Replacing vintage game batteries is a must. Not only will this restore the save function but will preserve your game. When game batteries age they will eventually leak on the motherboard and cause damage.
Batteries are easy to replace if you have soldering skills. This CR2025 is the exact size of battery used in many original GameBoy games. Many installation tutorials can be found online
Tools needed for most games:
3.8mm or 4.5mm Game Bit (sold in my store)
Solder and Iron
Note: This install is easy but requires basic soldering skills.