Standby Battery Sizing at Retail Handover
A 1,000 m² single-storey retail unit is being handed over with a 12-device addressable fire panel. The technician measured 310 mA standby current at 24 V with all devices connected and the AC supply simulated as failed, and 3.10 A with every notification appliance sounding. The panel carries two 12 V, 7 Ah batteries in series, dated 14 months ago. The agreed design basis is 24 h standby, 5 min full alarm, plus a 20% allowance for battery aging. The site technician argues the batteries are "basically new" and that no battery trouble has appeared in a week of testing, because mains power has never been off for more than a few minutes. The owner wants the handover certificate signed today so the final payment can be released.
1. Using the measured currents, determine whether the installed batteries satisfy the agreed design basis. Show the calculation and state the shortfall.
Read the worked answer
- Standby: 0.310 A × 24 h = 7.44 Ah
- Alarm: 3.10 A × (5 ÷ 60) h = 0.26 Ah
- Subtotal: 7.44 + 0.26 = 7.70 Ah
- With the 20% aging allowance: 7.70 × 1.20 = 9.24 Ah required
Two 12 V batteries in series add voltage, not capacity, so the installed bank delivers only 7 Ah at 24 V. That is roughly 2.2 Ah short - the installation does not meet the agreed design basis, even though the batteries are relatively new and no trouble has been logged.
The technician's reasoning confuses "no observed failure" with "adequate capacity." A battery only demonstrates standby capacity under a genuine full-duration discharge; a week of short mains interruptions tests nothing except the charger.
2. Recommend a remedy and list the checks you would complete before ordering replacement batteries.
Read the worked answer
- Select the next standard capacity at or above 9.24 Ah - a 12 Ah pair gives useful margin, but the choice is not automatic.
- Confirm the panel manufacturer's maximum permitted battery capacity and the charger's rated output; an oversized bank that the charger cannot recharge within its listed time is a new defect, not a fix.
- Check physical space in the panel enclosure, or specify a listed remote battery enclosure.
- Confirm the batteries are listed for fire alarm use and are a matched pair from the same batch.
- Verify the panel's low-battery and battery-missing thresholds still operate correctly with the larger bank.
- Update the battery calculation in the as-built documents and record the new install date on the battery label.
Reasoning: capacity, charger compatibility, and enclosure are a single system. Solving only the arithmetic is the common failure mode here.
3. The owner asks you to sign the handover certificate now and resolve the batteries at the next service visit. How do you respond?
Read the worked answer
- Do not certify the system as compliant; the deficiency is measurable and documented.
- Issue a written deficiency notice to the owner and the general contractor, stating the calculated requirement, the installed capacity, and the risk (loss of detection and notification during an extended mains failure).
- Offer two concrete paths: replace the batteries before handover, or hand over with a documented, time-limited impairment that the owner accepts in writing and that is managed under the site's impairment procedure.
- Confirm whether the monitoring provider needs notification if the system is handed over in an impaired state.
Reasoning: a signature certifies the state of the system at the time of signing. Signing first and fixing later transfers a known life-safety risk to the owner without their informed consent, and puts your name on a record that is not true.