Fire alarm battery calculation: the method, step by step

Calculate each battery bank separately. Add the charge used in standby to the charge used in alarm, apply the documented capacity corrections, then select a compatible bank. A remote supply with its own batteries needs its own worksheet.

The method below follows Fire-Lite's MS-9200UDLS manual, §§7.3–7.4. That manual is a 2015 revision: use its worksheet structure, then reconcile its assumptions with the project's enforced edition. Do not carry an older manual's correction factor into a newer-code calculation without resolving the difference.

Establish durations and the code basis first

Record the authority having jurisdiction's (AHJ's) enforced edition, local amendments and system arrangement on the calculation. For NFPA 72 (2022), start at §10.6.7.2.1, including the applicable subsections, for operating durations; use §10.6.7.2.2 for the load basis and §10.6.7.2.3 for capacity correction.

For the ordinary protected-premises arrangement, the baseline is 24 hours of standby followed by 5 minutes of alarm. For in-building fire emergency voice/alarm communication, use 24 hours of standby followed by 15 minutes of alarm. The reference is NFPA 72 (2022), §10.6.7.2.1, including its applicable subsections. Other system arrangements, exceptions and amendments can change the applicable duty; the enforced edition establishes the project basis. The code is cited by section number, without reproducing its text or forms. NFPA access provides the reference route.

Identify whether the bank serves ordinary notification, voice evacuation, communications equipment or another arrangement before entering time. An exam reference edition and the edition enforced on a job are separate decisions. NICET's Level II reference list names NFPA 72 (2022) and NFPA 70 (2020); that list does not establish a jurisdiction's adoption.

Build two current totals

Use the selected manufacturer's battery-powered standby and alarm currents. Fire-Lite's worksheet distinguishes those from normal AC-powered current. List quantities, settings and the source beside each load. Include every load supported by this bank; avoid counting components again when their current is already included in an assembly total.

Alarm current is the full alarm-state current, not the increase above standby. Standby and alarm are consecutive operating periods. Convert milliamperes to amperes before multiplying by hours: 180 mA ÷ 1,000 = 0.180 A.

This fictional equipment schedule demonstrates the arithmetic; none of its currents represents a real product:

• Control assembly: 0.120 A standby; 0.180 A alarm.
• Thirty addressable devices: 30 × 0.001 = 0.030 A standby; 30 × 0.002 = 0.060 A alarm.
• Annunciator: 0.040 A standby; 0.060 A alarm.
• Communicator: 0.110 A standby; 0.150 A alarm.
• Notification loads: 0 A standby; 1.350 A alarm.

Total standby current is 0.120 + 0.030 + 0.040 + 0.110 = 0.300 A. Total alarm current is 0.180 + 0.060 + 0.060 + 0.150 + 1.350 = 1.800 A.

Calculate amp-hours before applying corrections

Let Is and Ia be the total standby and full alarm currents in amperes, Ts the standby duration in hours, and Ta the alarm duration in minutes:

Base Ah = Is × Ts + Ia × (Ta ÷ 60)

Use fictional exercise durations of 10 hours standby and 12 minutes alarm. These are arithmetic inputs, not code requirements:

1. Standby: 0.300 × 10 = 3.000 Ah.
2. Alarm: 1.800 × (12 ÷ 60) = 0.360 Ah.
3. Base: 3.000 + 0.360 = 3.360 Ah.

Separate aging, temperature and extra margin

An additive margin of m percent multiplies capacity by 1 + m ÷ 100. A retained-capacity fraction f instead divides the load requirement by f. A percentage added to the load and a percentage of capacity remaining are different quantities. Adding 20% gives 3.360 × 1.20 = 4.032 Ah. Allowing for 80% retained capacity gives 3.360 ÷ 0.80 = 4.200 Ah. The first result would retain only 4.032 × 0.80 = 3.2256 Ah under that assumed loss, below the 3.360 Ah load.

Fire-Lite's cited revision uses a 1.2 factor. NFPA's official development record identifies the correction-factor issue at §10.6.7.2.3. Resolve the enforced-edition requirement before accepting a worksheet's default. Do not stack two factors that cover the same allowance.

For a second, entirely fictional exercise, assume the battery selection method specifies an aging retention fraction of 0.80, a temperature capacity fraction of 0.90 at the design condition, and an additional 10% project margin:

1. Aging: 3.360 ÷ 0.80 = 4.200 Ah.
2. Temperature: 4.200 ÷ 0.90 = 4.666666… Ah.
3. Extra margin: 4.666666… × 1.10 = 5.133333… Ah.

These assumed fractions are not manufacturer battery data or code-prescribed values. For a real selection, obtain the correction method for the battery technology, design temperature and discharge rate, and document what each factor covers. NFPA's development record discusses aging and temperature separately; Silent Knight's equipment precautions also identify temperature as a battery-life consideration.

Finish the equipment checks

Keep full precision until selecting the next permitted capacity that meets the calculation. The selected bank must meet the manual's charger range, recharge requirements, battery chemistry, cabinet capacity and connection requirements. A larger Ah rating does not raise the panel's output-current rating.

Voltage is a separate calculation. Establish the supply's available NAC-terminal voltage at the specified battery discharge condition, then calculate cable loss and compare appliance voltage with its listing. A capacity result alone cannot establish usable voltage.

The battery calculator checks standby-plus-alarm arithmetic with an additive margin. It does not model temperature and aging separately. Use the voltage-drop guide for the circuit check and submittals guide to organize the evidence. Return to Alarm Ladder or take the Level II diagnostic.

Sources

• MS-9200UDLS Series Manual, 52750 Rev M, Fire-Lite, §§7.1–7.4, 20 Nov 2015. Retrieved 17 Oct 2026; battery-powered current, worksheet method and equipment checks. Its historical factor is not a substitute for the adopted edition.
• NFPA 72 (2022), NFPA, §§10.6.7.2.1–10.6.7.2.3, references by number only. Access route retrieved 17 Oct 2026; final section text was not accessible in the public reader.
• NFPA 72 development record, SIG-FUN public-input responses, NFPA, responses to public inputs 560 and 576. Retrieved 17 Oct 2026; correction-factor background, not the final adopted code.
• IFP-2100/ECS Manual, LS10143-001SK-E Rev E, Silent Knight, installation precautions and §3.5. Retrieved 17 Oct 2026.
• Level II Selected General References, NICET. Retrieved 17 Oct 2026; exam editions only.