There's no denying the 12V Battery is a bit of an Achilles heel on Toyota Hybrids, but certainly the 2 times I've needed a new one (once on a Gen 1 Prius, once on a Gen 4), my dealer replaced under warranty with no quibble at all. Because they don't have to start the engine, and to save weight and space they use a small Battery.  With care, they can last a long time - my 2nd Gen 1 Prius still had the original Battery when I sold it at 9 years old with 163,000 miles on it. It's best to