About:ECM
Equivalent Circuit Model - fast, accurate voltage prediction for accelerated pack concept design
Overview
About:ECM is an Equivalent Circuit Model (ECM) representing a battery as a network of electronic components. This model predicts the battery current-voltage relation as a function of state-of-charge (SOC) and temperature. Battery properties depend on State-of-Health (SOH), as defined by inputs for capacity fade and resistance increase. The model is capable of evaluating battery heat dissipation rate and battery temperature change in conjunction with a 0-dimensional lumped thermal model.
Technical Description
About:ECM solves Kirchhoff's laws for the current-voltage relation as applied to a network of electronic components. About:ECM is compatible with any specification of current, voltage or power load on the battery. The About:ECM consists of the following:
Figure 1: Schematic of a 2-RC pair circuit.
Open Circuit Voltage (OCV)
OCV represents the equilibrium voltage of a cell when no current flows and all transient effects have settled. This voltage directly reflects the thermodynamic potential difference between the positive and negative electrodes at a given state of charge. OCV varies nonlinearly with state of charge and temperature, serving as the fundamental voltage source in the equivalent circuit model.
Series Resistance (R0)
The series resistance captures the instantaneous voltage drop that occurs when current begins to flow through the cell. R0 causes an immediate voltage change proportional to the applied current, with no time delay, making it responsible for the sharp voltage discontinuity observed when switching between charge, discharge, and rest conditions.
Resistor Capacitor (RC) Pairs
RC pairs model the time-dependent voltage relaxation processes within the cell, capturing phenomena such as charge transfer kinetics and diffusion limitations. The resistor determines the magnitude of the voltage response, while the capacitor controls how quickly this voltage builds or decays, creating the characteristic curved voltage response observed during current steps or after current interruption.
A 2-RC pair circuit is the standard implementation within About:ECM - this provides a good resolution across multiple timescales within a cell, whilst mitigating risks of over-fitting and excessive complexity of parameterisation, along with maintaining a fast-executing simulation.
Key features
Features of About:ECM include:
- Compatibility with any About:Energy thermal model
- Compatibility with distributed electronic networks and 3D cell/module/pack models
- Compatibility with user-defined inputs for capacity fade and resistance increase
Key applications
About:ECM provides fast, accurate estimation of a single set of cell states (voltage, temperature, etc.) - typically representing a single battery cell (or scaled into an averaged representation of multiple cells or packs). It is fast enough to run numerous instances simultaneously, allowing prediction of distributed behaviour across battery packs.
About:ECM applications include:
- Cell comparison and cell selection
- Rapid system prototyping
- Beginning-of-life and aged-cell scenario analysis
- Analysis of battery pack variance and fault states
- Generation of heat generation rates for 3D thermal model inputs