Could be battery, could be a failing fuel gauge... Switch batteries rarely fail tbh but in the rare case it has you are better with an original donor. They are also a nightmare to remove and go super easy on the IPA as it loves to seep through to the screen.
Thanks...
I see kits available on Amazon. Are they worth it?
at £14 its probably worth a go. If it continues not playing ball after the battery replacement then most likely the fuel gauge is at fault. Honestly I have so many spare switch batteries I haven't ever bought an aftermarket replacement.
Could you fix it if i sent it to you? Obviously with payment...