I just bought a new Gpsmap 66i and I charged it and used once down there road… then went to use it the next day for a trail run and it won’t turn on ? I then thought it was flat and plugged it into power and it came on and shows it’s fully charged.

I just bought a new GPSmap 66i and charged it and then took it down the road for a trial.  All seemed ok.  Then the next day went to take it for a trail run and could not get the unit to turn on.  I then plugged it into a power charger thinking maybe battery seemed flat which it it shouldn’t be as was fully charged the day before.  It did turn on when plugged into power, however when I disconnected the power it shut off and still won’t turn on unless plugged into power ??