Your nickel-metal hydride (NiMH) battery should slowly degrade over time but according to what I’ve read, they generally appear to be lasting much longer than their warranty period. If this were a dealer response only, I’d start pushing it up the ladder to see if the warranty could be extended retroactively. Their reputation needs repair more than your car. My question to you for potential hybrid buyers is…do you think it saved over $3700 in gas the last 9 years over a Corolla and is it worth the bill now to keep and replace the battery?