I saw on one of the threads that the main battery warranty had been extended for 10 years for certain model years. They did not have any documentation to validate the warranty extension.
Today I setup an account at Mitsubishi. After entering the VIN and getting setup I perused the site. They have one button for warranty and it crosschecks against the VIN #. I was pleasantly surprised to see that my 2012 iMiev has a 10 year or 100,000 mile warranty for the main battery.
If you are unsure of the warranty setup an account and check the warranty against your VIN#.
I was looking since the car is acting up. From the forums, it looks like the 12 volt battery is the issue and not the main battery.