Nickel-Metal Hydride — Electrochemical model of a nickel-metal hydride battery
Basic Thermal Parameters
The Nickel-Metal Hydride component is a model of a nickel-metal hydride battery based on a planar electrode approximation. The mass-balance of active materials, the kinetics of electrochemical reactions, internal resistance, and the energy balance of the cell are incorporated. See .
There are two main redox reactions at the positive and negative electrodes: the reaction of the nickel active material at the positive electrode and the reaction of the metal hydride material at the negative electrode. Besides that, Ni-MH cells are also known to have side reactions which cause gases to form inside the cell casing. The most significant side reactions are the oxygen evolution reaction at the positive electrode and the oxygen reduction reaction at the negative electrode. The main and side chemical reactions are described by the following equations:
Positive nickel electrode:
Negative metal hydride electrode:
Butler-Volmer's equation describes the kinetics of reactions for both positive and negative electrodes:
Φp,Φn are the electrical potentials in the positive and negative electrodes,
Φeqk is the equilibrium potential of reaction k at the standard conditions,
i0,k is the exchange current density of reaction k,
i4,ref is limiting current density of the oxygen reduction reaction, and
pO2,pO2,ref are the pressure and reference pressure of oxygen in the cell.
The exchange current densities vary with the nickel hydroxide concentration ( + ) in the nickel active material, and the hydrogen concentration in metal hydride material ( ) and are described by the following equations.
The open-circuit potential curves based on the Nernst equation are utilized:
The charge and mass balances on the electrodes are given by
Select the thermal model of the battery from the heat model drop-down list. The available models are: isothermal, external port, and convection.
The isothermal model sets the cell temperature to a constant parameter, Tiso.
The external port model adds a thermal port to the battery model. The temperature of the heat port is the cell temperature. The parameters mcell and cp become available and are used in the heat equation
where Pcell is the heat generated in each cell, including chemical reactions and ohmic resistive losses, Qcell is the heat flow out of each cell, and Qflow is the heat flow out of the external port.
The convection model assumes the heat dissipation from each cell is due to uniform convection from the surface to an ambient temperature. The parameters mcell, cp, Acell, h, and Tamb 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 Qcell given by
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 SOCmin sets the minimum allowable state-of-charge; if the battery is discharged past this level, the simulation is terminated and an error message is raised. This prevents the battery model from reaching non-physical conditions. A similar effect occurs if the battery is fully charged so that the state of charge reaches one.
The parameter SOC0 assigns the initial state-of charge of the battery.
The capacity of the battery can either be a fixed value, CA, or be controlled via an input signal, Cin, if the use capacity input box is checked.
The resistance of each cell can either be a fixed value, Rcell, or be controlled via an input signal, Rin, if the use cell resistance input box is checked.
Internal temperature of battery
Current into battery
Voltage across battery
State of charge [0..1]
Sets capacity of cell, in ampere hours; available when use capacity input is true
Sets resistance of cell, in Ohms; available when use resistance input is true
Temperature of cell, in Kelvin; available with convection heat model
Thermal connection; available with external port heat model
Number of cells, connected in series
Capacity of cell, in ampere-hours
Initial state-of-charge [0..1]
Minimum allowable state-of-charge
Internal resistance of one cell; available if use cell resistance input is not enabled
Constant cell temperature; used with isothermal heat model
Specific heat capacity of cell
Mass of one cell
Surface coefficient of heat transfer; used with convection heat model
Surface area of one cell; used with convection heat model
Ambient temperature; used with convection heat model
Activation energy of reaction 1
Activation energy of reaction 2
Activation energy of reaction 3
Activation energy of reaction 4
Loading of nickel active material
Loading of metal hyrdroxide material
Apparent open-circuit potential of the redox reaction of nickel active material at standard conditions during the whole range charge process
Apparent open-circuit potential of the redox reaction of nickel active material at standard conditions during the whole range discharge process
Equilibrium potential of reaction 2 at standard condition
Equilibrium potential of reaction 3 at standard condition
Equilibrium potential of reaction 4 at standard condition
Specific surface area of negative electrode
Specific surface area of positive electrode
Maximum concentration of nickel hydroxide in nickel active material
Reference concentration of nickel hydroxide in nickel active material
Maximum concentration of hydrogen in metal hydride material
Reference concentration of hydrogen in metal hydride material
Concentration of KOH electrolyte
Reference concentration of KOH electrolyte
Temperature coefficient of reaction 1, from Wang (2000) Thermal-Electrochemical Modeling of Battery Systems
Temperature coefficient of reaction 2, from Wang (2000) Thermal-Electrochemical Modeling of Battery Systems
Temperature coefficient of reaction 3, from Wang (2000) Thermal-Electrochemical Modeling of Battery Systems
Temperature coefficient of reaction 4, from Wang (2000) Thermal-Electrochemical Modeling of Battery Systems
Exchange current density of reaction 1 at reference reactant concentrations
Exchange current density of reaction 2 at reference reactant concentrations
Exchange current density of reaction 3 at reference reactant concentrations
Exchange current density of reaction 4 at reference reactant concentrations
Thickness of negative electrode
Thickness of positive electrode
Reference oxygen pressure in cell
Density of metal hydride
Density of nickel active material
Gas volume in cell
Volume of cell
 Wu, B., Mohammed, M., Brigham, D., Elder, R., and White, R.E., A non-isothermal model of a nickel-metal hydride cell, Journal of Power Sources, 101 (2001) pp. 149-157.
 Dao, T.S. and McPhee, J., Dynamic modeling of electrochemical systems using linear graph theory, Journal of Power Sources, No. 196, pp.10442-10454, 2011.
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