Is this your key type?
http://tinypic.com/view.php?pic=2qtczyt&s=7
If so it has a rechargeable battery. A little
trick that works wonders is to place or
Tape the key on top of an electric tooth brush charger!! The key (like an electric tooth brush) is charged by induction, while it is in the ignition slot. Putting it on top of the tooth brush charger will give it a good boost, trust me, it works!
If this is your key then you have a replaceable battery:
http://www.transponderautoparts.com/...uuton-shel.jpg