I see no one has answered you. I do not know the exact cutoff voltage for maintaining radio presets in the Prius. From past experience, it is usually something around 9+ volts. Remember, a 12V system is actually 13.2 V and with a running alternator is close to 14V. They sell an inexpensive kit that you can connect to the battery cables to maintain presets when changing batteries. If you give me some more background on your issue, I might be able to offer better advice.
