Unlike most other brands, KUKAbots do not use a battery in the robot arm. Instead, an EEPROM chip is used (mounted to the RDW card inside the robot base). The batteries inside the cabinet are used to keep the "brain" running long enough to update that EEPROM chip before letting the controller finish shutdown. This is why the KRC will appear to stay on for a minute or so, even after main power is removed.
If the batteries get too old, or lack sufficient charge, this "save" to the EEPROM will not be completed, and the robot will lose Mastering for all axes (on a KRC1 and most KRC2s -- KRC4s are less vulnerable to this).
Most other brands that I'm familiar with (Kawasaki, Nachi, Fanuc, ABB) have a battery-backed SRAM or similar memory in the robot base, and the batteries must be swapped with the main power on, in order to ensure the SRAM contents are not lost.