The Plantronics CA12CD-S replacement PTT remote is designed specifically for the CA12CD-S unit and will not work with the older CA12CD.
This unit comes with the battery so one is not needed to make this work.
Once you receive this new unit you will need to resubscribe it to the base by doing the following:
Plantronics CA12-CD-S Re-subscription Process:
1) Hold down both talk-volume controls on the base until the power indicator starts blinking
2) Hold the replacement remote's volume in either the up or down position
3) This should make the remote units power indicator turn on without any flashing
4) The remote unit should now subscribe to the base making the power indicator stop blinking and stay on which also will make the remote units power light turn off.
5) Take the power cord out of the base unit and plug it back in
6) Lastly, press the replacement remotes power button which will allow it to connect and create a link.