Energy Storage Devices

Electrical Equivalent Cell Modelling of a Commercial AA Rechargeable Battery

In this project I developed an equivalent electrical cell model for a commercial NiMH rechargeable secondary cell is developed using MATLAB Simulink. To conduct the parameter estimation I built a simple testbench with an Arduino interfaced with MATLAB. This project was with the guidance of Prof. R. Jayaganthan from the Department of Engineering Design, IIT Madras.

Arduino Testbench

A very simple circuit that allows for controlled discharge of the AA cell was made with an Arduino MEGA as the controller.

A relay controlled the discharge of the cell while two analog input pins on the Arduino MEGA were used as a shunt resistance ammeter and a voltmeter.

Different discharge resistances, discharge pulse widths and spacings were conducted and the data was all logged directly to a MATLAB workspace variable. Also to monitor the progress of the experiment as it happened, a real time plot of the current and voltage signals were plotted in a MATLAB figure.

All this data was consolidated with the specific discharge pattern so that the parameter estimation could be easily accomplished.

A sample of the voltage data collected is shown below in the graph during discharge with the least external resistance possible with the testbench.

Electrical Model of the Cell

I used a cell model with 5 parameters, which include one series resistance and two sets of RC sets. The single series resistance with the other two resistances define the total internal resistance of the cell. The two RC sets give two time constants to the cell's transient response. An ideal voltage source (which is a function of the state of charge, SoC) is attached to this whole arrangement.

This electrical model was implemented in MATLAB Simulink using the Simscape Electrical toolbox. This cell model could then be used with other Simulink blocks and models.

Electrical Model in Simulink

Electrical Model with Voltage-SoC curve

All figures are mine.