DIY Battery Rebuild Success… Except the Red Triangle

I've been wanting to get a Gen 2 Prius for 12 years now. A few months ago it finally happened. I bought a Gen 2 Toyota Prius 2009. The car would not start; it had a dead 12V battery. I was able to get the dashboard to light up with a jump-start battery. I brought the car home and connected Dr. Prius. The hybrid battery seemed in very bad shape—some modules had 0 voltage. The owner said it sat for 6 months in the winter, so it made sense.

So I decided to try to refurbish it myself. I bought the EV-Peak CQ3 with temperature probes and followed all the guidelines here (BTW, thanks to the great community here). I ended up replacing 10 modules since they had 0 voltage, but the rest I was able to bring back to around ~5 Ah discharge. After I finished charging all modules, I then balanced them: connected all "-" terminals with wire and all "+" terminals with another wire. I left it like this for around 10 days to balance out.

Then the moment of truth arrived. I put the battery back in the case and in the car, connected the brand-new 12V battery, and the triangle of death was still showing… After so much work, money, and time spent reading.

I connected Dr. Prius, and I was seeing all modules were green, but the values were fluctuating continuously. One or two modules seemed pretty steady, but the rest were all over the place. The fluctuation did not seem that large to me, but it was very frequent, which did not seem right. Since some modules seemed steady, I thought there had to be some modules that might be unstable that are causing this fluctuation. So I rearranged some of the modules: I kept the 10 modules I bought refurbished together, and then I rearranged the older ones based on how many cycles I had to run to get it fixed. I was thinking that the more cycles I had to do, the worse the module, so I was trying to put the bad modules in the same pack, and hopefully I can correlate the fluctuation with the bad modules. This kind of worked; I isolated like two modules that seemed to fluctuate the most, but others still have some fluctuation….

Some of the old modules were charged 2 months apart. (I was busy with other work), so maybe they got discharged too much.

When I first took it apart, the plastic around some of the bus bars looked melted. There was corrosion everywhere. It seemed like the battery was refurbished before, but whoever put it back did a very poor job since some nuts were not properly fastened. I got new bus bars and the whole plastic with sensor wiring harness.

Also, 4 of the modules seemed to corrode pretty fast. While I was charging, I could see corrosion build up on the terminals. When I balanced them, I saw corrosion starting to build on the wire around their negative terminal. I got them to around 5Ah discharge, but maybe they have something wrong going on in them causing this and maybe causing the fluctuation I see on Dr. Prius.

I am a bit unsure of what to do next. I have a few ideas:

• Try to narrow down the modules that cause the large swings and do a couple cycles on them to see if that fixes it.
  
  
• Replace the 4 modules with corrosion issues with refurbished ones from eBay. I plan to get them from the same person I got the 10 ones from.
  
  
• The 12V battery had been sitting for 3 months now, and it was showing in Dr. Prius that it was at 12V, and my voltmeter was showing 12.4V, so I am giving that a charge as well. Probably not related, but I wanted to make sure I ruled that out.

The screenshot is after I rearranged the modules. I also have a video but I'm too new here to post the link. I'll try to share that soon.