Development of a small power converter for testing small lithium batteries

Introduction:

Experimental research on degradation of big battery systems, e.g. in electric vehicles, is expensive and slow. Many commercial battery systems contain a power converter that does not support all the features needed for standard battery tests and don't have cell-level voltage and temperature measurements.

However, even the biggest commercially available lithium battery systems use a big number of small cells, the 18650 format is among the most common. Therefore, we may scale down the experiment to individual 18650 cells, which are inexpensive and widely available (used in flashlights, laptops...).

The goal of this project is to develop a complete experimental setup for measuring characteristics of 18650 batteries, and use it to explore battery degradation processes.

Tasks:

1. The converter shall enable charging and discharging of the 18650 cell with currents ranging from 150 mA to 6 A. Design an unipolar, current-bidirectional converter that enables this functionality, taking into account requirements for current ripple and rise/fall times. Confirm the functionality of the designed converter by simulating its behaviour in a free tool LTspice.
2. Draw an electric schematic of the converter based on the design from Task 1 and design the PCB. Add the functionalities of voltage and current measurements. The final product shall consist of one microcontroller and four converters with voltage and current measurement, so that four individual cells may be tested simultaneously and independently. Make the PCB and the converter.
3. Write a program for the microcontroller that enables setting reference charging/discharging current and reference tracking in a closed loop. The microcontroller communicates with the PC via serial link, data is exchanged in plain text.
4. Write a program for the PC for communication with the converter. The program shall enable running standard test procedures for lithium batteries with easily changeable parameters. Output measurements shall be exported as .csv or a similar format. The program may be written in Python or MATLAB.
5. For more detailed modeling of lithium batteries, test with short pulses or AC current are needed. Perform necessary software modifications to enable such tests.
6. A variety of 18650 cells is available. Perform automated testing of those cells and use obtained data to develop an empirical model of battery degradation.

Additional information:

• Number of students: 1-2
• Keywords: power electronics, power converters, PCB design, embedded software, MATLAB Real-time, batteries, model identification
• Possible work splitting to hardware and software parts
• Long-term project, students may start as undergraduates and contiune work towards graduation