The device will not take readings when there is no power but all readings it has taken prior to the loss of power will be synchronised.  Once recharged, the device will reconnect to the FilesThruTheAir™ Cloud, start a new recording session, and continue recording.

If using battery power you must set suitable sample and transmission rates as these will affect your battery life.  You can power the device from mains using a suitable USB power supply if fast transmission rates or long term, uninterrupted logging is needed.