Hybrid battery stopped working and a cell got.... burned?

All your picture links are broken. It's better to just upload the files as you compose your post.

It also sounds like you haven't researched the process and this is not something you can just figure out as you go. Nor can you "fix" a module. Techstream will tell you which modules have failed. That would have been step one before taking anything apart. There are plenty of threads here about reconditioning batteries. Not many people do it anymore because it takes quite a bit of skill, time, and effort to get it right.

 

FIRST! You shouldn't be working on the battery while it's still in the car and grounded. Even with the safety plug out, it's not a safe idea.

Fried modules are rare but do happen. Some leak and cause faults. A very rare exception explode from abuse.

 

I'm glad nothing bad had happened...Yet

Make sure all connected wire/cable are disconnected from the traction battery before you proceed farther...
And that battery in question have seen it's better days, and need replacing..
Good luck on rest of your battery pack, and hopefully you'll able to get them all balanced out before you put them back..

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Assuming that the battery fan works and air can flow around the battery, then nothing "caused" this except for statistics. Toyota has made many tens of millions of hybrid battery modules so it is not impossible for a few to have the kind of fault that you experienced- just extremely rare.

If you are going to do this level of work on your prius I suggest you get a copy of the Techstream software and mini VCI cable on amazon or ebay to install on an old windows laptop so you can get codes and data from all of the electronic control systems on the car. Also consider the Dr Prius app plus a OBD2 interface dongle specifically for the HV battery and battery ecu.

Your options for repair are:

Replace all modules with new Toyota modules. (likely good for 10+ years)

Replace all modules with new NewPriusBatteries.com "modules". (likely good for 5 to 10+ years)

Replace some modules and pray. (1 year? 1 month? 1 week?)

Install a "rebuilt" battery and pray. (1 to 3 years? 1 to 3 months?)

Replace all with "matched" set from junkyard and pray. (???)

Replace some modules and cycle the complete pack (discharge - charge) with a grid charger like the Prolong system. (more $$$ but less time.)

Replace some modules and cycle all individually with hobby chargers. (less $$ but MUCH more time)

The last two options require that all the modules are clamped together for the entire process and continuously cooled. And clean or replace the bus bars and check the orange wiring harness for damage at the battery studs and at the battery eu connnections.

To do a decent job with the hobby chargers also requires keeping track of the (discharge) capacity of each module for each cycle. Some might need only 3 cycles, some might need 5 or 6. And you want to monitor module voltage after sitting to find any with self discharge problems. And you want to perform load tests on each module to test for excess drop when stressed. Been there - done that. Took over a month and can't really recommend that if it's your only car or you need it to be reliable .

The problem is that old / used modules have different capacities, and will have different voltage levels when under the same discharge or charge rates. If the voltage difference gets too high, then the battery ecu will set a code and put the car in a limp in mode (or just not work at all). In addition a used module can fail at any time. You can do everything "right" but still might have more problems a few months (or years) down the road.

Each module is made up of six 1.2V cells for a total of 7.2V. the usual failure mode is that a cell shorts internally and the module will be over 1V lower under high discharge but show higher V under high charge. (testing the static voltage of each module with the battery pack apart on the table won't necessarily tell you much).

Just replacing a few module can work but the chances of have a lasting repair are slim . Often another module fails or sets a code & you have to replace another, then another, etc. (it's called "whack a mole")

So what you want to do will depend on: your budget, your available time and space (don't know how secure you parking is- I've read alot about the increase of catalytic converter thefts) and realistic expectations for how long this will last.

Good luck

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