Gen 1 Prius Assistance?

Hi all, new to this site but not new to the Prius. I am a 4 prius family (2001, 2x 2004, 2012V). Unfortunately it's my gen 1 that's giving me fits. (205k miles, new brakes, body in excellent condition, clean inside)

I recently replaced the traction battery with a rebuilt using Gen 2 cells. Drove the car for a bit and then got the "red triangle of death." My mechanic read the code and said one of the traction battery cells was bad. I called the remanufacturing company and had them send me a couple of new cells. I opened the battery, found the bad cell and replaced it. Drove the car for a while (10-20 miles ish) and got the red triangle again.

Assuming dead cells again, I opened up the battery, found two more with low voltages and replaced them. Buttoned everything back up. Drove the car ~20 miles again, and guess what - Red Triangle AND the Yellow Turtle on the dashboard. It would appear that my cells are not recharging. Not sure why it took me so long to figure this out... sigh.

Having just put $1200 into the car for the rebuilt battery, I'd like to keep it running for another year.

If I am in fact having recharging problems, how do I go about diagnosing the issue or where do I look for problems?

(I have no issues pulling the battery again because I so love lifting 100lbs while hunched over

Thanks in advance for your insights.
Mark