Hello Electrified Miata Fans. I hadn't planned to write you this week but plans change, sometimes in a hurry. This week was shaping up to be an epic week. Bruce welded up the bell house adapter, Ed finished the coupler and I finished assembling the mid sized Miata battery pack. It was going to be the first time to spin the motor. Due to a mechanical design flaw on my part, two battery cells helped celebrate election day with a bang, or rather a hiss followed by shooting white hot flames and acrid smoke.
Ed finished the coupler on Saturday, which is enough time for me to get the coupler to Bruce for welding.
Bruce told me it would be ready for car night! Oh my gosh. We'll be able to spin the motor and finally mate the Leaf motor and Miata transmission. I remember this as being a major milestone with Joe's truck project and celebrated with joy. I was so excited and feeling grateful for everyone's help in getting to this major milestone. All I had to do was finish the battery and make sure the software could work outside my office.
The mid-sized Miata battery was almost done, and I finished it on Monday. The plan was to run the system standalone in my office before I carted the system off to Ed's shop. I went to Home Depot and got some new connecting hardware for the battery terminals that were the right sized nuts and bolts.
I connected the contactor box to Leaf motor #1 with both the positive and negative 2/0 cable. The mid-size Miata battery is the largest of the all the packs so I placed the pack on the bottom as a base. I decided to connect the positive of that pack to the contactor box. The logic being that it's less dangerous to connect the pack now, with no other packs connected, because there is very little voltage present compared to when all packs are connected. I loosely put the bolts through connectors and everything seemed fine.
I got my wrenches and went to tighten that connection. Loose connections cause heat and I *don't* want heat around these batteries. As I tightened the connection, I heard a sizzle. Having recently heard the sound of electricity in action with the BMS connector, I knew that sound spelled trouble. Sure enough, in the span of 2 seconds, flames started erupting from the pack! It was similar to roman candle with about 15 inches of flame as measured by the black spray on the carpet. I yelled 'Run' to Teresa. Of course she was in her office and didn't hear me. It was one of those paralyzing moments where I wanted to video the event, get the heck away and get Teresa safe all at the same time.
After no response from Teresa, I went to find her. Then I realized, all 440 cells weren't on fire at once. However the single 2/0 cable was enough to keep the battery in the office. I put the socket back on the connection and took off the cable as fast as I could. Once disconnected, I could move the battery pack outside and let it finish that display of raw power.
When I picked up the battery to take it out side, the two burning cells fell out. Except for those, the pack wasn't flaming too much. Of course the acrid smoke was epic. The fire alarm went off and Teresa dealt with our monitoring system, who called us to find out if the fire trucks were needed.. I grabbed a baking tray and Teresa found some BBQ tongs to remove the two burning cells that were making a gooey mess of that now smoking carpet. Did you know that carpet really stinks when it burns?
Teresa and I set up some fans to blow the special smell out. Thank goodness the weather was great. As I was waiting for the smell to dissipate, I called Bruce. He's been with me for more near disasters than I care to remember and is a calming influence. My state of mind wasn't very good because I had no idea what went wrong. As I talked with Bruce, I figured it out, which helped a lot. A problem understood can be dealt with. Unknown problems cause me a lot of fear, uncertainty and doubt, which are not helpful for moving forward.
So what went wrong? Only one wire was connected to the battery so there shouldn't have been a short. It turns out the nut, if turned just the right way could rub through the insulation on the cells because the bus bar hole is too close to the pack.
This wouldn't matter too much on the negative terminal, as the bus bar and the terminal are the same electrically. On the positive bus bar, the cell casing is negative. So the nut touching the cell casing caused a short. While there was only one cell shorted, all 22 cells in that line tried to support the current. One side had the bus bar connection rated for 550 amps. The other side only had the balance lead. The number 14 wire isn't rated for a lot of current, but that rating is only for a certain temperature rise. Copper conducts quite well and so the single cell received a lot of current as well as its own stored energy. This resulted in 'flaming out'. Electric cars have their own problems. This was like lighting a pressurized fuel
line on fire and lucky for me the pump stopped, rather than burning the whole tank. So what do we do now?
Teresa rather strongly suggested that I move this phase of the construction to the garage where it will be easier and quicker to deal with things like unexpected
fires. On car night, we'll be exploring options for relocating the bus bars instead of spinning the motor. That's a pretty big let down for me, but I've still got my house and a loving wife. I'll deal with the let down by working through this with the rest of the team.
To give some perspective on how these events can effect us, the very next day there was a big pop, with Teresa saying "what was that"!?!? I think both of us panicked for a bit before we realized it was the shower organizer that fell down as the suction cups seem to lose suction once every three months. Dang, giving my blood pressure medication a real world test. Whew.
Thanks for reading. Hope y'all are staying healthy and having fun.