The battery is available at Wal-Mart in the coin sized batteries in the battery section. I don't remember the number but its the only one they sell that is the same diameter and thickness. We replace the battery in both keys at least once a year for good measure. The procedure to synchronize it with your AAM is in the manual - one caveat - your driver door MUST be CLOSED when you do the procedure or the code doesn't take. I found this out the hard way after fooling with it for 30 minutes.