I bought a new 2005 RX-8 couple years and the battery has died.
what is the extend of the warranty coverage of the battery?

 

Battery warranties are usually pro-rated. That is, within a certain timeframe they'll cover a portion of the cost based on what percentage of the time has passed. For a 2005 I doubt if you'd get much, if anything. But it doesn't hurt to ask.

 

you're better off to upgrade to an optima yellow or red top