6.3 Battery Sizing and Secondary Power
Key Takeaways
- Secondary power questions require both load totals and duration assumptions.
- Battery amp-hour capacity must be compared to the calculated demand for the served system or supply.
- Remote power supplies may have their own battery calculations.
- Rounding, units, and manufacturer ratings are frequent calculation traps.
Battery Sizing and Secondary Power
Fire alarm systems need a planned response when primary power is lost. On NICET FAS exams, secondary power questions usually ask whether a battery set or power supply can support the listed loads. The calculation depends on load, time, capacity, and which equipment is actually served by the supply being evaluated.
Battery capacity is commonly expressed in amp-hours. One amp-hour means one amp for one hour, before applying the details required by the listing, manufacturer data, or referenced standard. For exam prep, understand the relationship: current multiplied by time gives amp-hours. Then compare the result to the available rated battery capacity in the scenario.
| Item | What it means | Why candidates miss it |
|---|---|---|
| Amp-hours | Current times time | Unit conversions are easy to shift by a decimal. |
| Standby duration | Time the system must monitor on secondary power | Candidates may use alarm duration by mistake. |
| Alarm duration | Time the system must operate in alarm after standby | Candidates may omit it or apply it to the wrong load. |
| Battery rating | Available capacity from listed equipment or exhibit | Candidates may compare to only one battery in a series pair. |
| Remote supply | Separate supply and battery set | Candidates may add its loads to the panel battery. |
For NICET FAS scenario guidance, draw a box around the supply being checked. If the question asks for the main fire alarm control unit batteries, do not include notification appliances powered by a separate remote supply unless the exhibit says the main panel supplies that load. If the question asks for the remote supply batteries, include the NACs and modules served by that supply.
A battery sizing list is:
- Confirm the supply or panel under review.
- Gather standby current served by that supply.
- Gather alarm current served by that supply.
- Convert all current to the same units.
- Multiply each current total by its required duration.
- Add the amp-hour demands.
- Apply required margin or derating only when supplied by the problem or reference.
- Compare to the available battery rating and select the next acceptable option if choices are given.
Exam trap: treating two 12-volt batteries in series as double the amp-hour capacity. In a series arrangement, voltage adds, but amp-hour capacity does not simply double. Another trap is comparing a calculated demand to a charger rating rather than to battery capacity when the question asks for battery sizing.
The official NICET references by level include NFPA 72 (2022), with NFPA 70 (2020) included for Levels I, II, and III and also Level IV. Use the listed standards and equipment instructions for exact requirements during study. This chapter keeps the discussion at exam-prep level so you build the calculation structure without reproducing protected standard text.
Battery questions may be embedded in a shop drawing review. The exhibit might show battery size, standby current, alarm current, NAC load, power supply model, and wire distance. Read the stem carefully. If it asks what is missing, the answer may be a calculation worksheet or battery capacity note, not a new device.
On exam day, use the built-in calculator and write intermediate results on the provided scratch method if available. Keep amps, hours, and amp-hours labeled. If the answer choices differ by a factor of ten, suspect a milliamp conversion error before choosing.
What basic relationship supports battery amp-hour calculations?
A question asks for the remote power supply battery size. Which loads should be included?
What is a battery sizing trap involving series batteries?