@Yoshikkko
It's possible to replace the battery in the cartridge and it will save again. It's about a $1-$2 fix, depending where you get the battery from. There's a seller on eBay that sells the batteries $1 with Free Shipping in the U.S. Definitely worth it. :D
Works for all GBC and GBA pokemon games that won't save anymore. From what I've heard, the battery keeps the cartridge powered constantly for ~15 years (~6 for Gen II) and THAT is what keeps your save on the cartridge. As soon as the battery is removed or dies, the save is gone forever. There is also a gadget you can get from Asia that will let you dump your save file to your computer (but it's like $20. Worth it for me to keep my childhood alive xD ).
Changing the battery is easy, and it uses Scotch tape (easiest, not the MOST secure hold) or a soldering iron (slightly more difficult if you don't have or don't know how to use one, but should last for another ~15 years). Really, the hardest part is getting the cartridge open. I don't think there's a screwdriver for the screw in GBC games, though there IS one for the GBA ones.
There's many tutorials out there, just Google it :D
Sorry, back on topic. I'm looking to buy some first gen Japanese cartridges :D I'm gonna "Collect 'em all!"