Battery Sizing for a 24-Hour Standby With 5-Minute Alarm
You are designing the secondary (battery) power for a new 24 VDC addressable panel serving a small office building. The panel's published standby current is 0.180 A and its alarm current is 0.520 A. Connected NAC appliances draw an additional 1.40 A in alarm only. The authority having jurisdiction requires 24 hours of standby followed by 5 minutes of alarm. You select two 12 V, 18 Ah sealed lead-acid batteries in series. A colleague argues that because 18 Ah exceeds the 24-hour standby draw (0.180 A x 24 h = 4.32 Ah), the batteries are obviously adequate and no further calculation is needed. The installation must also account for the battery's derating and aging.
1. Compute the required battery capacity using the standard standby-plus-alarm method, and determine whether the 18 Ah batteries are adequate.
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Use the standard method: required capacity = (standby current x standby hours) + (alarm current x alarm hours), then apply a derating/aging factor (commonly 1.20, i.e., 20% margin).
- Standby: 0.180 A x 24 h = 4.32 Ah
- Alarm: total alarm current = panel 0.520 A + NAC 1.40 A = 1.92 A; for 5 min = 1.92 A x (5/60 h) = 0.16 Ah
- Subtotal = 4.32 + 0.16 = 4.48 Ah
- With 1.20 aging/derating factor: 4.48 x 1.20 = 5.38 Ah required
An 18 Ah battery pair provides ample capacity (5.38 Ah required vs. 18 Ah available), so the batteries are adequate - but the colleague's reasoning is wrong. The standby-only figure (4.32 Ah) ignores the alarm period and the derating factor. The correct conclusion happens to match, but only because the battery is generously oversized. The calculation, not the shortcut, is what must be documented.
2. The panel's listed battery-charger output is 0.75 A maximum. After a full 24-hour discharge, roughly how long to recharge, and does this create any code concern?
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Recharge time is governed by the charger's ability to replace the energy withdrawn plus maintain the load. A rough estimate: the battery must recover the standby energy used (about 4.48 Ah, ignoring the small alarm portion) plus losses. At a 0.75 A charger output, with part of that current serving the panel's continuous standby load (0.180 A), the net charging current is roughly 0.75 - 0.18 = 0.57 A. Time to replace ~4.48 Ah at 0.57 A is about 7.9 hours, plus the extra needed to reach full charge and account for charging inefficiency - realistically well under the 24-hour recharge expectation but not instantaneous.
The concern to flag: a charger sized too small relative to the battery can fail to restore full charge within the required recharge window, leaving the system under-protected after an extended outage. Confirm the charger's published recharge capability matches the selected battery and the code's recharge requirement, and document the calculation. The judgment point is that battery Ah and charger ampacity must be evaluated together; a large battery on a small charger is not automatically a good design.
3. During acceptance testing the batteries read 26.4 V open-circuit after charging. A helper says this means they are overcharged and should be removed. Evaluate.
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For a 24 V system using two 12 V lead-acid batteries in series, a healthy float voltage is typically around 27.0–27.6 V at room temperature (about 13.5–13.8 V per battery), and a rested open-circuit reading near 26.4 V (13.2 V per battery) is normal and healthy - not evidence of overcharge. Overcharge would show as excessive gassing, heat, electrolyte loss, or a float voltage well above the manufacturer's specified range, not a normal rested reading.
So the helper's conclusion is incorrect. The correct action: compare the measured voltage to the manufacturer's published float and recharge voltage ranges at the ambient temperature, verify the charger is in float (not equalize) mode, and load-test the batteries to confirm they deliver the required standby/alarm current. Voltage alone does not prove capacity; a load test does. The reasoning being tested is that a single open-circuit voltage reading must be interpreted against the manufacturer's specification and the battery chemistry, not against a guess.