Station Battery Sizing: IEEE 485 and IEEE 1115

Short answer

Sizing a stationary battery bank starts from the duty cycle: which loads it must supply and for how long when AC power fails. With that cycle, the cell capacity curve and the factors for temperature, aging and margin, IEEE 485 (lead-acid) and IEEE 1115 (nickel-cadmium) give the capacity in ampere-hours. The charger is calculated separately, with IEEE 946.

What is being calculated

The battery bank is the last energy reserve of a substation: it supplies protection, control, communications and the trip and close coils when AC power is lost. If it is undersized, it fails exactly when it is needed. If it is oversized, it costs more, takes more space and does not improve safety.

Sizing means answering a specific question: how many ampere-hours must the bank have to sustain the loads for the required time? The voltage cannot fall below the minimum the equipment accepts, not even at the end of the bank’s service life or at the lowest temperature of the site.

Two IEEE standards solve the calculation with the same method: IEEE 485 for lead-acid and IEEE 1115 for nickel-cadmium. The charger follows another standard, IEEE 946. To understand the complete system, see the guide on the substation DC system. With your duty cycle at hand, you can try the calculation in the battery sizing tool. For more detail on BAE OPzS and OPzV stationary batteries, read BAE OPzS and OPzV batteries.

The starting point: the duty cycle

The whole calculation starts from the duty cycle, which is the profile of the load over time from the moment AC is lost. It is built from three kinds of load:

Continuous: relays, controllers, communications and emergency lighting. They draw during the whole autonomy period.
Intermediate duration: loads that act during a part of the cycle, such as some lamps or emergency equipment.
Momentary: breaker tripping and closing and spring-charging motors. They last seconds, but with a high current that pulls down the battery voltage.

Autonomy has no universal value. It is set by the owner or the utility according to how long it takes for AC to return or for personnel to arrive. The longer the autonomy, the bigger the bank. This is why a poorly defined cycle is the most costly mistake: everything that follows inherits its error.

From the duty cycle to the capacity

Duty cycle. Current of each load and duration of each period
Capacity by sections. Each load increase is weighted with the cell capacity factor
Corrections. Temperature, aging and design margin
Bank capacity. Ampere-hours to request from the manufacturer

Why a cell does not always deliver the same

The rated capacity of a cell is given for one discharge rate, for example eight hours for lead-acid or five hours for nickel-cadmium. If it is discharged faster, the cell delivers fewer ampere-hours than that value. If it is discharged slower, it delivers more.

The capacity factor Kt captures that behavior. For a given discharge duration and end-of-discharge voltage per cell, Kt says how many times the rated capacity of the cell is needed for each ampere of load. Manufacturers publish it in tables or curves for each model. A Kt is specific to the cell and to the end voltage: you cannot take that of another one, or of a different cell type.

The section method

The standardized method divides the cycle into sections: the first contains the first period, the second the first two, and so on until the whole cycle is covered. For each section the capacity it requires is calculated and the largest is taken, because that section is the one that governs. The method takes into account when the load rises and when it falls: an increase adds capacity and a decrease subtracts it, since the cell partly recovers.

To the largest section, the capacity for random loads is added, when there are any: those whose exact moment is not known, such as a breaker operation. It is a more detailed calculation than it seems, and its result is sensitive to the manufacturer’s data, so it should be done with the real curves of the chosen model.

The corrections

The capacity obtained is adjusted with three factors, each with a physical reason:

Temperature. In the cold, the cell delivers less capacity. The minimum expected temperature of the battery room is used. Substation cells are sized with it, not with the nominal temperature of 25 °C.
Aging. An aged battery loses capacity. IEEE 450 recommends replacing a vented lead-acid bank when its capacity falls to 80 % of rated. Sizing includes a factor so that the bank meets the cycle even near that limit.
Design margin. A margin for future loads and for uncertainty in the input data.

The product of the three factors multiplies the calculated capacity. The result is the capacity requested from the manufacturer, rounded to commercial cell sizes.

Number of cells and voltage

Besides capacity, sizing fixes the number of cells in series. The system voltage, for example the 125 V DC widely used in substations, is obtained with many cells: about 2 V nominal per cell in lead-acid and 1.2 V in nickel-cadmium.

The number of cells must meet two limits at once:

The maximum voltage that the equipment can withstand limits how many cells fit, because during equalizing charge each cell raises its voltage.
The minimum voltage that the equipment requires limits how far each cell can fall at the end of the discharge.

With the minimum voltage and the number of cells, the end voltage per cell is obtained, which determines which Kt curve is used. This is why the calculation always looks at both extremes: high voltage on charge, low voltage on discharge.

The charger

The charger has two simultaneous tasks: to supply the continuous loads and to recharge the bank in the set time after a discharge. Its current is that of the continuous loads plus the current that recharges the capacity discharged in the cycle, with a factor that recognizes that recharging needs more ampere-hours than were removed. IEEE 946 covers this calculation, and IEEE 485 excludes it from its scope. The details are in the guides on the substation battery charger and the rectifier charger.

What to do when loads change

Sizing is a snapshot of the system at one moment. When bays, communications equipment or heating are added, the cycle changes and the bank can end up short without anyone noticing until AC fails. For that reason it is reviewed at every expansion. The guide on batteries that do not hold the load explains how to tell an undersized bank from an aged one, and the one on the capacity test explains how to check its real condition. On Saft, EnerSys and Hoppecke substation batteries: Saft, EnerSys and Hoppecke substation batteries.

The battery loads (trip and close coils, relays and signaling) are defined in the drawings; see Secondary design: protection and control drawings.

Calculate it here

To estimate the capacity, the number of cells and the charger current from your duty cycle, use our free battery sizing tool.

We can help with your DC system

At Tiza Group we size and verify DC systems for substations and industry, and test banks and chargers during commissioning. See our secondary engineering and testing and commissioning services.

References

IEEE 485, recommended practice for sizing lead-acid batteries for stationary applications.
IEEE 1115, recommended practice for sizing nickel-cadmium batteries for stationary applications.
IEEE 946, recommended practice for the design of DC auxiliary power systems, which includes charger sizing.
IEEE 450, recommended practice for maintenance, testing and replacement of vented lead-acid batteries.

Frequently asked questions

Why is it not enough to multiply the current by the hours of autonomy?

Because a cell does not deliver the same capacity at every discharge rate: the faster it is discharged, the fewer ampere-hours it delivers. In addition, loads change during the cycle, there are brief peaks such as breaker tripping, and the battery ages and loses capacity in the cold. The standardized method accounts for all of that.

What is a Kt factor?

It is the cell capacity factor for a given discharge duration and end-of-discharge voltage. The manufacturer publishes it as a table or curve. It is multiplied by the load increase of each period of the cycle to obtain the capacity that period requires.

Are the bank and the charger sized together?

They are calculated separately, but from the same input data. The bank answers the discharge during the AC failure. The charger must supply the continuous loads and recharge the bank in the time set by the project, and its method is covered by IEEE 946.