Bought used 2005 Prius, potential battery failure?

A simple test with a Volt meter will not tell the whole story.

If you had software or a helper you could watch voltage under load while driving.
Could also try the torque app with a bluetooth/wifi connector.

edit: WHOA! didn't realize the youtube link would create such a GIGAGNTIC video preview pane. Wow. Sorry about that.

 

You bought an 11 year old car, and no vehicle parts last forever.

What did you pay for the car? How many miles?

If the miles are low enough, consider a NEW HV Battery from a Toyota dealer who sells online ($1900-$2200); you would have to will call pick-up. Going new is the greatest chance for trouble free operation. Most original new vehicle owners seem to get 100K+ miles before a failure; some get less than this, some get more. You could also take this HV Battery to another Gen2, should something happen to your current Gen2 (accident, super expensive vehicle repair bill).

 

FYI,
New Battery from the dealer is more work to install than a rebuilt.
It is not a case of simply taking out the the old one out and put in the new.
Beginning in January there are even more components that must be swapped from old battery pack, plus some additional connection pieces, adding to the possibility of error.

 

If you Google Prius Optima batteries you can find them all over the place at less than $200.00.

In case this helps...