The Four-Year-Old Battery Pair and the Fifth NAC
A 24 V addressable panel protects a four-story office building. The system is not a voice system. With all input devices normal, you measure 0.60 A total draw at the batteries. You then trip an alarm and measure 2.40 A, a figure that includes the panel's alarm current and all four notification circuits at full appliance load. The panel carries two 12 V, 18 Ah sealed lead-acid batteries in series. They are dated four years ago and sit in a closet that reaches about 85 °F each August. The owner wants to add a fifth notification circuit and a cellular communicator. The electrical contractor reports the batteries are 'fine because the charger floats at 27.6 V and never faults,' and proposes to skip any further calculation.
1. Compute the required secondary power capacity on the standby/alarm basis and compare it to what is installed. Which time basis applies, and why?
Read the worked answer
Standby: 0.60 A × 24 h = 14.4 Ah Alarm: 2.40 A × (5 ÷ 60) h = 0.20 Ah Subtotal: 14.6 Ah With the 25% aging factor: 14.6 × 1.25 = 18.25 Ah required
Installed capacity is 18 Ah, roughly 99% of the minimum - short before any new load is added.
The 24 h standby plus 5 min alarm basis is the correct one here precisely because the system is not an emergency voice/alarm communication system; a voice system would demand 15 minutes of alarm load instead of 5, which would make the deficit far worse.
Two facts make the shortfall worse than the arithmetic suggests: the batteries are four years old and live in an 85 °F closet. Capacity falls with age and with sustained temperature, so the calculation (which describes a healthy new battery) overstates what is actually installed.
2. The contractor argues that correct float voltage proves the batteries are good. How do you respond, and what evidence would you require?
Read the worked answer
Float voltage proves the charger is working, not that the battery can deliver its rated capacity. A lead-acid battery can sit at 27.6 V, accept charge current, show no charger trouble, and still have lost most of its usable capacity - the failure only appears under discharge.
Evidence that actually supports adequacy:
- A sizing calculation from measured standby and alarm currents, with the safety factor applied.
- A load test conducted per the battery manufacturer's instructions, with terminal voltage recorded under load for the specified duration.
- Verification that the charger can carry the standby load and recharge the battery within the required time after a full discharge.
- A documented installation date and expected service life for the environment it sits in.
'It floats at 27.6 V and never faults' is not evidence and should not appear in a record of completion or an inspection report.
3. The owner proceeds with the fifth circuit and the communicator, and the new loads are 0.65 A standby and 3.30 A alarm. Select a battery and list what else must be verified before the work is signed off.
Read the worked answer
Standby: 0.65 × 24 = 15.6 Ah. Alarm: 3.30 × 0.0833 = 0.275 Ah. Total 15.875 × 1.25 = 19.8 Ah.
A 22 Ah pair satisfies the calculation with little margin; a 26 Ah pair gives useful margin and better tolerance for the hot closet. Take the larger size if physical space permits.
Verify before sign-off:
- Panel alarm current vs. its total and per-circuit NAC ratings. If the fifth circuit pushes the panel past rating, the answer is a booster supply near the load - and that changes the calculation, because the booster's own standby draw and the relocated appliance current move.
- Charger recharge capacity for the larger battery. A bigger battery on an unchanged charger is a classic defect; confirm the charger restores the battery within the required time while carrying the standby load.
- Enclosure fit and listing - the panel's battery-charger listing and the physical space.
- Communicator current counted in both standby and alarm totals, and listed for fire service.
- Documentation - revised battery calculation, updated battery label with install date, and the record of completion.