Batteries die or stop holding as much charge, its a fact of life.
While checking my batteries the other week to get ready for a shoot, I noticed that one of my batteries is no longer charging fully, so it was time to buy a new one.
In my lunch break I popped into the local camera shop, London Camera Exchange. I got an excellent deal from London Camera Exchange, sometimes it pays to have a good relationship with your local camera shop.
Its always a good idea to check your batteries regularly, for example one of my batteries for my Nikon D200 developed a weird fault last year.
The symptoms were odd. The battery showed full charge but when you used it, it would work fine for two to five frames and then the camera would hang.
On removing the batteries and then putting them back, the camera would again work for a few frames before again hanging.
A similarly odd battery problem occurred to a friend of mine.
He recently purchased a new battery for his Sony NX5, but it would not work, but when the battery was tested in thecamera shop it worked fine in there Sony NX5. It turned out that he had to update his camera firmware to use the latest batteries. On updating his firmware the battery worked fine.
Odd things batteries. So, if you are having strange problems, try a different battery and if your a Sony NX5 user, check your firmware!