Josh,
The Cobra 3 is a user change-able battery. The only difference is when the service center performs a battery service, they will also test dive the computer to 130 ft in a pressure pot. They will drive the computer into decompression and then bring it to the safety stop to to clear and make sure all of the functions are working properly. If you would like to change the battery yourself, there are how-to videos on Suunto.com as well as directions in your manual.
As for your D6, I strongly urge you to ignore prior posts about replacing the battery by yourself. There is delicate circuitry inside these units that can be damaged very easily. By opening your D6 you void any warranty. The service centers have proper equipment for ESD protection. Static discharges cannot be seen by the human eye and will cause a computer to malfunction. The service centers have machines to test the integrity of the computer and like I have explained before to test dive them, which ensures the computer is free from leaks and that all functions are working as designed by the manufacture.
If you have any other questions please ask.
Best Regards,
Chris from Suunto