The difficult thing is, you’d likely have to buy another player, to replace the battery with. Although you can replace the battery, the player wasn’t designed for that or to be opened (although people do so).
A separate possibility: if your battery has degraded to that point, SanDisk might replace the player under the player’s warranty (1 year in the U.S., 2 in the EU). I would try that avenue first and telephone SanDisk Customer Service about that.
http://www.sandisk.com/about-sandisk/contact-us/
By the way, what type of files are you using? If “basic” mp3 files, that battery charge level is low. But if FLAC files, which consume more energy, that might be more normal.
