Fix my flat battery in my 2000 VTII Calais today.
Just get get a new battery for it, got mine from jaycar for $4.50
Remove the case and remove the battery, don't break off the metal bit holding the battery in, can be a pain in the ass but they do come off.
I put the new battery in and then used so eleco tape to hold it down because I didn't have a soldering iron to solder it back in.
Put it all back together and went out to my car and tried to use the remote buttons on the key i just fixed to open the car and it didn't work, had a felling it wouldn't
I used my spare key to unlock the car (using the remote buttons) then once I was in the car I inserted the key I just fixed into the ignition and turn it on, left it for a minute then turned it off, the key should now be reprogrammed.
While still in the car I used the remote buttons (on the key i fixed) and locked the car, unlocked it,locked it and then unlocked it again and then started it just to make sure things were ok and now she is working like a treat. Now she is all working again I super glued the case back together to stop it from falling apart.
Total cost was only $4.50 and about an hour of my time.
I hope this helps anyone that doesn't have the cash to get a new key or just want's to change the battery