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OTCexpoly

One time-constant expoly-based equivalent-circuit model of a battery

Basic Thermal Parameters

General Parameters

References

Description

The EquivCircuit.OTCexpoly component is an equivalent-circuit model of a generic battery. The open-circuit voltage depends on the state-of-charge (SoC) based on a polynomial with an exponential term. The transient response is modeled by an SoC-dependent resistor-capacitor network.

$Voc = expoly (V_{oc}, soc)$

$R_{0} = expoly (R_{out}, soc)$

$R_{1} = expoly (R_{otc}, soc)$

$R_{1} C_{1} = expoly (T_{tc}, soc)$

Degradation

The gradual decay, with use, of a cell's capacity and increase of its resistance is modeled by enabling the include degradation effects boolean parameter. Enabling this feature adds a state-of-health (soh) output to the model. This signal is 1 when the cell has no decay and 0 when is completely decayed.

The soh output is given by

$soh = {(1 - \frac{s}{R_{s}})}^{3}$

where

$s$ is thickness of the solid-electrolyte interface (SEI),

$R_{s}$ is radius of the particles of active material in the SEI.

The decay of the capacity is

$C = C_{\max} soh$

where

$C$ is the effective capacity, and

$C_{\max}$ is the specified capacity equal to either the parameter $CA$ or the input $C_{in}$ .

The additional series resistance added to a cell is

$R_{sei} = \frac{s}{κ}$

with $κ$ a parameter of the model.

The following equations govern the increase in the thickness of the SEI layer ( $s$ ).

$k = A_{e} \exp (- \frac{E_{a}}{R T})$

$\frac{ds}{dt} = {\begin{array}{c} \frac{k c M}{(1 + \frac{k s}{D_{diff}}) ρ_{sei}} & charging \\ 0 & otherwise \end{array}$

Thermal Effects

Select the thermal model of the battery from the heat model drop-down list. The available models are: isothermal, external port, and convection.

	Isothermal
	The isothermal model sets the cell temperature to a constant parameter, $T_{iso}$ .

External Port

The external port model adds a thermal port to the battery model. The temperature of the heat port is the cell temperature. The parameters $m_{cell}$ and $c_{p}$ become available and are used in the heat equation

$m_{cell} c_{p} \frac{d T_{cell}}{d t} = P_{cell} - Q_{cell}$

$Q_{flow} = n_{cell} Q_{cell}$

$P_{cell} = i_{cell} (v_{cell} - v_{oc})$

where $P_{cell}$ is the heat generated in each cell, including chemical reactions and ohmic resistive losses, $Q_{cell}$ is the heat flow out of each cell, and $Q_{flow}$ is the heat flow out of the external port.

Convection

The convection model assumes the heat dissipation from each cell is due to uniform convection from the surface to an ambient temperature. The parameters $m_{cell}$ , $c_{p}$ , $A_{cell}$ , $h$ , and $T_{amb}$ become available, as does an output signal port that gives the cell temperature in Kelvin. The heat equation is the same as the heat equation for the external port, with $Q_{cell}$ given by

$Q_{cell} = h A_{cell} (T_{cell} - T_{amb})$

	Capacity
	The capacity of a cell can either be a fixed value, $CA$ , or be controlled via an input signal, $C_{in}$ , if the use capacity input box is checked.

	Resistance
	The resistance of a cell can either be a fixed value, $R_{cell}$ , or be controlled via an input signal, $R_{in}$ , if the use resistance input box is checked. This resistance is in addition to the resistance of the equivalent circuit.

State of Charge

A signal output, soc, gives the state-of-charge of the battery, with 0 being fully discharged and 1 being fully charged.

The parameter ${SOC}_{\min}$ sets the minimum allowable state-of-charge; if the battery is discharged past this level, the simulation is either terminated and an error message is raised, or, if the allow overdischarge parameter is true, a warning is generated. A similar effect occurs if the battery is fully charged so that the state of charge reaches one; the simulation is terminated unless allow overcharge is true.

The parameter ${SOC}_{0}$ assigns the initial state-of charge of the battery.

Connections

Name	Type	Description	Modelica ID
$p$	Electrical	Positive pin	p
$n$	Electrical	Negative pin	n
$soc$	Real output	State of charge [0..1]	soc
$C_{in}$	Real input	Sets capacity of cell, in ampere hours; available when use capacity input is true	Cin
$R_{in}$	Real input	Sets resistance of cell, in ohms; available when use cell resistance input is true	Rin

Variables

Name	Units	Description	Modelica ID
$T_{cell}$	$K$	Internal temperature of battery	Tcell
$i$	$A$	Current into battery	i
$v$	$V$	Voltage across battery	v

Basic Parameters

Name	Default	Units	Description	Modelica ID
$N_{cell}$	$1$		Number of cells, connected in series	Ncell
$CA$	$1$	$A·h$	Capacity of cell; available when use capacity input is false	C
${SOC}_{0}$	$1$		Initial state-of-charge [0..1]	SOC0
${SOC}_{\min}$	$0.02$		Minimum allowable state-of-charge	SOCmin
$R_{cell}$	$0.005$	$Ω$	Fixed cell resistance, if use cell resistance input is false	Rcell
allow overcharge	false		True allows simulation to continue with $1 < SoC$	allow_overcharge
allow overdischarge	false		True allows simulation to continue with $SoC < {SoC}_{\min}$	allow_overdischarge
use capacity input	false		True allows enables the $C_{in}$ input port	useCapacityInput
use cell resistance input	false		True allows enables the $R_{in}$ input port	useResistInput

Basic Thermal Parameters

Name	Default	Units	Description	Modelica ID
$T_{iso}$	$298.15$	$K$	Constant cell temperature; used with isothermal heat model	Tiso
$c_{p}$	$750$	$\frac{J}{kg K}$	Specific heat capacity of cell	cp
$m_{cell}$	$0.014$	$kg$	Mass of one cell	mcell
$h$	$100$	$\frac{W}{m^{2} K}$	Surface coefficient of heat transfer; used with convection heat model	h
$A_{cell}$	$0.0014$	$m^{2}$	Surface area of one cell; used with convection heat model	Acell
$T_{amb}$	$298.15$	$K$	Ambient temperature; used with convection heat model	Tamb

General Parameters

Name	Default	Units	Description	Modelica ID
$V_{oc}$		$V$	expoly array for open-circuit voltage	Voc
$R_{out}$		$Ω$	expoly array for series resistance	Rout
$R_{tc}$		$Ω$	expoly array for time-constant resistance	Rtc
$T_{tc}$		$s$	expoly array for time-constant duration	Ttc

An exponential-polynomial (expoly) is a polynomial with an exponential term included. Its coefficients are given by a one-dimensional array, $k$ , such that $&ExponentialE;xpoly (k, soc) = k_{1} &ExponentialE;xp (k_{2} soc) + k_{3} + k_{4} soc + k_{5} {soc}^{2} + \dots$ .

References

[1] Chen, M. and Rincón-Mora, G.A., Accurate electrical battery model capable of predicting runtime and I-V performance, IEEE Transactions of Energy Conversion, Vol. 21, No. 2, 2006.

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