BlueTrek COMPACT CLASSIC Manual page 5

You can also connect the charging cable to a USB socket.
Diagram 3
While charging, the indicator light on the BLUETREK® COMPACT CLASSIC should be orange.
The green indicator light will turn on when the unit is fully charged. Subsequent charges
should take approximately 3 hours.
When the indicator light on your BLUETREK® COMPACT CLASSIC flashes in red at regular
intervals (or the voice alert is activated), the unit needs to be recharged.
BLUETREK COMPACT CLASSIC I 4