If the tracker is discharged for a longer time, the tracker will keep in the turned off mode from safety reasons to protect the battery. The tracker won't light and will look dead even during charging.
In that case, plug the tracker into the charger for 5 minutes, unplug it, leave it unplugged for at least 1 minute and then plug it again. The tracker should light blue and after a short while the tracker lights red as being charged. A green light means the tracker is fully charged and ready to use.
How to fix the tracker that won’t turn on when charging.
