Sometimes an external source of power does not supply enough current to operate the device. Make sure the current output of your car charger is sufficient if it is not the original power supply cable for your model.
Thought battery was exhausted but now won't even turn on when connected to DC power in auto.
Take it back.
I am on my THIRD 3790t !!!
Garmin will not reimburse so am stuck with the product.
Having said that the third is operating fine so somewhat mollified , but difficult to recommend to others as a failure rate of 2 in 3 is hardly inspiring.