Had this same problem with 3 SP’s I picked up. It’s not likely a big issue at all so don’t worry. It means you need a new battery. Luckily nintendo made it really easy. All you need to do is buy a replacement, you can get cheap ones from amazon that’ll do the trick. Though before you buy the new battery, check the back of your console, you’ll see the battery compartment held in with a phillips screw. It’s better if you can open it and take the old battery out to check, but even if you can’t at least check for any bumps/ a bulge in the plastic. If there is a slight bulge in the plastic around the battery compartment, this means the battery is starting to expand, and it can damage your gameboy if left unchecked. Best of luck! (by the way most batteries for it come with a little screwdriver so don’t worry if you don’t have the right size)