Aging and Undersized Secondary Batteries
During the annual inspection of a 24-volt addressable fire alarm control unit in a small office building, you check the secondary power supply. The panel has two 12-volt, 18 Ah sealed lead-acid batteries connected in series, dated four years ago. The charger output measures 27.1 VDC. The battery open-circuit voltage is 24.8 VDC. You perform a load test at 6.0 A for five minutes; battery voltage drops to 19.5 VDC and recovers to 23.9 VDC after the load is removed. The panel's battery calculation, dated at installation, shows a required capacity of 24 Ah for 24 hours of standby plus 5 minutes of alarm. The owner says the batteries 'still work' because the panel has no trouble indication. The panel is monitored by a central station, and the building has no sprinkler system.
1. Interpret the battery load test and recommend action.
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The battery failed the load test. A 24-volt lead-acid battery at 6 A dropping to 19.5 V and recovering to only 23.9 V shows high internal resistance and reduced capacity. The open-circuit voltage of 24.8 V looks normal, which is why a voltage check alone is misleading. Recommend replacement. Also check the charger; 27.1 V is a reasonable float voltage, but the battery is four years old and has been stressed. Retest after replacement.
2. What does the battery calculation reveal about the installed batteries?
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The calculation requires 24 Ah for 24-hour standby plus 5 minutes of alarm. The installed batteries are 18 Ah, so they are undersized even when new. NFPA 72 also requires the battery to be sized with a margin above the calculated load, so 18 Ah cannot be justified. Replacing with another 18 Ah battery would repeat the error. The correct action is to install batteries that meet or exceed the calculated capacity, including the required margin, and update the calculation if the load has changed.
3. The owner refuses replacement because the panel shows no trouble. How do you respond?
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A lack of trouble indication only means the charger is present and the battery voltage is above the trouble threshold; it does not prove capacity. The load test is the evidence. Explain the risk: during a power outage the panel may fail before the required standby period, leaving the building without fire detection. Document the failed test, the undersized batteries, and the owner's refusal in writing. Notify the monitoring company and, where required, the authority having jurisdiction. A system with inadequate secondary power is not compliant.