Well... im pretty sure the internal battery is used to keep track of time in-game, a feature not in red/blue/yellow games, and besides that, the games are still playable, i have a pokemon ruby with no internal battery, berries dont grow, daily events dont take place, you cant catch regice due to the need to stand still for 2 minutes, but overall the game is still playable. If your game isnt saving, that would be another problem altogether It will say upon starting up "The internal battery has run dry. The game can still be played, however, clock based events will no longer occur" The battery can be replaced though, with the proper tools. Save data, like the rest of the game, except the clock, doesnt run on that battery. Of course this is just my personal experience, since I can still save pokemon ruby (my brothers save game tho, not mine) For all I know, there may be another battery that runs the saving though.