The battery is actually very closely monitored and any lessening of capacity is known immediately.
Some batteries have failed, but very few.
The warranty is quite long on the battery anyway.
And the prices for replacement batteries have come down, plus you can find pulls from wrecks now.
Not that a Prius makes any economic sense yet, mind you.
The batteries are the last thing you need to worry about with the Prius, imo. They aren’t likely to fail on you at all.
Thanks for the clarification.