IIRC it's just a cr2032 watch battery, common size available at supermarkets etc.
Pop the key open to check.
Do you have the car handbook, it's all explained in there.
Have the new battery ready to go in before removing the old one. If the battery is out too long the key will lose its memory and will have to be re-synchronised.
Re-sync is actually really easy on these, just turn the ignition on (dash lights on but dont start engine) and push the lock button on the remote. The doors should lock then unlock.
You might have to try it a few times before it works.
vaux, October 2009
