10 Sep Reflection on a virtual design summer
This cycle has been unlike any other the team has experienced. The recent turn of events around us has created quite a bit of uncertainty for us regarding our education and activities at Stanford. Despite all this, the leadership has been able to strategize ways to continue design work virtually, continue on-boarding new members and continue to build the new team. We are actively preparing for the race to come in 2021. The challenges we have seen during this design cycle have ultimately presented us with new opportunities to push the boundaries of our imaginations and continue in our quest to build the most incredible race car yet.
We are proud of our members for stepping up these past few months and delivering on even the most ambitious goals we have set. Below is an update on the progress of our design cycle.
Aero: The Aero team worked in parallel pipelines over the summer to decide between a catamaran or bullet car configuration. After many iterations and brainstorming sessions with Solar Car alums, the team is close to converging on a design that so far promises to have the best aero performance we’ve seen. Our final design has taken advantage of the new regulations to explore a range of track width options. Though the new license plate placement requirements and departure from gallium arsenide arrays (creating the necessity to have a larger plan form area for the silicon array) have presented challenges, the aero team has found creative solutions and is excited to unveil the final design when the time comes.
Array: Over the past few months, the Array team has been researching normalization, encapsulation, and optimization techniques to employ while designing and manufacturing our team’s array. Our goal is to use a mixture of team encapsulated and pre-encapsulated array cells to maximise performance and learning for the members of the team. We are currently gearing up to begin testing our encapsulation process. Recently, we have been in contact with several generous companies and have acquired multiple samples of backsheets, topsheets, encapsulants, and solar cells. We are excited to use our computational placement models on the finalised Aero body and decide on the best array configuration for the cycle.
Electrical: The electrical subteam is currently developing our next generation battery management system, one of the key blocks in the electrical system, and investigating innovative additions that we could bring into our system this cycle, maximizing performance while maintaining a high standard of safety. One exciting addition is a system similar to an airplane flight recorder. Such a system, which is contained in a fire and crash proof box, silently captures important vehicle information in order to ensure that such data survives a catastrophic event like a crash, roll-over, or fire. The team decided to take a more modular approach to the BMS design and we are happy to report that we have successfully designed, manufactured and tested 85% of the new BMS which includes safety critical components, sensors, power converters and communication.
Mechanical: With the release of the 2021 World Solar Challenge regulations, the mechanical team is working to get ahead in the early part of the build cycle by making important system level decisions about the car’s architecture and design. In addition to learning how to surface aero body concepts, the new members of the team are also getting their first mechanical projects that will eventually make it onto our 2021 car! While we have not been able to work on campus this summer, the mechanical team is working hard in this virtual environment to make sure we stay on schedule.
We want to extend our heartfelt gratitude to all our advisors and sponsors, especially Pointwise, Siemens, ASSU, Stanford Engineering, Sierra Circuits, Digikey, Altium and IAR Embedded Workbench, who have enabled our team to maintain the integrity of our designs even when working virtually.
Phil Karn
Posted at 01:22h, 18 DecemberI just saw the multi-part documentary on the 2019 Australia race on Youtube. It was so very sad to see your early battery fire. I’m curious – did you determine a cause? With two catastrophic battery fires in the same race, I began to suspect that these cars are not using battery management systems to ensure that li-ion cell temperatures and terminal voltages are kept within safe limits. Can you talk about your battery management system, at least the safety related aspects? Is this addressed by the race rules?
I am an electrical engineer in San Diego, retired from Bell Labs, Bellcore and Qualcomm. Although my specialty is computer networking and digital communications I have long had a strong side interest in power systems and electric vehicles. I’ve had rooftop PV since 1999, I drove two GM EV1s between 1998-2003 and we have two Teslas now so i am very familiar with electric land propulsion. Thanks!
Maisam Pyarali
Posted at 15:53h, 01 JanuaryWe suspect the cause of the fire occurred due to intermittent short-circuiting, likely due to debris that might have somehow been overlooked in our BMS during final sealing. This resulted in overcurrent failure -> local overheating of internal cell components and/or electrolyte. When a short circuit occurs, the pack voltage will sag substantially low in response to high load. Such a sag will follow the behavior of a roughly first order system, and will produce a transient, then steady response shaped like an exponential decay. We see this sag response upon closer examination of our telemetry data, zooming in on these impulsive voltage sags indicates that indeed they are shaped like exponentials. This shows that the sags are a response to load dynamics, and their severity points to the fast switch and release between tremendous load and no load, load that is generally only seen in the case of a short circuit.