Thabks for the advice, the thing is, I've just bought it, the battery should be brand new. I can see the save files on the micro SD too, just can't access them on my GBA SP
I've seen a thread about that from 3 years ago, but I've just bought one and still the same problem. I have a GBA SP, idk the version since i switched shell, but it's on old north American one.
Please help