Standby Battery and an Added Notification Circuit
An existing 24 V addressable panel serves a three-story office building. Field-measured loads are 0.275 A standby and 1.85 A full alarm. The panel carries two 12 V, 7 Ah sealed lead-acid batteries in series; the date code is five years old. The manufacturer lists a 0.5 A charger and a maximum battery size of 18 Ah. The owner wants to add 14 horn/strobes drawing 0.145 A each on a new notification circuit. The technician proposes to keep the existing batteries because they read 26.4 V at the terminals with a voltmeter.
1. Calculate the required secondary power capacity for the existing system (24-hour standby plus 5-minute alarm, 20 percent derate).
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Commercial occupancy, not a voice system, so the basis is 24 hours standby followed by 5 minutes of alarm.
- Standby: 0.275 A x 24 h = 6.60 Ah
- Alarm: 1.85 A x (5/60) h = 0.154 Ah
- Subtotal: 6.75 Ah
- Aging/temperature derate: 6.75 x 1.20 = 8.10 Ah
So the existing system alone needs about 8.1 Ah of usable capacity. One more subtlety matters when comparing to a nameplate: a "7 Ah" battery is rated at the 20-hour discharge rate. At a 24-hour draw its available capacity is lower than nameplate (roughly 80 percent), so a new 7 Ah battery is worth only about 5.6 Ah on this duty cycle - already below the requirement before any additions or aging.
2. Is the existing battery acceptable? What does the 26.4 V reading prove?
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The existing battery fails for two independent reasons:
- Capacity. A 7 Ah battery does not meet the 8.1 Ah derived requirement at the 24-hour rate, even when new.
- Age. At five years it is at or beyond typical sealed lead-acid service life, and real capacity may be half of nameplate.
The voltmeter reading proves only that the battery is not open or deeply discharged. Open-circuit voltage says nothing about capacity under load, and a sulfated battery can read 26 V and collapse the instant the panel draws alarm current.
The correct evaluation is a load or capacity test at the calculated load for the required duration, done per the manufacturer's instructions and documented. Recommendation: replace with 12 Ah (two 12 V units in series), record the install date on the battery and in the panel, and verify the charger output after replacement.
3. Evaluate the added notification load. What must be checked besides the battery calculation?
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New alarm load: 1.85 + (14 x 0.145) = 1.85 + 2.03 = 3.88 A.
Battery: 6.60 + 3.88 x (5/60) = 6.60 + 0.323 = 6.92 Ah, x 1.20 = 8.31 Ah. A 12 Ah battery covers this, applying the 24-hour-rate caveat.
The battery math is the least likely thing to fail. Check separately:
- Panel and circuit ratings. If the panel's total alarm output or the new circuit's rating is below 3.88 A, the load must be split across circuits or served by a listed booster/auxiliary supply. A battery calculation does not cure an over-current circuit.
- Charger capability. A 0.5 A charger against an 18 Ah battery may not restore the battery within the recharge time required by the listing after a full discharge; check the manufacturer's recharge table rather than assuming.
- Voltage drop. Run the drop calculation on the new circuit and confirm the last appliance sees at least its listed minimum operating voltage (commonly about 20.4 V) with the battery at end-of-discharge voltage, not at a nominal 24 V.
Only after all four checks pass should the added circuit be approved.