There’s not really much you can do if the battery is wearing out. It’s hardware you can’t replace.
On the very distant possibility that something about how the Fuze controls the charging is going awry, you might try reinstalling the firmware. Download the firmware for your version (1 or 2) from here:
http://forums.sandisk.com/t5/Sansa-Fuze/Sansa-Fuze-Firmware-Update-01-02-31-amp-02-03-33/td-p/139175
Don’t use the updater, just get it from one of the All Regions links.
It’s a zip file. Unzip it and drag it onto the Sansa Fuze driveletter, and when you disconnect it will install itself. If will reset to factory defaults, so if you were using a fixed USB mode (MTP or MSC rather than Auto Detect) you’ll have to change it. Choose Other (rather than US) for region on the initial setup if you want to never see the Slot Radio icon.
But I doubt it will help. The battery has both a limited (though very high) number of charging cycles and decreasing efficiency with age whether you use it or not. So if you can’t regularly recharge the aging Fuze after a few hours of use, another player might be your only solution.
