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Station Battery Sizing: IEEE 485 Duty Cycle and Margins

Size a stationary lead-acid battery from its duty cycle using IEEE 485 section calculations, manufacturer Kₜ factors, random loads and temperature, aging and design margins.

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Published 29. October 2026 · version 2 of 6. November 2026

#capacity rating factors #design margins #duty cycle #IEEE 485-2020 #lead-acid batteries #station batteries #substations

A station battery must carry steady controls, temporary auxiliaries and switching operations throughout an outage. This station battery sizing calculator implements the constant-current section method of IEEE Std 485-2020 for stationary lead-acid batteries. It suits substation and industrial DC-system engineers, students learning battery duty-cycle calculations, and reviewers checking a supplier's sizing worksheet.

Enter the total battery current and duration of each scheduled period, in chronological order. The standing load belongs in every period. Up to six periods can represent the initial switching event, sustained load, intermediate operations and final switching event. For short momentary lead-acid loads, use the manufacturer's approved short-duration rating and a minimum one-minute sizing period here. Do not average a brief peak into the standing current.

Choose the built-in C&D DCU-17 data or enter two to six points from the selected manufacturer's capacity rating factor curve. Kₜ is rated ampere-hours divided by the amperes deliverable for a stated time, temperature and end voltage; its unit is hours. It is not a universal battery constant. The built-in curve uses a 200 Ah nominal, eight-hour rating at 25 °C and 1.75 V per cell. Custom points must use one consistent rating basis and voltage endpoint. The tool interpolates linearly in time and blocks results outside the supplied duration range.

Each section ends after one more duty period. The calculation applies Kₜ to signed current changes using the remaining time to that section end. A load decrease therefore contributes a negative term. The largest section requirement controls the scheduled battery size; it need not occur at the end of the full cycle. The section table and chart expose this comparison.

If an additional load can occur at an unknown time, enter its extra current and duration. Its independent capacity requirement is added to the largest scheduled section as a conservative random-load allowance. Do not enter a load both here and in the scheduled periods. Repeated events or events with a restricted operating window need explicit scheduling and a more detailed review.

Apply the temperature sizing multiplier for the lowest expected electrolyte temperature, the aging multiplier and the design multiplier. For example, 1.25 represents an 80% retained-capacity criterion, while 1.10 adds a 10% design allowance. Compare the required rated capacity with a candidate on the same rating basis. Ampere-hours removed are shown separately because the charge integral alone cannot capture high-rate performance.

The optional voltage-window check compares the curve endpoint with the required bus voltage plus discharge-drop allowance and checks the highest connected charging voltage. It does not predict a voltage-versus-time trace. This tool does not size chargers, cables, protection, ventilation or racks, model constant-power loads, or cover lithium and nickel-cadmium batteries. Installation work belongs to a qualified person. Verify the final cell's own discharge data, short-duration voltage behavior and applicable design requirements; check against the standard in force.

How it is calculated

Uses IEEE Std 485-2020, §§6.3–6.4, the Kₜ form of the constant-current section method. The implementation and drawings are original; no standard tables or figures are reproduced.

For section $s$ , with $I_0=0$ , calculate

Q_s=\sum_{p=1}^{s}(I_p-I_{p-1})K\!\left(\sum_{j=p}^{s}d_j\right).

Retain negative current changes. Evaluate every section and select $Q_\mathrm{base}=\max_s Q_s$ . Kₜ is a capacity-to-current ratio in hours, so amperes times Kₜ gives rated Ah.

For one independent constant-current random load, $Q_r=I_rK(d_r)$ . The conservative sizing requirement is

Q_\mathrm{required}=(Q_\mathrm{base}+Q_r)f_Tf_Af_D.

The charge removed is calculated separately by integrating the scheduled current; it is not substituted for the section requirement.

Built-in factors are $200\,\mathrm{Ah}/I(t)$ from C&D Technologies DCU performance ratings, revision 9/26/24, p.2, for DCU-17 at 25 °C and 1.75 V/cell. Published rates include connector drop. Linear interpolation in time is an implementation choice; final selection should use manufacturer-approved factors for the exact durations. Custom Kₜ points replace that curve.

The optional voltage checks require $(V_\min+\Delta V)/N\le V_\mathrm{curve}$ and $NV_\mathrm{charge}\le V_\max$ . Increasing series cells changes the usable end voltage; obtain the appropriate discharge data rather than rescaling an unrelated curve.

Assumptions

Fully charged stationary lead-acid battery at the start; no charger contribution or recharge during the duty.
Currents are nonnegative, piecewise constant TOTAL battery currents. Period durations are at least one minute.
Kₜ values describe one cell design, end voltage and rated-capacity basis. The built-in basis is nominal 200 Ah at the eight-hour rate; custom results inherit the entered curve's basis.
Temperature, aging and design factors multiply required capacity and are at least 1. Warm-temperature capacity credit is not taken. Enter a cell-specific temperature multiplier for the site's lowest expected electrolyte temperature; the tool does not infer it from room temperature.
The random-load allowance is independent and conservative, not a claimed optimized placement. Its duration must fit within the whole duty cycle. Explicitly schedule repeated or time-restricted events.
Candidate headroom is a capacity comparison. Scaling a rating curve to another cell size assumes similar performance and must be checked with the final cell's data.

Where it applies

For stationary lead-acid constant-current duties with one to six periods and a rating curve covering all time-to-section-end intervals. Built-in data cover 1–720 min at 25 °C and 1.75 V/cell. Custom curves have two to six points, use linear time interpolation and do not extrapolate; add points near important discharge durations.

A one-minute representation does not resolve subsecond voltage dip, motor inrush or coup de fouet. Confirm these with the manufacturer. Constant-power loads require an appropriate current envelope or a separate constant-power analysis. The voltage-window check excludes a time-resolved discharge model, charger regulation and transient cable drops.

A positive candidate headroom is not a complete battery selection verdict. Recalculate using the selected model's actual factors, verify deliverable initial capacity and confirm aging behavior at the applicable discharge rate. Check against IEEE Std 485-2020 and the standard in force. Charger recovery, redundancy, protection, cable sizing, ventilation and installation design are outside this tool.

References

IEEE Std 485-2020, IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications, §§6.3–6.4. DOI: https://doi.org/10.1109/IEEESTD.2020.9103320. Licensed reference checked privately; its tables and figures are not reproduced.
C&D Technologies, DCU battery performance ratings, 12-311-T, REV 9/26/24, p.2: https://assets.ctfassets.net/3inf9wk05c1h/1XezoIYDOGCe5gwZbken4m/817d5db6cb9d956a0fe0d27516f020f4/202410-12-311-T-DCU.pdf
Lead-Acid Battery Sizing for a DC Auxiliary System in a Substation by the Optimization Method, Energies 12(22), 4400 (2019), Table 2, p.16: https://doi.org/10.3390/en12224400
Saft, Battery Sizing Considerations, IEEE Houston (2020), lead-acid example p.34: https://r5.ieee.org/houston/wp-content/uploads/sites/32/2020/03/Battery-Sizing-Considerations-IEEE-2020.pdf

Inputs

Input	Symbol	Unit	Default
Number of duty periods Each period is a constant TOTAL battery current. Include the standing load in every period; use at least one minute for momentary lead-acid loads.	`n_periods`		3
Period 1: total current Total current, not the change from the previous period.	`I_1`	A	100
Period 1: duration	`d_1`	min	1
Period 2: total current Total current, not the change from the previous period.	`I_2`	A	20
Period 2: duration	`d_2`	min	118
Period 3: total current Total current, not the change from the previous period.	`I_3`	A	100
Period 3: duration	`d_3`	min	1
Period 4: total current Total current, not the change from the previous period.	`I_4`	A	20
Period 4: duration	`d_4`	min	30
Period 5: total current Total current, not the change from the previous period.	`I_5`	A	20
Period 5: duration	`d_5`	min	30
Period 6: total current Total current, not the change from the previous period.	`I_6`	A	100
Period 6: duration	`d_6`	min	1
Additional random-load current Extra current above the scheduled duty cycle. Leave zero if all loads are already scheduled.	`I_random`	A	0
Random-load duration Independent constant-current random load. Repeated or restricted-time events should instead be placed explicitly in duty periods.	`d_random`	min	1
Capacity rating data Kₜ has units of hours: rated Ah divided by discharge A. Use one cell type, rating basis and end voltage throughout.	`curve_mode`		C&D DCU-17: 200 Ah, 1.75 V/cell, 25 °C
Number of Kₜ curve points Enter points in increasing time order. Include points near every time-to-section-end used by your duty cycle.	`n_knots`		6
Kₜ point 1: discharge time	`t_k1`	min	1
Kₜ point 1: rated Ah / discharge A Manufacturer's Kₜ value; this is not a dimensionless multiplier.	`k_1`	h	0.6688963210702341
Kₜ point 2: discharge time	`t_k2`	min	15
Kₜ point 2: rated Ah / discharge A Manufacturer's Kₜ value; this is not a dimensionless multiplier.	`k_2`	h	1.0040160642570282
Kₜ point 3: discharge time	`t_k3`	min	60
Kₜ point 3: rated Ah / discharge A Manufacturer's Kₜ value; this is not a dimensionless multiplier.	`k_3`	h	1.9193857965451055
Kₜ point 4: discharge time	`t_k4`	min	120
Kₜ point 4: rated Ah / discharge A Manufacturer's Kₜ value; this is not a dimensionless multiplier.	`k_4`	h	2.985074626865672
Kₜ point 5: discharge time	`t_k5`	min	240
Kₜ point 5: rated Ah / discharge A Manufacturer's Kₜ value; this is not a dimensionless multiplier.	`k_5`	h	4.8076923076923075
Kₜ point 6: discharge time	`t_k6`	min	480
Kₜ point 6: rated Ah / discharge A Manufacturer's Kₜ value; this is not a dimensionless multiplier.	`k_6`	h	8.097165991902834
Custom curve end voltage per cell Voltage endpoint for all entered Kₜ points. Changing this field does not recalculate the factors.	`V_custom`	V	1.75
Temperature sizing multiplier Required capacity multiplier at the lowest expected electrolyte temperature. Use the manufacturer's data; if available capacity is 0.84, enter 1 / 0.84. No warm-temperature capacity credit.	`f_temp`		1
Aging multiplier 1.25 corresponds to an 80% end-of-life capacity criterion. Confirm the appropriate factor for the cell and discharge rate.	`f_age`		1.25
Design multiplier Enter 1.10 for a 10% design allowance; 1.00 for none. Avoid counting load growth twice.	`f_design`		1.1
Candidate rated capacity Use the same rating basis as the Kₜ curve. This compares capacity only; rerun with the final candidate's own factors before selection.	`Q_candidate`	Ah	200
Check the DC voltage window	`check_voltage`		true
Series cells	`n_cells`		60
Minimum required load-bus voltage	`V_min`	V	105
Discharge cable and connection allowance Added to the minimum load-bus voltage to obtain the required battery-terminal voltage.	`V_drop`	V	0
Maximum permitted bus voltage	`V_max`	V	140
Highest charging voltage per cell Use the highest float or equalize voltage while the battery remains connected to the bus.	`V_charge`	V	2.33

Results

With the default inputs above, the calc gives:

Result	Value
Required rated capacity	157.62 Ah
Largest section before margins	114.63 Ah
Random-load capacity allowance	0 Ah
Controlling scheduled section	3
Scheduled charge removed	42.67 Ah
Candidate capacity headroom	42.38 Ah
Required / candidate capacity	0.788

Checked against sources

Each case below was calculated by this calc and agrees with the value from its source (within the tolerance where one is shown).

Published small substation with random load

Source: Energies 2019, 12, 4400, Table 2, p.16, DOI 10.3390/en12224400. Independent published case: 3.83 A × 8 h plus 127.14 A × 1.1 h, then ×1.19×1.25×1.15. Small differences reflect rounding. Factors apply at the paper's 1.81 V/cell endpoint.

Inputs: Capacity rating data = Enter manufacturer's Kₜ curve; Number of duty periods = 1; Period 1: total current = 3.83 A; Period 1: duration = 480 min; Additional random-load current = 127.1 A; Kₜ point 1: rated Ah / discharge A = 1.1 h; Kₜ point 2: discharge time = 480 min; Kₜ point 2: rated Ah / discharge A = 8 h; Number of Kₜ curve points = 2; Temperature sizing multiplier = 1.19; Design multiplier = 1.15; Candidate rated capacity = 300 Ah; Check the DC voltage window = no; Custom curve end voltage per cell = 1.81 V; the others as above.

Result	Source	This calc
Largest section before margins	30.64 Ah	30.64 Ah
Random-load capacity allowance	139.9 Ah	139.9 Ah
Required rated capacity	291.7 Ah	291.7 Ah

DCU-17 published one-minute current rating

Source: C&D Technologies DCU brochure 12-311-T-DCU, October 2024, p.2: DCU-17, 200 Ah nominal, 299.0 A for one minute at 25 °C and 1.75 V/cell. This is an independent manufacturer endpoint check.

Inputs: Number of duty periods = 1; Period 1: total current = 299 A; Aging multiplier = 1; Design multiplier = 1; Check the DC voltage window = no; the others as above.

Result	Source	This calc
Required rated capacity	200 Ah	200 Ah

DCU-17 published eight-hour current rating

Source: C&D Technologies DCU brochure 12-311-T-DCU, October 2024, p.2: DCU-17 supplies 24.7 A for eight hours to 1.75 V/cell. Nominal capacity is rounded to 200 Ah; integrated delivery is 197.6 Ah. Kt uses the published nominal rating.

Inputs: Number of duty periods = 1; Period 1: total current = 24.7 A; Period 1: duration = 480 min; Aging multiplier = 1; Design multiplier = 1; Check the DC voltage window = no; the others as above.

Result	Source	This calc
Required rated capacity	200 Ah	200 Ah
Scheduled charge removed	197.6 Ah	197.6 Ah

Saft switching duty: initial and final high-current pulses

Source: Saft Battery Sizing Considerations, IEEE Houston 2020, p.34, own LSe worked example (not an IEEE standard table): 325/5/325 A for 1/478/1 min. Published Kt values 0.5589, 7.997, 8.009 h. The source rounds the three section-3 terms to 2603, -2559 and 178.85 Ah, obtaining 222.85 Ah and 324.8 Ah corrected. Keeping the supplied factors unrounded gives 222.733 Ah and 324.63335 Ah. Tolerances explicitly allow that source arithmetic rounding; the formula is not adjusted. Intermediate section 2 is linearly interpolated and does not control.

Inputs: Capacity rating data = Enter manufacturer's Kₜ curve; Number of Kₜ curve points = 3; Kₜ point 1: rated Ah / discharge A = 0.5589 h; Kₜ point 2: discharge time = 479 min; Kₜ point 2: rated Ah / discharge A = 7.997 h; Kₜ point 3: discharge time = 480 min; Kₜ point 3: rated Ah / discharge A = 8.009 h; Period 1: total current = 325 A; Period 2: total current = 5 A; Period 2: duration = 478 min; Period 3: total current = 325 A; Temperature sizing multiplier = 1.06; Candidate rated capacity = 350 Ah; Check the DC voltage window = no; the others as above.