The purpose of the calculation
A battery calculation estimates the ampere-hour capacity needed to support the fire alarm system during loss of normal AC power. The result is not simply the panel nameplate current multiplied by a number of hours. It must account for every load powered by the battery set, including control equipment, annunciators, loop devices, notification circuits, relays, communicators, and auxiliary equipment that remains energized during standby or alarm.
Build a complete load list
Start with the approved equipment list and manufacturer current data. Separate each device’s standby current from its alarm current. Some devices have several operating states, and the highest applicable state may need to be used. Include remote power supplies and their connected loads in the correct calculation rather than combining unrelated battery sets.
Field additions are a common source of error. Compare the calculation to the installed system: extra annunciators, cellular communicators, network cards, door-holder relays, auxiliary power modules, and replacement devices may not appear on the original submittal.
Apply standby and alarm time correctly
The general structure is straightforward: standby amp-hours equal total standby current multiplied by standby duration; alarm amp-hours equal total alarm current multiplied by alarm duration. Add the two results, then apply the required derating or safety factor.
The actual durations depend on the adopted code, system type, supervising-station arrangement, project specifications, and AHJ requirements. Do not assume every system uses the same standby period. Voice systems and certain special applications may also require a different alarm load scenario.
Worked field example
Assume a system has 0.42 amperes of total standby current and 2.80 amperes of total alarm current. Using 24 hours of standby and 5 minutes of alarm, the standby portion is 10.08 amp-hours. Five minutes is 0.0833 hours, so the alarm portion is approximately 0.23 amp-hours. The subtotal is about 10.31 amp-hours. If the project uses a 20 percent safety factor, the adjusted requirement is about 12.37 amp-hours.
The next standard battery size must meet or exceed the requirement and must also be listed for use with the equipment. A pair of 12-amp-hour batteries would not satisfy a 12.37-amp-hour requirement; the next approved size would be needed.
Check charger and enclosure limits
A mathematically adequate battery may still be unacceptable. Verify the control unit or power supply is listed to charge the selected capacity within the required time. Confirm the cabinet or remote battery enclosure is approved for the quantity and size, has appropriate ventilation and separation, and does not exceed mounting or weight limitations.
Do not parallel extra batteries or install a larger set merely because it fits physically. The manufacturer’s documentation controls the permitted battery arrangement.
Field testing and replacement decisions
Battery age alone does not prove capacity, and an open-circuit voltage reading does not prove performance. Inspect terminals, date codes, swelling, corrosion, temperature, and charger voltage. Use the testing method permitted by the manufacturer and applicable inspection standard. Replace batteries as a matched set when required, and investigate repeated early failures rather than treating batteries as the only problem.
Recurring battery trouble may point to excessive load, a failed charger, high cabinet temperature, loose terminals, or an AC-power problem.
Field checklist
- Confirm the installed equipment matches the load list
- Separate standby and alarm current for every powered device
- Use the project-required standby and alarm durations
- Apply the specified safety or derating factor
- Round up to an approved standard battery size
- Verify charger capacity and cabinet listing
- Record assumptions and manufacturer data sources
Important limitation
This guide is a field-oriented starting point, not a substitute for the adopted code, approved drawings, project specifications, manufacturer instructions, site safety procedures, or the authority having jurisdiction. Do not bypass a listed protection feature or leave a required system impaired without following the approved impairment process.