In the Equivalent Circuit Model (ECM), battery electric behavior is mimicked by an electrical circuit. Ansys Fluent has adopted the six parameter ECM model following the work of Chen [[105]]. In this model, the circuit consists of three resistors and two capacitors (Figure 19.1: Electric Circuits Used in the ECM Model).
The voltage-current relation can be obtained by solving the electric circuit equations:
 | (19–9) |
Here, 
is the battery cell voltage that can be either obtained directly from the
circuit solution in the Circuit Network solution method or calculated as 
-
from the MSMD solution method. For a given battery, the open circuit voltage,
resistors' resistances, and capacitors' capacitances are functions of the battery state of
charge (SOC) and temperature. These functions could be expressed in three different ways in
Ansys Fluent:
The fifth order Polynomial form:
(19–10)
The function form proposed by Chen [[105]]:
(19–11)
2D table
This method allows to account for the temperature effect, which cannot be considered in the above two methods. It uses a two dimensional structured table to define each parameter as a function of SOC and temperature.
The source terms for Equation 19–3 and Equation 19–4 are computed as:
 | (19–12) |
 | (19–13) |
where 
is the current, and 
is the open circuit voltage.
Similar to the NTGK/DCIR model, you can use model parameters from a tested battery with
capacity 
for a battery with different capacity 
as long as the battery's material is the same. If the model parameters are
from the battery that is analogous to the one you are simulating, then capacities
and 
will be the same.
Optionally, you can account for the battery property dependencies on other factors through
user-defined functions. See Battery Model DEFINE Macros in the Fluent Customization Manual for details.
The battery parameters and their dependencies on SoC and temperature can be either obtained from a battery OEM vendor or evaluated from a physical prototype by using the parameter estimation tool provided by Ansys Fluent. The tool is available via the text user interface (TUI) (see Battery Model Text Commands in the Fluent Text Command List). The parameter estimation tool can be used to accept the recorded inputs from the more computationally expensive Newman’s P2D model.
The ECM model allows you to simulate battery discharging and recharging processes. The ECM model is economical, versatile, and suitable for simulating both a single battery cell and a battery pack. It can be used for various types of solid batteries, not only for Li-ion types.
For more information about setting up the ECM Model, see Inputs for the Equivalent Circuit Model.