For the current supply of the terminal Lithium-Ion batteries are used. A rechargeable, not exchangeable by the user, backup battery secures the main memory if the capacity of the main battery is low or during the change of the main battery. The backup battery is charged by the main battery.
 

Further information, especially about calculating the running time can be found in the Hardware Manual.

Backup Battery

If both main and backup battery are fully depleted, you might not be able to switch the device on. Insert a charged main battery and wait for the backup battery to recover.