13.2 Battery Sizing with 20% Aging Derating

13.2 Battery Sizing with 20% Aging Derating

Quick Answer: After you compute calculated load Ah (standby Ah + alarm Ah), multiply by 1.20 for 20% aging: Required Ah = Calculated Ah × 1.20. Choose a battery (or bank) with at least that capacity at the system voltage, meeting manufacturer minimums. Forgetting the 1.20 factor or using the wrong alarm minutes are the highest-yield traps in blueprint 2.3.4.

Section 13.1 produced the load basis. This section turns that number into an ordered battery and a reviewable calc sheet.

The Aging Formula

Batteries lose capacity over life. Industry fire-alarm practice and exam expectations apply a 20% aging derating:

Required Ah = (Standby Ah + Alarm Ah) × 1.20

Equivalent wording: multiply calculated amp-hours by 120%, or add 20% of the calculated total to itself.

Trap A: Selecting a 5 Ah battery because “calculated was about 5.3” without × 1.20 → undersized.
Trap B: Applying 1.20 only to standby Ah and adding raw alarm Ah (or the reverse) → inconsistent and often wrong. Apply 1.20 to the sum.
Trap C: Multiplying currents by 1.20 before converting to Ah and mixing units—keep the factor on the Ah total (or apply consistently to both Ah terms, which is algebraically the same).

Continuing the 13.1 Example (5 min)

From 13.1:

• Calculated Ah = 5.259 Ah
• Required Ah = 5.259 × 1.20 = 6.311 Ah

If the control unit requires two 12 V batteries in series for a 24 V system, each battery in that series string must be sized for the same Ah rating (for example two 12 V, 7 Ah batteries in series = 24 V at 7 Ah). Series connection adds voltage, not amp-hours.

EVACS Path on the Same Currents

From 13.1 alternate (15 min alarm): calculated Ah = 5.746 Ah
Required Ah = 5.746 × 1.20 = 6.895 Ah → select at least 7 Ah (or 8 Ah if that is the next standard or required minimum).

Compare: same hardware currents, longer alarm, higher required Ah. A worksheet that stamped “5 min” on a voice project fails both code intent and exam scrutiny.

Series vs Parallel Battery Banks

Example: Need 24 V and ≥ 14 Ah.

• Two 12 V, 18 Ah in series → 24 V, 18 Ah (one string).
• Two strings of (two 12 V, 7 Ah in series), strings paralleled → 24 V, 14 Ah (if paralleling is permitted by manufacturer and done correctly).

Rules of thumb for Level II:

• Prefer manufacturer-documented bank configurations.
• Parallel only identical voltage/capacity types as allowed; poor paralleling causes unequal charge/discharge.
• Charger and cabinet must support the selected Ah and number of batteries.
• Label voltage and date; maintenance chapters already stressed battery ITM—sizing must match what is installed.

Manufacturer Minimum Size & Charger Limits

Even if math yields 6.3 Ah, the control unit data sheet may say “minimum battery 7 Ah” or “12–18 Ah” for a given cabinet. Always take the greater of:

1. Required Ah after 1.20 factor, and
2. Manufacturer minimum listed battery capacity for that panel/power supply.

Also verify:

• Maximum battery size the charger can recharge in the required time.
• Cabinet space and battery lead sizing.
• Whether remote power supplies need separate battery sets (size each secondary supply on its own load basis).

Temperature Derating Awareness

Battery capacity falls in cold environments and life shortens in heat. Manufacturer tables may require additional capacity factors below a stated temperature. Level II exam items may not always give a temperature table, but submittal-quality calcs note:

• Assumed ambient for the battery location.
• Any manufacturer temperature multiplier applied in addition to the 20% aging factor when required by the product instructions.

Do not “double-count” random safety factors without a basis—but do not ignore a stated temperature derating on the cut sheet.

Second Worked Example (Clean Numbers)

Given:

• I_standby = 0.50 A
• I_alarm = 6.00 A
• Non-voice protected premises: 24 h + 5 min

Standby Ah = 0.50 × 24 = 12.00 Ah
Alarm Ah = 6.00 × (5/60) = 6.00 × 0.08333 = 0.50 Ah
Calculated = 12.00 + 0.50 = 12.50 Ah
Required = 12.50 × 1.20 = 15.00 Ah exactly.

Select batteries ≥ 15 Ah at system voltage (for example two 12 V, 18 Ah in series for 24 V if 15 Ah is not a standard SKU and 18 Ah is listed).

If the same currents were EVACS (15 min):

Alarm Ah = 6.00 × 0.25 = 1.50 Ah
Calculated = 12.00 + 1.50 = 13.50 Ah
Required = 13.50 × 1.20 = 16.20 Ah → choose ≥ 16.2 Ah (e.g., 18 Ah series pair).

Common Exam & Field Failure Modes

1. Forgot 20% — calculated 12.5 Ah, ordered 12 Ah batteries.
2. Wrong minutes — used 5 min on voice system (or 15 on plain NAC-only when the question specified 5).
3. mA left unconverted — 500 mA × 24 = “12,000 Ah.”
4. Series/parallel confusion — two 12 V, 7 Ah in series claimed as “14 Ah at 24 V” (false: series keeps Ah, raises volts).
5. Only NAC load counted — omitted FACU/SLC standby that dominates multi-day amp-hours.
6. Only standby counted — omitted alarm Ah entirely.
7. Ignoring manufacturer minimum — math says 6.3 Ah, panel requires 12 Ah minimum.

Documentation for Submittals

A complete battery calc sheet typically shows:

• Project name, panel/power-supply ID, system voltage.
• Device or circuit current table (standby and alarm).
• Standby hours and alarm minutes used (and system type: voice vs non-voice).
• Calculated Ah, × 1.20, required Ah.
• Selected battery voltage, Ah, quantity, series/parallel description.
• Cut-sheet references and revision date matching the drawing package.

Chapter 12 notes should not contradict the wire and load assumptions; Chapter 13.3 will use alarm currents again for voltage drop—same load basis, different formula.

Bridge to 13.3–13.4

Batteries can be large enough on paper while NAC voltage at the last appliance is too low (13.3) or raceways are overfilled (13.4). Power-supply loading is a package: capacity, voltage drop, and wire/raceway geometry together.