since then, a wing was added, two notification appliance circuits were extended, and six devices were installed. You measure with a calibrated meter: 0.620 A steady standby after the panel stabilizes, and 3.20 A with all notification appliances sounding. The system is not voice evacuation, so the required alarm duration is the standard 5 minutes. The charger is the original unit. The panel has been on normal AC with no battery trouble, and the owner points to that as proof the batteries are fine.
1. Calculate the required battery capacity and determine whether the installed batteries comply. Show each step.
Read the worked answer
Use the standby-plus-alarm method with a 20 percent aging/derating factor applied to the computed capacity.
Standby energy: 0.620 A times 24 h equals 14.88 Ah. Alarm energy: 3.20 A times (5 min divided by 60 min per hour, i.e., 0.0833 h) equals 0.267 Ah. Subtotal: 14.88 plus 0.267 equals 15.15 Ah. With the 1.20 derating factor: 15.15 times 1.20 equals 18.18 Ah required.
The installed pair provides 18 Ah, so the arithmetic shows a shortfall of about 0.18 Ah, roughly one percent. That looks trivial, but the margin is worse in practice. Published capacity is quoted at the 20-hour discharge rate; at a 3.2 A alarm draw the effective delivered capacity is lower than the nameplate figure, and the batteries are six years old, so their real capacity is well below 18 Ah. The conclusion is non-compliance, and the owner's "no battery trouble" argument is irrelevant, since a battery trouble indication tests the charger and charging circuit far more than it tests capacity under load.
2. What are the acceptable corrective options, and what must be verified for each?
Read the worked answer
Three routes exist, and each has a verification step that is often skipped.
First, reduce the load: trim unnecessary standby draw and confirm the alarm current with all appliances active, then recompute. Only accept this if the measured numbers, not the cut sheets, support it.
Second, increase capacity: install batteries sized to the computed 18.18 Ah with margin (typically a 33 Ah pair for headroom and aging), and verify the charger is listed and rated to recharge that capacity within the required time. A larger battery on an unchanged charger produces a system that cannot recover after a full-duration event, so the charger must be checked against the battery manufacturer's charging current and voltage.
Third, add a second power supply or battery cabinet if the existing enclosure cannot hold the larger batteries.
In every case, replace the six-year-old batteries regardless of the calculation, since they are at or beyond typical VRLA service life, perform a load test on the replacement set, update the calculation sheet in the panel with the measured currents and the date, and record the battery date of manufacture on the maintenance tag.
3. How would you address the discrepancy between the original calculation sheet and your measurements in the report?
Read the worked answer
State both sets of numbers and explain the gap. The original sheet shows 0.180 A standby and 1.10 A alarm; the field measurements show 0.620 A and 3.20 A. That is roughly a 3.4-fold increase in standby and a 2.9-fold increase in alarm current, consistent with a wing addition, extended notification circuits, and six added devices without a re-run of the battery calculation.
Record the meter used, the conditions of measurement (steady state, all appliances sounding), and the calculated capacity. Identify the deficiency as an inadequate secondary supply for the present load, not as a failed battery, because these are different findings with different corrective actions.
Note also that the charger was never evaluated for the increased load. Recommend that the owner's service provider re-run the calculation after any future change, since the pattern here is that additions were made without revisiting the power budget. The report should make clear that the annual inspection finding is a design and power-supply deficiency, and that correction requires both new batteries and a charger verification, with the replacement documented before the next inspection.