K4J1 - Does not work unless plugged in

I got this keyboard years ago so a bit of an older model, but I was wondering if anyone might have any insight on why this keyboard won't work unless it's plugged in. I used to be able to use it wirelessly, but now it seems it doesn't even hold a charge.

Any ideas or is this plain and simple just the battery has conked out? Thanks in advance for any help received.