Hello Electrified Miata Fans! This week's blog will be a short one as charging and balancing batteries were not very eventful. Everything worked as needed. This is very good news as this means we'll be able to move ahead with the next revision of our BMS boards, which is needed to complete the Miata batteries! Last weeks goals were:
1. Upgrade firmware on BMS boards.
2. Get battery balancing code working.
3. Install hardware switch to cut power.
4. Start on logging and integrating a 120GB Solid State Drive (SSD) on the cart
The first step in testing any product is the get the newest firmware. Upgrading firmware was easy! I generated a shell script to do one battery pack at a time. I have two packs, a mini-pack (4 cells) for powering the Pi's and the headlight, and main pack( 12 cells) for powering the traction motor. After the upgrade, I needed to work on balancing as the mini-pack was deliberately left out of balance to test the BMS boards.
To balance the battery cells, I needed to turn on the 'shunt' resistor on the BMS boards. Turning the shunt resistor on will draw power from the cell. Since the boards can be addressed individually, this allows certain cells to be drained, while others are not. Let me show you how Joe made battery balancing very easy. The Pi's send a command to turn on the shunt. The BMS boards monitor the cells and start gently engaging the shunt resistor when the voltage rises above a certain level, (a programable 3.9 volts in this case). The shunt will be further engaged as the voltage level on the cell rises. The shunt is fully engaged at 4.1 volts, subject to thermal limitations of the board. So the boards do all the hard work, while the Pi's decide to balance or not. Thank you Joe for making balancing easy!
I now have a visceral understanding of the purpose of the DC/DC converter in electric cars. The purpose of the DC/DC converter is to keep the low voltage systems charged while the car is in operation. For the go-kart, we don't have a DC/DC converter and one charger but two battery packs. Having only one charger, but two packs to charge can be an annoyance, because the mini-pack needs charged up to run the Pi's for an extended period of time while the main pack gets charged. Then the mini-pack needs re-charged after being run for several hours while the main pack charges. That means I have plenty of practice moving the charger back and fourth. I like the Anderson connectors. A little pricey, but seem to work really well. I'm looking forward to using the Leaf's DC/DC converter on the Miata!
Car night arrived and the biggest work task was to make some Ethernet cables. I'd put an Ethernet jack in the garage, so I didn't have to run a cable from my office through the house out to the kart. Brian made two custom cables, one for the office to garage jack, and one to go inside the Pi box. Thanks Brian! With the hard work out of the way, we looked at Dave and Broch Evans's design for mounting the Leaf motor to the trans-axle and mounting that assembly into the Formula Mazda.
Broch is a Formula Mazda driver and hadn't driven the kart yet so we took a break from discussion to have some fun.
Discharging these battery packs is such hard work, but somebody has to do it! It never fails to bring a smile to my face when someone enjoys the acceleration of the cart. Broch adapted well to the fact it doesn't slow down that fast and old tires that don't corner that well. Thanks Broch for the smiles.
The goals for next week are:
1. Investigate why the BMS values seem noisier on the mini-pack vs the main pack.
2. Integrate 120GB SSD into the system so we can store the BMS data.
3. Write BMS logging software.
4. Cut some metal for the Formula Mazda project.
As always, thank for reading! Hope you are staying healthy and having fun.
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Bill likes cars that understand the 'go fast now' pedal