Battery Capacity and Charger Performance at an Annual Inspection
During an annual inspection of a 12,000 sq ft warehouse, the technician opens the fire alarm control unit and finds two 12 V, 7 Ah sealed lead-acid batteries connected in series. The panel is rated for 24 VDC secondary power. The technician measures the standby load at 0.35 A and the alarm load at 1.68 A. The system is not a voice system and is required to provide 24 hours of standby followed by 5 minutes of alarm. The batteries are dated four years old. One battery measures 12.1 V open-circuit after the charger is disconnected; the other measures 12.9 V. The charger output with the batteries connected is 27.4 V. The panel is not reporting a battery trouble. The building owner says the system has had two brief power outages in the past year, and both times the panel stayed normal. The technician must decide whether the batteries are adequate and what to record.
1. Calculate the required secondary power capacity and determine whether the installed batteries are adequate. Show the calculation and state the conclusion.
Read the worked answer
The required capacity is the standby load for 24 hours plus the alarm load for 5 minutes, with an aging margin applied. Standby energy: 0.35 A x 24 h = 8.4 Ah. Alarm energy: 1.68 A x 5/60 h = 0.14 Ah. Total before margin = 8.54 Ah. Applying the commonly required 20% aging margin gives 8.54 x 1.2 = 10.25 Ah.
The installed batteries provide 7 Ah at 24 V, which is less than the 10.25 Ah requirement. The batteries are therefore not adequate even before considering that a 7 Ah battery delivers less than its nominal rating at the higher discharge rate and at end of life. The installer appears to have sized the battery for a shorter standby period or a smaller load. The correct replacement is at least a 12 Ah battery, subject to the manufacturer's discharge data and the panel enclosure space. The calculation should be repeated with the actual alarm current of all notification appliances, including any added since the original installation, and with the manufacturer's derating for the 5-minute alarm period.
2. What do the open-circuit voltage readings and charger voltage indicate, and what actions should the technician take before returning the system to service?
Read the worked answer
The 12.1 V reading on one battery after charger disconnect indicates a weak or failing battery. A healthy fully charged 12 V lead-acid battery normally rests near 12.6 V or higher after surface charge is removed. The 12.9 V reading on the other battery does not rescue the pair because the batteries are in series and the weaker battery limits the string. The 27.4 V charger output is in the normal float range for a 24 V lead-acid string, so the charger is probably trying to charge, but the battery cannot hold the charge. The absence of a battery trouble is not proof of capacity; many panels monitor voltage, not capacity.
The technician should replace both batteries as a matched pair, not just the weak one. Before replacement, verify that the charger is rated for the new battery capacity and that the enclosure can accommodate the larger batteries. After replacement, confirm the charger returns to float voltage, perform a load test or capacity test per the manufacturer's instructions, and verify that a battery trouble annunciates when the batteries are disconnected. The technician should also check that the panel's low-battery threshold and charger settings match the new battery type. If the batteries cannot be replaced immediately, the deficiency should be reported in writing and the owner should be informed of the reduced secondary power capacity.
3. What should be recorded on the inspection report, and how should the deficiency be communicated and tracked?
Read the worked answer
The report should record the battery make, model, date code, and quantity; the measured standby and alarm currents; the calculated 24-hour standby and 5-minute alarm capacity; the 20% margin result; the open-circuit voltages; the charger float voltage; and the conclusion that the installed 7 Ah batteries are inadequate. It should state that the panel did not annunciate a battery trouble and explain why that does not demonstrate capacity. The report should also list the required corrective action, such as replacement with 12 Ah batteries or larger, and any charger or enclosure limitations.
The deficiency should be written on the inspection report and provided to the owner or responsible party, with a copy retained by the contractor. The owner should sign an acknowledgement of the deficiency, especially if replacement is deferred. The contractor should schedule a follow-up visit, and the replacement should be documented with the new battery date code, the repeat calculation, and a load test result. If the system remains on inadequate secondary power, the owner should be advised of the risk during a power outage and the contractor should follow the applicable impairment and notification procedures. The record should be retained with the inspection file so the next annual inspection can verify that the correction was made.