Safety has become an important issue in battery design. Under abuse conditions, thermal runaway may occur in a battery. Increased temperature could trigger thermal runaway reactions. Excessive of heat released as a result of such conditions could damage a battery cell, or even cause fire or explosion. Thermal runaway reactions are very complicated and material-specific. For modeling thermal runaway reactions and simulating a battery thermal behavior under thermal abuses, Ansys Fluent offers two semi-empirical models:
One-Equation Model
Four-Equation Model
For details on how to set up and use these model, see Specifying Advanced Options.
One-Equation Model
In the one-equation model, thermal runaway reactions are lumped together as one reaction that can be described by the following kinetics equation [406]:
 | (19–33) |
| where: | |
 = degree of conversion | |
 = pre-exponential factor of the reaction
(s-1) | |
 = activation energy of the reaction (J/mol) | |
 = universal gas constant | |
 = temperature | |
 and  = reaction order parameters |
Equation 19–33 is solved to keep track of the progress of the thermal
runaway reaction. 
changes from 0 to 1, with 0 denoting “no reaction”, and 1
denoting “completion of reaction”.
With 
computed from Equation 19–33, the heat generation
rate due to thermal runaway can be calculated as:
 | (19–34) |
where 
is the specific heat release (J/m3).
Four-Equation Model
In the four-equation model, thermal runaway reactions are put into the following four categories [311]:
solid electrolyte interface (SEI) decomposition reactions
negative electrode-electrolyte reactions
positive electrode-electrolyte reactions
electrolyte decomposition reactions
To keep track of the progress of each type of reactions, a lumped Arrhenius kinetics rate is used:
 | (19–35) |
 | (19–36) |
 | (19–37) |
 | (19–38) |
| where: | |
 ,  , and various  are reaction kinetics parameters, representing pre-exponential factor,
activation energy, and reaction order, respectively | |
subscripts  ,  ,  , and  denote parameters associated with SEI decomposition reaction, negative
electrode-electrolyte reaction, positive electrode-electrolyte reaction, and electrolyte
decomposition reaction, respectively | |
 ,  ,  , and  are dimensionless variables that can be interpreted as the fraction of the
remaining reactant associated to each type of reactions in the medium as reactions
proceed | |
 is the dimensionless measure of a SEI layer thickness | |
 is the reference SEI layer thickness | |
 is the temperature | |
 is the universal gas constant |
The values of 
, 
, and 
change from 1 (representing the state when nothing has reacted yet) to 0
(denoting the state with reactants being completely consumed). 
, on the other hand, varies from 0 to 1, as in the One-Equation model.
can be computed as:
where 
and 
are two initial values. Note that the SEI layer is growing as the negative
electrode-electrolyte reaction progresses.
The total heat generation due to the thermal runaway reactions (in units of W/m3) can be computed as:
 | (19–39) |
where 
(kg/m3) is density of reactants in the medium, and
is the heat of reaction. All heats of reaction are in units of J/kg.
If the internal short reaction is considered, the heat due to internal short,
where 
is the heat of reaction [J/m3], is added
to the thermal abuse heat.
Abuse reaction due to the internal short circuit
In both the one-equation and four-equation models, another abuse reaction due to the internal short circuit can be optionally added [120]. In this model, the internal short circuit reaction is governed by the following kinetics rate equation:
 | (19–40) |
where 
is the progress variable for the internal short reaction, 
is a pre-exponential factor, and 
is activation energy of the reaction. The subscript 
refers to the short-circuit electrochemical reaction. The factor
is used to make sure that the reaction is activated only after the
user-specified trigger temperature 
is reached:
 | (19–41) |