From my personal experience, a dead GBA cartridge battery will not erase a save at all. I have Pokemon Emerald which has a dead battery, each time I load the game it always tells me that clock based events will no longer occur. That is all I have ever seen happen with Pokemon games for GBA. Now I think the GBC games work a little different, I think when a battery dies in those you can't save anymore. I have a Pokemon Silver cartridge that erases saves if you remove it from the Game Boy.