The internal battery is the battery in the game cartridge that powers the games clock. If the battery runs dry, you can no longer have time based events occur on your game. That means lottery tickets, secret base battles, berries, apprentices at the battle frontier, berry master, clear out sales, game corner service days, energy guru sales, and the guy who gives TM's in pacifidlog town every week. If your battery runs out, you can send it to Gamestop where they can replace your battery, so far as I know. You can also send it to Nintendo where they can fix it, but they reset your file in the process, once again, as far as I know.