Batteries

The Battery option is accessed by selecting the Battery category on the Storage dialog. Batteries can be used to store charge in an Electric load centre. The battery bank is a collection of one or more individual battery modules. Given the surplus or deficit power from the electrical system and the state of charge from the previous time step, this object can model the voltage, current, and energy losses with charging and discharging during each time step. The cumulative battery damage can be also modelled and reported at the end of each simulation run.

The battery component allows both Lead-Acid and Nickel Cadmium batteries to be simulated. With input parameters derived from specific battery tests, the object is expected to support other battery types such as Lithium-ion batteries.

The kinetic battery model assumes that part of the battery’s energy storage capacity is immediately available in discharging or charging while the rest is chemically bound. As a function of constant current, the battery capacity is related to three parameters: the maximum capacity at infinitesimal current, the capacity ratio of available charges, and the conversion ratio between available charges and bound charges. These parameters are usually obtained via curve fitting based on battery data sheets or test data.

Each individual battery module is modelled as a voltage source in series with an electrical resistance. KiBaM assumes that the internal resistance is constant and the open circuit voltage varies with the electric current, the state of charge and the operation mode (charging or discharging). For an individual battery module, the open circuit voltage at any time is correlated to the voltage at fully charged/discharged state and three other regression coefficients. These regression coefficients are usually obtained via curve fitting based on battery test data.

The object offers user the option to perform battery life calculation. If battery life is modelled, the user needs to provide a group of coefficients for the correlation between the number of cycles for battery failure and the corresponding cycle range. More detailed information can be found in the Engineering Reference.

Name

This alpha field contains the identifying name for the battery bank.

Category

There are 2 categories of electrical storage device to choose from:

The Category selected affects the type of model used and the data to be entered on the Storage dialog.

Availability schedule

This alpha field contains the schedule name (ref. Schedule) that describes when the battery is available. A schedule value greater than 0 (usually 1 is used) indicates that electrical energy can be stored or drawn from the battery. A value less than or equal to 0 (usually 0 is used) denotes that the battery is not available. If this field is blank, the schedule has values of 1 for all time periods.

HEAT GAIN TO ZONE

Attach to a zone

If the losses from the storage device should be represented as heat gains to a zone then check this box and select the zone in the next field.

Zone

This field contains the name of the thermal zone where the battery is located. Entering a valid name of zone here will direct EnergyPlus to include the energy storage losses as heat gains to the named thermal zone. If the battery is not within a thermal zone, this field can be left blank and the thermal energy associated with storage losses is removed from the building model.

Radiative fraction for zone heat gains

This field contains the fraction of storage losses that enter the zone as long-wave thermal radiation. This numeric filed should have a value between 0.0 and 1.0. The balance of the losses is convective. This data is only required when the Attach to a zone option is checked.

BATTERY STORAGE

Number of battery modules in parallel

This field defines the number of modules connected in parallel in the battery bank.

Number of battery modules in series

This field defines the number of modules connected in series in the battery bank. The total number of modules in a battery bank is equal to the number of modules in parallel times the number of modules in series.

Maximum module capacity

This field indicates the maximum capacity of one battery module. It is evaluated as the total Amp-hours available when a full battery is discharged at infinitesimal current. This capacity is for an individual battery module, not for the whole battery bank. The maximum capacity can be found from manufacture’s data or derived from test data by curve fitting.

Initial fractional state of charge

This field describes the initial state of charge in terms of the fraction of maximum capacity as defined in the previous field.

Fraction of available charge capacity

The kinetic battery model assumes that battery is a two-tank electrical energy storage device: an available tank and a bound tank. The available tank can be immediately charged or discharged, while the bound tank can only be charged or discharged via the available tank.

This field specifies the fraction of total charge in the battery that is part of the available tank. The ratio of available charge capacity to total capacity is a parameter usually derived from test data by curve fitting.

Change rate from bound charge to available charge

This field specifies the rate at which the charge flows between the available tank and the bound tank. It is a parameter used to calculate the conversion between available charge and chemically bound charge when charging and discharging the battery. This parameter is usually derived from test data by curve fitting.

Fully charged module open circuit voltage

This field indicates the open circuit voltage for a fully charged battery module. It can be found from manufacture’s data or battery tests.

Fully discharged module open circuit voltage

This field indicates the open circuit voltage for a fully discharged battery module. It can be found from manufacture’s data or battery tests.

Voltage change curve for charging

This field specifies the name of a Rectangular Hyperbola 2 performance curve that determines the change of open circuit voltage (?E ) as a function of the battery state of charge in charging. The change of open circuit voltage is relative to a fully discharged battery. The curve has the following form:

where X is the battery charge divided by the maximum capacity at a given current. More details can be found from the Engineering Reference.

Voltage change curve for discharging

This field specifies the name of a Rectangular Hyperbola 2 performance curve that determines the change of open circuit voltage (?E ) as a function of the battery state of charge in discharging. The change of open circuit voltage is relative to a fully charged battery. The curve has the following form:

where X is the removed charge divided by the maximum capacity at a given current. More details can be found from the Engineering Reference.

Module internal electrical resistance

This field specifies the battery internal resistance in ohms. In theory, the electrical resistance within the battery varies with the state of charge and depends on whether it is in charging or discharging. In the kinetic battery model, the internal resistance is assumed as constant and the terminal voltage varies with current and state of charge. The internal resistance may be obtained from the battery manufacture or battery test data. Note that the field is for an individual module, not for the whole battery bank.

Maximum module discharging current

This field indicates the maximum current at which the battery can be discharged continuously. The limit on discharge current is usually defined by the battery manufacture to avoid battery damage. The limit is for an individual battery module.

Module cut-off voltage

This field specifies the minimum allowable voltage, below which the battery is generally regarded as empty. The cut-off voltage is usually defined by the battery manufacture and it is for an individual battery module.

Module charge rate limit

This field specifies the limit on charging current relative to the remaining charge until the battery is full. This limit reflects the common practice that the charge rate is reduced as the battery gets more charged.

BATTERY LIFE

Battery life calculation

This checkbox indicates whether the battery life model is activated in the simulation. If the battery life model is activated, the following 2 inputs are required.

Number of cycle bins

This field specifies the number of equally ranged cycle bins in battery life simulation. If 10 bins are used, the cycle ranges will include 10%, 20%, …, 100%.

Battery cycle life curve

This field specifies the Double Exponential Decay Curve that correlates the cycles of battery failure (CF) and fractional depth of discharge (R). The curve is: