IEEE 1185 — Battery Sizing for Stationary Applications: What the Standard Requires

IEEE 1185 — IEEE Recommended Practice for Sizing Nickel-Cadmium and VRLA Batteries for Stationary Applications — provides the methodology for determining the number and size of battery cells required to support critical loads during utility power outages. It is a companion standard to IEEE 485 (which covers vented lead-acid batteries) and follows a similar but distinct sizing methodology.

Stationary battery systems are the last line of defence for critical infrastructure: data centres, hospitals, telecommunications, substations, and emergency power systems. An undersized battery fails when it is needed most. An oversized battery wastes capital and floor space. IEEE 1185 provides a rigorous, repeatable method for getting the size right.

This guide explains the three-step sizing method, the correction factors that account for real-world conditions, and the critical differences between IEEE 1185 and IEEE 485 that engineers working across both battery types must understand.

The IEEE 1185 sizing process follows three sequential steps, each building on the previous:

Step 1: Define the Duty Cycle

The duty cycle is a time-sequenced profile of all loads the battery must support during the design discharge period. It defines what the battery must deliver, when, and for how long. Per IEEE 1185, Clause 6.2, the duty cycle must account for:

• Continuous loads: Loads present for the entire discharge duration (e.g., control systems, emergency lighting, communication equipment).
• Momentary loads: Short-duration high-current loads that occur at specific times (e.g., circuit breaker trip coils, motor starting, switchgear spring charging).
• Non-continuous loads: Loads that start or stop at defined points in the discharge (e.g., alarm annunciators that activate 5 minutes into an outage).

Step 2: Size the Cells

Using the duty cycle, determine the required ampere-hour (Ah) capacity of each cell. This involves selecting cells from manufacturer data that can deliver the required current profile while maintaining voltage above the minimum threshold throughout the discharge. Per IEEE 1185, Clause 6.3:

Cell sizing equation:
  C_required = MAX over all periods [ S_period / K_t(period) ]

Where:
  C_required = minimum cell capacity (Ah at 25 deg C, 8-hour rate)
  S_period   = amperes required for the period
  K_t        = capacity factor from manufacturer data for the
               discharge rate and time to end of period

The cell must be sized for the worst-case period — the period during the duty cycle where the ratio of required current to available capacity is highest. This is not always the first period or the highest-current period; it depends on the interaction between current magnitude and remaining discharge time.

Step 3: Determine Battery Configuration

Calculate the number of cells in series to achieve the required system voltage, accounting for the minimum cell voltage at end of discharge:

Number of cells (N):
  N = V_system_min / V_cell_min

For a 125 V DC system with lead-acid cells:
  N = 125 V / 1.75 V per cell = 71.4 → round to 60 cells
  (Standard configuration: 60 cells × 2.0 V nominal = 120 V nominal)

For a 48 V DC telecom system:
  N = 43.2 V / 1.75 V per cell = 24.7 → 24 cells
  (Standard: 24 cells × 2.0 V nominal = 48 V nominal)

The number of cells is typically standardised to common configurations (24, 60, 120 cells) based on the nominal DC system voltage. The actual minimum voltage at end of discharge must be verified against the minimum input voltage of the connected DC loads (inverters, rectifiers, control circuits).

Real-world batteries do not operate at laboratory reference conditions. IEEE 1185 requires three correction factors to account for the difference between reference and actual conditions. These factors are applied multiplicatively to the calculated cell capacity:

Corrected capacity:
  C_corrected = C_required × K_t × K_a × K_d

Where:
  K_t = temperature correction factor
  K_a = aging correction factor
  K_d = design margin factor

Temperature Correction (K_t)

Battery capacity is highly sensitive to temperature. At low temperatures, the electrolyte viscosity increases and the chemical reaction rates decrease, reducing available capacity. Per IEEE 1185, Clause 6.4.1, the temperature correction factor is:

Critical note: While higher temperatures increase available capacity in the short term, they drastically reduce battery life. The Arrhenius rule of thumb states that every 10°C above 25°C halves the expected service life. IEEE 1185 recommends designing for the lowest expected temperature during a discharge event, not the average temperature.

Aging Correction (K_a)

Batteries lose capacity over their service life due to plate degradation, grid corrosion, and electrolyte stratification. IEEE 1185 recommends sizing for end-of-life capacity, typically defined as 80% of rated capacity. This means the aging factor K_a = 1/0.80 = 1.25 for most applications. Some utilities use K_a = 1.20, accepting replacement at 83% remaining capacity.

Design Margin (K_d)

The design margin accounts for load growth, unforeseen loads, and uncertainty in the duty cycle. IEEE 1185, Clause 6.4.3 recommends a minimum of 10% design margin (K_d = 1.10) for well-defined duty cycles, and 15–25% for duty cycles with significant uncertainty. Nuclear power plant battery sizing per IEEE 946 often requires K_d = 1.25.

The duty cycle (load profile) is the most labour-intensive part of an IEEE 1185 battery sizing study. Two approaches are available:

Method 1: Detailed Duty Cycle Table

The preferred method per IEEE 1185, Clause 6.2. Every load is listed with its current demand and the time period during which it operates. The duty cycle is divided into time periods, and the total current demand for each period is calculated.

The cell must be sized to handle the worst-case period. Period 1 has the highest current, but it only lasts 1 minute. Period 5 occurs at the end of discharge when the battery is most depleted. The sizing calculation must check each period to find the governing case.

Method 2: Simplified Peak Demand

For preliminary sizing or simple systems, the peak demand method uses the maximum instantaneous current and the total discharge time. This method is more conservative (oversizes the battery) but requires less detailed load analysis:

Simplified sizing:
  C_simple = I_peak × T_discharge × K_t × K_a × K_d

Example:
  I_peak = 185 A
  T_discharge = 2 hours
  K_t = 1.08 (15 deg C)
  K_a = 1.25 (end of life)
  K_d = 1.15 (15% margin)
  C_simple = 185 × 2 × 1.08 × 1.25 × 1.15 = 574 Ah

The simplified method can oversize the battery by 30–50% compared to the detailed duty cycle method, because it assumes peak current flows for the entire discharge period. For large battery installations where cell cost is significant, the detailed method is always worth the additional engineering effort.

A critical constraint in battery sizing is the minimum cell voltage at end of discharge. If the cell voltage drops below this threshold, the connected DC loads may malfunction or shut down, defeating the purpose of the battery system.

The 1.75 V/cell minimum for lead-acid batteries is the most commonly used value in IEEE 1185 calculations. Discharging below this voltage causes deep discharge damage, sulfation, and permanent capacity loss. Some manufacturers rate their cells to 1.67 V/cell for short-duration high-rate discharges (less than 15 minutes), but this should only be used if explicitly supported by the manufacturer’s data sheets.

The system minimum voltage must be calculated as:

V_system_min = N_cells × V_cell_min

Example (125 V DC substation battery):
  60 cells × 1.75 V = 105 V at end of discharge

The connected loads must operate down to 105 V DC.
If any load requires >105 V, either:
  (a) increase the number of cells, or
  (b) increase the minimum cell voltage (reduces available capacity)

This voltage constraint often governs the battery configuration for systems with voltage-sensitive loads such as inverters and DC-DC converters.

Engineers frequently confuse IEEE 1185 and IEEE 485 or assume they are interchangeable. They are not. The two standards cover different battery chemistries and have distinct methodologies:

The practical implication: if your project uses VRLA batteries (which are by far the most common in modern UPS and telecom installations), use IEEE 1185. If the project uses traditional vented (flooded) lead-acid batteries (common in utility substations and power plants), use IEEE 485. Using the wrong standard will produce incorrect sizing because the discharge characteristics of vented and VRLA cells are fundamentally different.

For projects using lithium-ion batteries, neither IEEE 1185 nor IEEE 485 directly applies. Lithium-ion battery sizing is typically performed using manufacturer-specific discharge data and the general principles from IEEE 1185, but there is currently no dedicated IEEE standard for lithium-ion stationary battery sizing.

A complete IEEE 1185 battery sizing study follows this workflow:

1. Define system parameters: Nominal DC voltage, minimum acceptable voltage, maximum float/equalise voltage, discharge duration (autonomy requirement).
2. Enumerate all loads: List every DC load with its current demand, start time, stop time, and criticality. Include momentary loads (CB trip, motor starting) and continuous loads (control, lighting, SCADA).
3. Construct the duty cycle: Organise loads into time periods. Calculate total current for each period. Identify the worst-case (governing) period.
4. Determine correction factors: K_t from lowest expected temperature, K_a = 1.25 for end-of-life, K_d per project risk tolerance.
5. Select candidate cells: From manufacturer catalogues, identify cells that can deliver the required discharge profile. Use the manufacturer’s discharge rate tables (not just the nominal Ah rating).
6. Verify voltage: Confirm the cell voltage remains above 1.75 V/cell (lead-acid) or 1.00 V/cell (NiCd) throughout the entire duty cycle, including during momentary high-current loads.
7. Document: The sizing report must include the duty cycle table, all correction factors with justification, cell selection data, and voltage verification at each period.

The entire sizing calculation should be repeatable and auditable. Any change to the load list, autonomy requirement, or environmental conditions requires re-running the sizing calculation to confirm the selected battery remains adequate.