Engineering - Cooperative Education
Bachelor of Science
Date of Expected Graduation
The senior engineering design project presented in the paper that succeeds this, outlines the steps taken to design and implement an electronic bicycle that is able to keep a user’s heart-rate in a selected “intensity zone” though the use of an electronic motor and custom control circuitry. My portion of this project was to allow for safe and consistent power delivery to the rest of the electronics. That is to say, I built a battery pack and battery management system (BMS) to safely supply sufficient power to the rest of the electronics that were required for this project. In order to design a BMS for a project, the theoretical battery specifications first need to be established. To come up with these specifications, three key aspects were analyzed: voltage level, capacity, and size. The voltage and capacity had to be large enough to supply the necessary power to the motor, while also minimizing the size so that unnecessary weight was not added to the bike. Next, the BMS was able to be designed to protect the battery mentioned above. When designing a BMS, three key aspects of the battery have to be continuously monitored: cell voltage, cell temperature, and total battery pack current. In order to get adequate measurements for cell temperatures and current, custom circuitry was designed to enable accurate measurements through the use of a central microcontroller’s analog-to-digital converter. To measure the pack voltage, an integrated-circuit was used as it allowed for quick measurements of every battery cell, effective passive balancing to elongate the lifespan of the battery pack, and could communicate with the central microcontroller through the use of the serial-peripheral-interfacing communication protocol. These three systems combined allowed for quick and accurate measurements of the battery, so that the BMS could determine if it was operating safely or not.
There were many takeaways from this project that relate both to this specific project, and what would be changed in the next iteration, as well as how to handle future design based projects. For this specific project, the design included two power MOSFETs on the BMS to enable and disable power flow to and from the battery. The way the MOSFET’s were oriented allowed for continual power flow to or from the batter, regardless of whether or not the BMS deemed it safe to do so. To solve this issue, one MOSFET was removed to make it possible for the BMS to disable discharging, but not charging (someone has to always watch the batteries while they are charging to ensure everything is safe). In the next iteration of this project, it would make sense to design for the use of an external relay instead. This would allow for easy customization of the BMS for use with different battery packs that have different power requirements, as well as more efficiently allowing bidirectional power flow, and being able to disable charging and discharging if the operation is deemed unsafe by the BMS. Also, the next iteration would implement active balancing instead of just passive balancing. This would allow the redistribution of charge around the battery pack so that the weakest cells could be propped up by the stronger cells, allowing the battery pack to last longer on a single charge. A final takeaway of this project was how to test and implement future design based projects. In this project, the various subsystems were tested on breadboards before a final PCB was ordered and implemented. This lead to unforeseen errors with the PCB manufacturing process as well as things that were overlooked on the PCB schematic. In the future, it would make sense to instead design and order a cheap, low power, version of the PCB to test with. Then, all of the issues could be diagnosed and dealt with on that board prior to ordering the final PCB. All in all, this design project lead to a lot of learning about not only the BMS design process, but the design process as a whole.
Mr. Gregory A. Lewis
Dr. Robert Veillette
Matthews, Tyler, "Cycle Assist" (2019). Williams Honors College, Honors Research Projects. 840.