In case of an internal battery failure and loss of power to the controller, it will display (self-check: Error 48) in the display.

In this way, the operator is notified of the battery failure in the controller. 
However, in this state, the controller is not operational as it was before the battery failure - the entire controller will be reset to factory setting, or not work at all depending on the age of the controller. 

In this situation, the controller's internal battery must be replaced.
Furthermore, the controller must be flashed with new application software (same version as the controller had already)
and a batch file must be downloaded to it again, if available. Otherwise, all parameters must be adjusted manually to restore them as they were before the battery failure.

To get the controller repaired you can make a RMA here:

if you want to try and change the battery yourself you can follow the guide linked in here

you can see the expected lifetime of the battery here: