>>105826277 (OP)I'm guessing that it boots up and plays normally, but if you shut off the Gameboy the save file is gone?
One thing to test: Start a game and save. Now push A+B+Start+Select. This does a soft reset -- you're rebooting the game without removing power from the RAM chip. You'll see one of 2 outcomes:
1. Your save is there and loads normally. The RAM chip works fine but loses data when the power goes out. Either you fucked up installing the new battery, installed a dead battery, or something is wrong with another component between where the battery power goes in and where it powers the RAM chip.
2. Your save is gone, or is there but it says it's corrupted. Fault inside the RAM chip or one of the RAM chip pins isn't soldered well. Or one of the contacts that go to it is still dirty -- I've had contact issues even after a thorough scrubbing sometimes.
Another test: If you have another Gameboy, GB Player, Super Gameboy, or N64 Transfer Pak + Pokemon Stadium 2 (I think; unsure if the emulator caches the save file in system RAM though), do the same test as above on that. If it behaves differently, the cartridge connector inside your Gameboy is dirty. You can also try another cartridge that has saving in the same Gemeboy.
Cleaning GB slot contacts: I've had the best luck with a piece of corrugated box cardboard. Cut it to the width of the edge connector on the PCB inside the cartridge. Wet the end with IPA rubbing alcohol. Push it in and out of the cartridge slot repeatedly to scrub the pins. Always in and out, never back and forth or you might bend a pin.
Cleaning a GB cartridge connector: Dip a q-tip in IPA, press out excess so you don't dribble it all over inside the cartridge. Stick it into the end (it's a tight fit) with the connector facing down so any drips go out not in. Scrub back and forth over and over. For stubborn cartridges, open the shell and use a pink pencil eraser like picrel to erase the dirt off the pins.