Back when Nintendo and Sega started adding batteries to their carts for game saves and the like, they said the batteries should be good for about 5 years. Has anyone actually had a battery die on them yet? I have not...
at least not one inside of a cartridge.

The internal battery inside my awesome very great neat-o Sega Saturn has died a couple of times, and it is easily replaced. But the battery inside my two official Sega Saturn back up RAM carts are still going and still have game saves from every game I ever played on the Saturn (and I' ve played most). Some games have saves from 1997. And the battery in my Miracle Warriors (ug) cart for the Sega Master System still works. Will these batteries last longer than the planet Erath itself, or am I just lucky?

My TurboGrafx-16 CD-ROM which saves games internally has never died either. The manual says to turn on the Turbo at least once every two weeks to ensure a charge remains. But I' ve left the system off and unplugged for over a year straight and the game saves were still there and they still worked.