Strange Clip+ behavior -- will replacing the battery help?

I have an 8GB Clip+. The length of play time per full charge has been gradually decreasing. I usually recharge it using my laptop’s USB ports, but I tried using a cell phone charger with an output of 5V 850 mA.

It worked fine, at first, then began to freeze, or to operate very slowly.

Resetting by holding the power and center buttons doesn’t seem to help. I updated the firmware to 01.02.18, and the display said the update was successful, after a couple of tries (the Clip+ turned itself off while the update-in-progress type of message was on).

Do you think I fried my Clip+ by using the cell phone charger, or could the battery have reached a point where it will only operate for a few seconds before shutting down, even when the charge level indicator reads close to full?

I’m willing to try to replace the battery, but don’t want to bother if the Clip+ has a scrambled brain.

Thanks for any input!