Behavioral battery model

**Library:**Simscape / Electrical / Sources

The Battery block represents a simple battery model.
The block has four modeling variants, accessible by right-clicking the block in your
block diagram and then selecting the appropriate option from the context menu, under **Simscape** > **Block choices**:

**Uninstrumented | No thermal port**— Basic model that does not output battery charge level or simulate thermal effects. This modeling variant is the default.**Uninstrumented | Show thermal port**— Model with exposed thermal port. This model does not measure internal charge level of the battery.**Instrumented | No thermal port**— Model with exposed charge output port. This model does not simulate thermal effects.**Instrumented | Show thermal port**— Model that lets you measure internal charge level of the battery and simulate thermal effects. Both the thermal port and the charge output port are exposed.

The instrumented variants have an extra physical signal port that outputs the internal state of charge. Use this functionality to change load behavior as a function of state of charge, without the complexity of building a charge state estimator.

The thermal port variants expose a thermal port, which represents the battery thermal mass. When you select this option, provide additional parameters to define battery behavior at a second temperature. For more information, see Modeling Thermal Effects.

The battery equivalent circuit is made up of the fundamental battery model, the
self-discharge resistance *R _{SD}*, the charge
dynamics model, and the series resistance

If you select `Infinite`

for the **Battery charge
capacity** parameter, the block models the battery as a series resistor
and a constant voltage source. If you select `Finite`

for
the **Battery charge capacity** parameter, the block models the
battery as a series resistor and a charge-dependent voltage source. In the finite
case, the voltage is a function of charge and has the following relationship:

$$V={V}_{0}\left(\frac{\text{SOC}}{1-\beta (1-\text{SOC})}\right)$$

where:

`SOC`

(state-of-charge) is the ratio of current charge to rated battery capacity.*V*_{0}is the voltage when the battery is fully charged at no load, as defined by the**Nominal voltage, Vnom**parameter.*β*is a constant that is calculated so that the battery voltage is*V1*when the charge is*AH1*. Specify the voltage*V1*and ampere-hour rating*AH1*using block parameters.*AH1*is the charge when the no-load (open-circuit) voltage is*V1*, and*V1*is less than the nominal voltage.

The equation defines an approximate relationship between voltage and remaining charge. This approximation replicates the increasing rate of voltage drop at low charge values, and ensures that the battery voltage becomes zero when the charge level is zero. The advantage of this model is that it requires few parameters, which are readily available on most datasheets.

For battery models with finite battery charge capacity, you can model battery
performance deterioration depending on the number of discharge cycles. This
deterioration is referred to as *battery fade*. To enable
battery fade, set the **Battery fade** parameter to
`Enabled`

. This setting exposes additional parameters
in the **Fade** section.

The block implements battery fade by scaling certain battery parameter values that
you specify in the **Main** section, depending on the number of
completed discharge cycles. The block uses multipliers
*λ*_{AH},
*λ*_{R0}, and
*λ*_{V1} on the **Ampere-hour
rating**, **Internal resistance**, and
**Voltage V1 when charge is AH1 (V1<Vnom)** parameter
values, respectively. These multipliers, in turn, depend on the number of discharge
cycles:

$${\lambda}_{AH}=1-{k}_{1}{N}^{0.5}$$

$${\lambda}_{R0}=1+{k}_{2}{N}^{0.5}$$

$${\lambda}_{V1}=1-{k}_{3}N$$

$$N={N}_{0}+\frac{1}{AH}{\displaystyle \underset{0}{\overset{t}{\int}}\frac{i\left(t\right)\cdot H\left(i\left(t\right)\right)}{{\lambda}_{AH}\left(t\right)}dt}$$

where:

*λ*is the multiplier for battery nominal capacity._{AH}*λ*is the multiplier for battery series resistance._{R0}*λ*is the multiplier for voltage_{V1}*V1*.*N*is the number of discharge cycles completed.*N*is the number of full discharge cycles completed before the start of the simulation._{0}*AH*is the rated battery capacity in ampere-hours.*i(t)*is the instantaneous battery output current.*H(i(t))*is the Heaviside function of the instantaneous battery output current. This function returns 0 if the argument is negative, and 1 if the argument is positive.

The block calculates the coefficients
*k _{1}*,

*k*= 1e-2_{1}*k*= 1e-3_{2}*k*= 1e-3_{3}

You can also define a starting point for a simulation based on the previous
charge-discharge history by using the high-priority variable **Discharge
cycles**. For more information, see Variables.

For thermal variants of the block, you provide additional parameters to define battery behavior at a second temperature. The extended equations for the voltage when the thermal port is exposed are:

$$V={V}_{0T}\left(\frac{\text{SOC}}{1-\beta (1-\text{SOC})}\right)$$

$${V}_{0T}={V}_{0}\left(1+{\lambda}_{V}\left(T-{T}_{1}\right)\right)$$

where:

*T*is the battery temperature.*T*is the nominal measurement temperature._{1}*λ*is the parameter temperature dependence coefficient for_{V}*V*_{0}.*β*is calculated in the same way as Battery Model, using the temperature-modified nominal voltage*V*._{0T}

The internal series resistance, self-discharge resistance, and any charge-dynamic resistances are also functions of temperature:

$${R}_{T}=R\left(1+{\lambda}_{R}\left(T-{T}_{1}\right)\right)$$

where *λ _{R}* is the parameter temperature
dependence coefficient.

All the temperature dependence coefficients are determined from the corresponding values you provide at the nominal and second measurement temperatures. If you include charge dynamics in the model, the time constants vary with temperature in the same way.

The battery temperature is determined from a summation of all the ohmic losses included in the model:

$${M}_{th}\dot{T}={\displaystyle \sum _{i}{V}_{T,i}{}^{2}/{R}_{T,i}}$$

where:

*M*is the battery thermal mass._{th}*i*corresponds to the*i*th ohmic loss contributor. Depending on how you have configured the block, the losses include:Series resistance

Self-discharge resistance

First charge dynamics segment

Second charge dynamics segment

Third charge dynamics segment

Fourth charge dynamics segment

Fifth charge dynamics segment

*V*is the voltage drop across resistor_{T,i}*i*.*R*is resistor_{T,i}*i*.

You can model battery charge dynamics using the **Charge
dynamics** parameter:

`No dynamics`

— The equivalent circuit contains no parallel RC sections. There is no delay between terminal voltage and internal charging voltage of the battery.`One time-constant dynamics`

— The equivalent circuit contains one parallel RC section. Specify the time constant using the**First time constant**parameter.`Two time-constant dynamics`

— The equivalent circuit contains two parallel RC sections. Specify the time constants using the**First time constant**and**Second time constant**parameters.`Three time-constant dynamics`

— The equivalent circuit contains three parallel RC sections. Specify the time constants using the**First time constant**,**Second time constant**, and**Third time constant**parameters.`Four time-constant dynamics`

— The equivalent circuit contains four parallel RC sections. Specify the time constants using the**First time constant**,**Second time constant**,**Third time constant**, and**Fourth time constant**parameters.`Five time-constant dynamics`

— The equivalent circuit contains five parallel RC sections. Specify the time constants using the**First time constant**,**Second time constant**,**Third time constant**,**Fourth time constant**, and**Fifth time constant**parameters.

This figure shows the equivalent circuit for the block configured with two time-constant dynamics.

In the diagram:

*R*and_{RC1}*R*are the parallel RC resistances. Specify these values with the_{RC2}**First polarization resistance**and**Second polarization resistance**parameters, respectively.*C*and_{RC1}*C*are the parallel RC capacitances. The time constant_{RC2}*τ*for each parallel section relates the*R*and*C*values using the relationship $$C=\tau /R$$. Specify*τ*for each section using the**First time constant**and**Second time constant**parameters, respectively.*R*is the series resistance. Specify this value with the_{0}**Internal resistance**parameter.

A quick plot feature lets you visualize the voltage-charge characteristic for the
battery model parameter values. To plot the characteristics, right-click a
Battery block in your model and, from the context
menu, select **Electrical** > **Basic characteristic**. The software automatically computes a set of bias conditions, based
on the block parameter values, and opens a figure window containing a plot of
no-load voltage versus the state-of-charge (SOC) for the block. For more
information, see Plot Basic Characteristics for Battery Blocks.

Use the **Variables** section of the block
interface to set the priority and initial target values for the block
variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables (Simscape).

Unlike block parameters, variables do not have conditional visibility.
The **Variables** section lists all the existing
block variables. If a variable is not used in the set of equations
corresponding to the selected block configuration, the values specified
for this variable are ignored.

When you model battery fade, the **Discharge cycles** variable
lets you specify the number of charge-discharge cycles completed prior to the start
of simulation. If you disable battery fade modeling, this variable is not used by
the block.

The self-discharge resistance is assumed not to depend strongly on the number of discharge cycles.

For the thermal variant of the battery, you provide fade data only for the reference temperature operation. The block applies the same derived

*λ*,_{AH}*λ*, and_{R0}*λ*multipliers to parameter values corresponding to the second temperature._{V1}When using the thermal block variants, use caution when operating at temperatures outside of the temperature range bounded by the

**Measurement temperature**and**Second measurement temperature**values. The block uses linear interpolation for the derived equation coefficients, and simulation results can become nonphysical outside of the specified range. The block checks that the internal series resistance, self-discharge resistance, and nominal voltage always remain positive. If there is a violation, the block issues error messages.

[1] Ramadass, P., B. Haran, R. E. White, and B. N. Popov.
“Mathematical modeling of the capacity fade of Li-ion cells.” *Journal of
Power Sources*. 123 (2003), pp. 230–240.

[2] Ning, G., B. Haran, and B. N. Popov. “Capacity fade study of
lithium-ion batteries cycled at high discharge rates.” *Journal of Power
Sources*. 117 (2003), pp. 160–169.