# Supercapacitor

Implement generic supercapacitor model

## Library

Electric Drives/Extra Sources

## Description

The Supercapacitor block implements a generic model parameterized to represent most popular types of supercapacitors. The figure shows the equivalent circuit of the supercapacitor:

The supercapacitor output voltage is expressed using a Stern equation as:

`${V}_{SC}=\frac{{N}_{s}{Q}_{T}d}{{N}_{p}{N}_{e}\epsilon {\epsilon }_{0}{A}_{i}}+\frac{2{N}_{e}{N}_{s}RT}{F}{\mathrm{sinh}}^{-1}\left(\frac{{Q}_{T}}{{N}_{p}{N}_{e}{}^{2}{A}_{i}\sqrt{8RT\epsilon {\epsilon }_{0}c}}\right)-{R}_{SC}\cdot {i}_{SC}$`

with

`${Q}_{T}=\int {i}_{SC}dt$`

To represent the self-discharge phenomenon, the supercapacitor electric charge is modified as follows (when iSC = 0):

`${Q}_{T}=\int {i}_{self_dis}dt$`

where

`${i}_{self_dis}=\left\{\begin{array}{l}\frac{{C}_{T}{\alpha }_{1}}{1+s{R}_{SC}{C}_{T}}\begin{array}{cc}& if\begin{array}{cc}& t-{t}_{oc}\le {t}_{3}\end{array}\end{array}\\ \frac{{C}_{T}{\alpha }_{2}}{1+s{R}_{SC}{C}_{T}}\begin{array}{cc}& if\begin{array}{cc}& {t}_{3}\prec t-{t}_{oc}\le {t}_{4}\end{array}\end{array}\\ \frac{{C}_{T}{\alpha }_{3}}{1+s{R}_{SC}{C}_{T}}\begin{array}{cc}& if\begin{array}{cc}& t-{t}_{oc}\succ {t}_{4}\end{array}\end{array}\end{array}$`

The constants α1, α2, and α3 are the rates of change of the supercapacitor voltage during time intervals (toc, t3), (t3, t4), and (t4, t5) respectively, as shown in the figure:

VariableDescription
AiInterfacial area between electrodes and electrolyte (m2)
cMolar concentration (mol/m3) equal to c = 1/(8NAr3)
iscSupercapacitor current (A)
VscSupercapacitor voltage (V)
CTTotal capacitance (F)
RscTotal resistance (ohms)
NeNumber of layers of electrodes
Np Number of parallel supercapacitors
NsNumber of series supercapacitors
QTElectric charge (C)
RIdeal gas constant
TOperating temperature (K)
εPermittivity of material
ε0Permittivity of free space

## Parameters

### Parameters Tab

Rated capacitance (F)

Specify the nominal capacitance of the supercapacitor, in farad. Default is `99.5`.

Equivalent DC series resistance (Ohms)

Specify the internal resistance of the supercapacitor, in ohms. Default is `8.9e-3`.

Rated voltage (V)

Specify the rated voltage of the supercapacitor, in volts. Typical rated voltage is equal to 2.7 V. Default is `48`.

Number of series capacitors

Specify the number of series capacitors to be represented. Default is `18`.

Number of parallel capacitors

Specify the number of parallel capacitors to be represented. Default is `1`.

Initial voltage (V)

Specify the initial voltage of the supercapacitor, in volts. Default is `0`.

Operating temperature (celsius)

Specify the operating temperature of the supercapacitor. The nominal temperature is 25° C. Default is `25`.

### Stern Tab

Use predetermined parameters

When this check box is selected, loads predetermined parameters of the Stern model into the mask of the block. These parameter values have been determined from experimental tests, and they can be used as default values to represent a common supercapacitor. Experimental validation of the model has shown a maximum error of 2% for charge and discharge when using the predetermined parameters. Default is cleared

When this check box is selected, the Number of layers, Molecular radius (m), Permittivity of electrolyte material (F/m), and Estimate using test data parameters appear dimmed.

Estimate using test data

When this check box is selected, you provide test data required for the estimation of the Stern model parameters. Default is cleared. This parameter is available only if the Optimization Toolbox™ is installed.

When this check box is selected, the Charge current (A) and Voltage @ 0 s, 20 s, and 60 s [V_0, V_2, V_3] (V) parameters are enabled. The Use predetermined parameters, Number of layers, Molecular radius (m), and Permittivity of electrolyte material (F/m) parameters appear dimmed.

Number of layers

Specify the number of layers related to the Stern model. Default is `1`.

Specify the molecular radius related to the Stern model, in meters. Default is `1e-9`.

Permittivity of electrolyte material (F/m)

Specify the permittivity of the electrolyte material, in farad/meter. Default is `6.0208e-10`.

Charge current (A)

Specify the charge current during a constant current charge test, in amperes. Default is `10`.

Voltage @ 0 s, 20 s, and 60 s [V_0, V_2, V_3] (V)

Specify the supercapacitor voltage, in volts, at 0 s, 20 s, and 60 s, when the supercapacitor is charged with a constant current equal to the value provided in the Charge current (A) parameter. Default is `[0.161 2.7 7.8]`.

### Self-discharge Tab

Simulate self-discharge

When this check box is selected, you provide test data required for modeling the self-discharge phenomenon. Default is selected.

Current prior open-circuit (A)

Specify the current prior to an open-circuit event, in amperes. Default is `10`.

Voltage @ 0 s, 10 s, 100 s, and 1000 s [V_oc, V_3, V_4, V_5] (V)

Specify the supercapacitor voltage, in volts, at 0 s, 10 s, 100 s, and at 1000 s, when the supercapacitor is open-circuit. The corresponding current prior to open-circuit is given in the Current prior open-circuit (A) parameter. Default is ```[48 47.8 47.06 44.65]```.

Plot charge characteristics

When this check box is selected, the block plots a figure containing the charge curves at the specified charge currents and time units. Default is cleared.

Charge current [i_1, i_2, i_3, ...] (A)

Specify the charge currents, in amperes, used to plot the charge characteristics. Default is `[10 20 100 500]`.

Time units

Specify the time units (seconds, minutes, hours) used to plot the charge characteristics. Default is `sec`.

## Inputs and Outputs

`m`

Outputs a vector containing measurement signals. You can demultiplex these signals using the Bus Selector block.

SignalDefinitionUnitsSymbol
1The supercapacitor currentA`Current`
2The supercapacitor voltageV`Voltage`
3The state of charge (SOC), between 0 and 100%`SOC`

The SOC for a fully charged supercapacitor is 100% and for an empty supercapacitor is 0%. The SOC is calculated as:

`$SOC=\frac{\underset{0}{\overset{t}{Qinit-\int i\left(\tau \right)d\tau }}}{{Q}_{T}}×100$`

## Model Assumptions

• Internal resistance is assumed constant during the charge and the discharge cycles.

• The model does not take into account temperature effect on the electrolyte material.

• No aging effect is taken into account.

• Charge redistribution is the same for all values of voltage.

• The block does not model cell balancing.

• Current through the supercapacitor is assumed to be continuous.

## Examples

The `parallel_battery_SC_boost_converter` example shows a simple hybridization of a supercapacitor with a battery. The supercapacitor is connected to a buck/boost converter and the battery is connected to a boost converter. The DC bus voltage is equal to 42V. The converters are doing power management. The battery power is limited by a rate limiter block, therefore the transient power is supplied to the DC bus by the supercapacitor.

## References

[1] Oldham, K. B. “A Gouy-Chapman-Stern model of the double layer at a (metal)/(ionic liquid) interface.” J. Electroanalytical Chem. Vol. 613, No. 2, 2008, pp. 131–38.

[2] Xu, N., and J. Riley. “Nonlinear analysis of a classical system: The double-layer capacitor.” Electrochemistry Communications. Vol. 13, No. 10, 2011, pp. 1077–81.