The hybrid battery is acting broken-then it isn't

Check the fault codes. You need Toyota hybrid capable scanner. Mini-vci is cheap and good if you want to do it yourself (but requires xp-laptop).

 

That is a faulty conclusion. The traction battery ECU will decide a module pair is bad based upon module pair voltage compared to the other module pairs - the voltage may be too low or too high. Although the battery is still capable of providing power, it needs to be replaced because continued use will lead to overheating, which creates a potential fire hazard.

As previously suggested, you need Mini VCI so that you can read the DTC which will provide a clue about what is wrong with your car. The problem may be unrelated to the traction battery but you won't know until you have the diagnostic trouble codes.

 

This is normal behavior when one of the modules has a failed cell. Like Patrick said, the battery ECU monitors the voltage differences between the modules and waits for a large enough difference before it activates the trouble code. You cleared the code and it just took a little while for it to come back. I recondition and rebuilt hybrid batteries in the DFW area I've actually had customers admit that they have gone months clearing the codes and driving until it came back. I replace 10-15 Prius batteries a month right now and they are almost all 05 and 06 models ranging from 90,000 miles to 250,000. Your battery lasted a lot longer than most. If you want more information let me know. Good luck.

Matt
Texas Hybrid Batteries LLC

 

And the "standard" thought about problems that come and go:
Might want to check the 12V battery before you do anything else.......but given the circumstances, the HV is probably on the way out.

 

Check the 12, though it is unlikely to be bad. But better to be sure.

Unhooking the 12v battery simply makes the car temporarily "forget" that it is broken.
As soon as the error presents itself, the car remembers that it is broken, and lights up the warning icons.

Get the car scanned and tell us the codes. Then we'll have something to go on.