The Backup Battery (a.k.a. PRAM Battery) disappeared from all desktop Macs beginning in 2009. It is not as clear with the laptops as the PRAM was kept charged by the main battery in some models (the iBook) as early as 1999. I am not privy to Apple's design internals, but I have always assumed they went to an EEPROM (electronically erasable programable read only memory) or similar device that does not require a continuous source of power to retain the data stored on it. It makes a lot more sense than relying on a battery that will inevitably have to be replaced and according to McGillicudy's Corollary to Murphy's Law will invariably occur at the worst possible time.
