The WDT60 & WDT90 mobile devices have an internal backup battery. Its purpose is to maintain data and programs in memory long enough to remove a discharged main battery and replace it with a charged one (or connect to a charger or other USB power source).

 

A new unit's backup battery can last up to about 30 minutes, though this number will come down as the unit ages. The backup battery should not be relied on for longer-term data preservation.