Hello Electrified Miata Fans! We know just enough now, all that stands in the way of progress is us. This week the team was busy and made great progress. Last week's goals were:
1. Cut the materials needed for assembling the battery packs.
2. Software design to alleviate some problems I found while testing PID.
Ed and I cut plastic, bus bars and wood. The plastic is used to cover the batteries, while the copper bus bars are used for the main connections from each battery pack. Here is Ed, figuring what comes next after cutting some 4x8 sheets of plastic into usable dimensions
We also cut some wood, to simulated the battery height plus spacers. Using that wood and the cut plastic, we can 'mock up' the three different battery pack sizes. I can get these battery pack mock ups to Bruce and we can make steel frames to hold the battery packs These frames will be attached to the car, making a new 'fuel tank' for batteries. Thanks Ed for sharing your wonderful shop and experience.
I worked on multi-hub software, which allows us to distribute the computing tasks among different Pi's. I found that I was using nearly all the real time capabilities of one of the Pi's when attempting to use PID. Knowing, we'll have many more computing tasks to do, it's a good idea to get these infrastructure enhancements done early, so the bugs can be found earlier, rather than when going around a twisty corner on Lime Creek Rd! The multi-hub software is estimated to be 50% done.
Joe re-designed the BMS boards to custom fit our current needs. Specifically, he eliminated the daisy chain communication path and replaced it with a multi-point path, removing some failure modes. He also modified the board layout to facilitate our battery pack size and replaced the tiny surface mount balancing resistors with a larger size wire-wound resistor that has enough heat tolerance and dissipation for use with our packs. Joe had Brian and I review the new board layout and we had fun doing a mini design review using Slack. Three of the new prototype boards were ordered so we can verify the design and are due back in early July. Here's a picture of the new board designed using KiCad
Bruce and I created milestones & dependencies for getting the 0.5 version Miata prototype running.
It's always amazing how many little details need to be done to get a car
working. In this instance, all the software was assumed to work correctly and be 'bug free'. However great our aspirations, I've never found software that is bug free, so we'll also create a dependency chart for the software. Thanks Bruce!
Now that the pack sizes are known, we need to start building them. While there are a lot of steps in building a pack, the first is to physically assemble them. Here is the big pack assembled.
The big pack weighs 75.4 lbs and is 1/3 of the total cells needed. The full set of batteries and holders will weigh 226.2 lbs. There will also be an estimated 75lbs of bus bar, connecting wire, battery holders, etc. This size battery allows us to have an estimated range of between 50 & 100 miles, keeping approximately the same weight as a stock Miata. A little lighter in the front, and a little heavier in the back. I'll weigh the bus bars when we've lightened them by drilling all the connecting holes.
Thank you team for a great week! Next week's goals will be to continue assembling the batteries, finish the drilling and tapping the bus bars and build 'mock up' battery boxes and getting them to Bruce for planning the batttery frames. I'll also work on more software cleanup and testing with the multi-hub feature. Thanks for reading and stay healthy!