Toyota Prius 2nd Gen, p0a80

The answer you seek is in the photo you posted, module (actually module pair) #13 is DOA; likely shorted internally, causing excessive current and heat.

 

To be consistent, that should be module block (which is a pair of modules) #13 ...

 

Correct, 2 modules = 1 block, 14 blocks = 1 pack.

 

Yes, it does, one of the 2 modules is definitely bad. You probably only need to replace the bad one, but most people just replace both with two other second-hand modules that closely match the characteristics of the remaining 26 modules. If you can do a load test (one way to do that here) on the remaining 26 modules and then also the replacement modules, this is a pseudo-capacity test. For good measure, it can be better for longevity to follow the load test with 2 (min) or 3 (better) charge/discharge cycles going down to 5.0 V/module for the first discharge, 3.6 V/module for the second discharge and 3.0 V/module for the last discharge. Then follow this with one last charge up at a slow (350 mA) charge until the voltage tops out and remains stable for 4-6 hours.

If you have a grid charger of some sort, e. g. the Hybrid Automotive Prolong charger that a lot of this community uses, you can reassemble the battery and install it in the car and then charge/discharge and final charge the whole battery as a unit.

If you haven't already come across the information, on a Gen 2, the block and module count begins on the non-ECU side of the battery and counts towards the ECU/Electrical bay as illustrated by this image:




One thing to remember, all the remaining 26 modules that are still functional at this time could also be close to failing (as they have had the same useage and are the same age), so you may need to go back in there and replace another module at another time. You will be doing this until it is no longer fun. At this point, you will need to consider a set of all-new OEM (from Toyota) or after-market (from newpriusbatteries.com). Whether this is an option depends on the overall condition of the car, what other major components have been replaced, and what is your end game as far as keeping the car for the long haul.