P3018, P0A80, and P3019 15.5V in HV battery low?

So recently my 2008 prius finally start showing signs of age for the first time since I got it (3 1/2 years ago now) and to my dread it has proven to be the HV battery. First thing I noticed was the battery fan was running high so I took it appart to cleaned and it was full of gunk from dust to dog hair. But after cleaning, putting it all back together, and reseting the codes they repoped up again. So I did the basic displasy screen 3 headlight flash check and my battery seemed okay it ran consistently at 13.9V total so wasn't sure it was the battery. I then did a torque pro check and all my battery blocks run at a consistant 16.8V except for block 8 which runs at 15.5V. While the voltage is not low and is still decent considering they are meant to run 7.5V each cell, it is a voltage lower that all the other 13 blocks. I guess my question is would it be better to attempt to replace the single block for replace to whole HV battery?