I thought my battery was bad too... the "charge" function would only work for 10mins and the battery didn't seem to hold a charge.
Here's how I fixed mine:
Go into the menu, go to "General" and select "Charge Function".
Select "Discharge & Charge".
When you activate the charging function now, it will first discharge the battery and
then charge it again. It will take a while but you can tell it's working.
I think the player reverts back to the default "charge" only option, so it might be worth giving the battery a few discharges to recover its capacity, and also discharge it every now and then.
Hope this helps
