Charging Prius cells with IMAX B6 Charger

Of course your Prius battery is NiMH, not lithium. You should still be ok here because the B6 does have a mode for NiMH; you just need to be sure it is set for that and not for lithium.

You won't really be doing any balancing to speak of, because you don't have access to the individual cells. Each of the 38 things you just bought is called a module, and there are six NiMH cells in each one, strung in series. You only have access to the two terminals at the ends. So you have to set the B6 to charge a string of six NiMH cells, and connect it to one module at a time. You have to just sort of hope that the six cells inside each module are in reasonably matched condition. A very slow, tightly controlled overcharge may help to bring them to similar states of charge.

The specs of your B6 say it can charge series strings of up to 15 NiMH cells, so in theory you could set it for a string of 12 and do two modules at a time, which would speed up your progress through the pack but you would learn less about the condition of the individual modules.

The specs I find online for the B6 give it an input voltage range of 11 to 18 volts, so if you have a question about the "input vol error" and you say you're feeding it 19.5 volts, you've sort of answered the question.

You'll probably find a lot of threads in this forum with information that will help your project go better, if you spend a bit of time searching. Batteries aren't as simple as they seem and there are a lot of details to be aware of.

Cheers,
-Chap