ZAGG Boost Plus - Manual (Page 7)

5.3 Battery Low Indication:
The blue LED flashes 3 times every 4 seconds to indicate the battery low status.
This happens only in Standby mode. Please note that when the batteries are
relatively low, the sound may be distorted when playing in high volume.
When the batteries are too low, the system will cut off power to prevent the
batteries from being damaged.
The speaker will return to Standby mode 1 minute after the music has stopped. When
using Alkaline batteries, the speaker can stay in Standby mode for over 6 months.
When the speaker is in Power On mode, it will switch automatically to Power Off 10
minutes after the music has stopped.
5.4 External DC 5V in:
The micro USB socket is for external power of DC 5V. When the external power is
plugged in, the AA batteries will not be used.