Methods to diagnose suspect HV battery performance

I starting noticing that my 2005 prius with 164k mi now runs the engine more often than previously; I notice it at stoplights, at slow speed cruise, etc. I have also noticed rapid SOC swing on the MFD, from green to purple within ~0.1 miles driven, but that only appears to occur when the car is cold and I take the first drive of the day. This has been a subtle change over time, and may have been occurring for some time (6mo-1yr).

I took a picture from torque showing an almost 1V difference in block 5:


After taking this picture, I turned on data logging, but could not reproduce the same readings once the car was warmed up. The log I have from a 5mile drive has all of the voltages within 0.2 V of each other.

I have no CELs (yet) and car mileage seems to be down maybe 3-4mpg (44) from my historical norm.

Is it time for me to consider rebuilding and rebalancing the HV cells, or let it go until the car starts throwing codes? Are their any additional methods to help diagnose how much battery performance is still remaining?

 

Yes, i learned from my initial logging attempt, and was able to log all reported 14 HV Blocks this morning. it recorded some 1V differences occurring only in block 5. I assume each block is really two of the 7V cells. I'll filter the data and post later this evening.

 

Here is a single data point for the HV Battery Blocks 1-14, after about 1 minute of driving:
13.97999954 13.93000031 13.85000038 13.81000042 12.92000008 13.84000015 13.85999966 13.84000015 13.84000015 13.81999969 13.93999958 13.94999981 13.88000011 13.89000034

I also color coded the data, based on calculating the difference between "voltage read" and "average voltage" for each time point of data; it produced this chart which highlights block 5 as the outlier:

This data starts from initial power on and extends for about 2.5 minutes. The first 1 minute of drive time has the most red/yellow than the rest of the 15minutes of data. Also, no other cells seem to show a significantly different voltage than the rest of the cell blocks, which I hope means that I have a maximum of 2 bad cells (within Block 5)..

My plan is to source two cells off ebay, and hope that its enough before I take apart my battery? Hoping to not have to wait an extra 3-4 days for shipping if it turns out I really need a couple more cells.. Also now going to research some battery balance chargers..