For storing settings in NVRAM there is a battery installed, called a Snaphat which consists of a battery and a crystal.

The type is: STMicroelectronics M4T28-BR12SH1.

Conrad (NL) - STMicroelectronics M4T28-BR12SH1 Clock/timing-IC - batterij 48 mAh Batterij en kristal Lithium SOIC-28