Im stumped. triangle of death again

Hi All long time reader, first time poster. I have a 2005 prius with 185k miles. At 150k miles, I did for the first time see the triangle of death and confirmed it was a bad hv battery. I'm relatively handy and was able to take the hv battery out and find the bad cell. Replaced it with new used cell and all was good. Fast forward 2 years I had the same issue again. All cells were reading 8.4 volts except 1. I replaced it and thought I was good to go again. 2 weeks later the triangle came back. I thought it was probably the cell I had replaced but after checking them all,10 times, I noticed they all registered the same at 8.4. I did notice some corrosion on the terminals so I did clean them all but I am still coming up with the same battery needs replaced code. Any ideas for what to try next. Is 8.4 a normal reading to be getting? Thanks in advance

 

Thanks for the incite and advice. I believe the time has come for me to wave the white flag and take it to a local shop and have them look it over. It is one that has dealt with these batteries and I trust them. If they say its shot too my plan "B" is to see what they offer as a fix. Plan "C" is to purchase a reconditioned/rebuilt one from a different place that redoes them at a cost of 750. comes with a 6 month warranty but they say they have not had any issues with batteries failing outside of that time frame. I'm not looking to keep the car for another 100,000 miles or anything.