7 Ah Battery on a 24-Hour Standby
A 24 VDC addressable panel protects a small mercantile building. Connected loads: panel electronics 0.085 A in standby; the SLC with 22 detectors and 6 monitor modules 0.120 A; an IP communicator 0.050 A in standby and 0.090 A when transmitting. On alarm, a single NAC draws 1.80 A. The specification requires 24 hours of standby followed by 5 minutes of alarm. The panel's charger is listed for batteries up to 18 Ah, and the cabinet will hold one 12 Ah battery or two 7 Ah batteries. The contractor installed a single 7 Ah battery and says the panel's manual table shows the panel "supports 7 Ah," so the installation is fine.
1. Calculate the required battery capacity and determine whether the installed 7 Ah battery is adequate.
Read the worked answer
Standby load: 0.085 + 0.120 + 0.050 = 0.255 A Standby energy: 0.255 A × 24 h = 6.12 Ah
Alarm load: panel 0.085 + SLC 0.120 + communicator 0.090 + NAC 1.80 = 2.095 A Alarm energy: 2.095 A × (5/60 h) = 0.175 Ah
Subtotal: 6.12 + 0.175 = 6.295 Ah
Apply the 1.20 safety/aging factor used in the battery calculation: 6.295 × 1.20 = 7.55 Ah
Conclusion: the requirement is 7.55 Ah; a 7 Ah battery is not adequate - it is below the calculated requirement before any consideration of cold temperature, battery age, or end-of-life capacity loss, all of which reduce delivered capacity in service.
The next commonly available size that exceeds 7.55 Ah is 12 Ah, and the panel's charger is listed for up to 18 Ah, so 12 Ah is acceptable from a charging standpoint. Confirm the enclosure and the battery terminal/lead size accommodate it, and record the calculation in the record of completion.
2. The contractor proposes to install two 7 Ah batteries in parallel to reach 14 Ah instead of buying a 12 Ah battery. Evaluate that proposal.
Read the worked answer
Reject it unless the batteries are a listed, matched assembly approved for parallel connection by the panel manufacturer.
Reasons:
- Two batteries in parallel share the load only if their internal resistances and states of charge match. In service they drift; the stronger battery carries more current, ages faster, and the pair fails earlier than either would alone.
- Charging is also shared unevenly, so one battery can be chronically undercharged - a condition that is invisible until a real outage.
- The AHJ and the manufacturer's listing address the battery as a system component. Unless the instruction manual explicitly permits two identical batteries in parallel (some panels do, with matched pairs from one lot and specific wiring), the installation is a deviation.
- It also doubles the number of terminations and connections inside the cabinet that must be inspected and maintained.
Correct answer: one 12 Ah battery, or two batteries only if the manufacturer's documentation expressly lists that configuration. Note that the parallel proposal still does not fix the underlying issue - the calculated requirement is 7.55 Ah, so the design must be documented, not improvised in the field.
3. What else must be verified before this battery installation is accepted?
Read the worked answer
- Charger capability versus installed capacity. A larger battery needs more recharge current. Verify the panel's listed maximum battery (here 18 Ah, so 12 Ah is allowed) and that the charger will recharge the battery within the manufacturer's stated time after a full discharge. If the charger is marginal, the battery never recovers between outages.
- Voltage drop in the battery leads and terminations. Long or undersized battery leads and poor terminations drop voltage under the 2.1 A alarm load; check them at the terminals, not at the battery posts.
- Actual measured loads, not nameplate guesses. Re-measure standby and alarm current with a meter after the system is programmed, since device counts and NAC loads change between design and installation. If the measured alarm load exceeds the calculation, redo the calculation before accepting the battery.
- Age and date coding. Record the manufacturer's date code on the battery and in the inspection record so the next technician can apply the replacement interval.
- Documentation. The battery calculation, the installed battery size, and the charger model belong in the record of completion and the panel's as-built documentation - otherwise the next inspector recalculates from the wrong assumption.