Why Does My Garmin Cannot Be Charged While Driving ?

I jsut bought a garmin GPS and it sucks.. how come i cannot charge it while its navigating at the same time like my other GPS..? does anyone have the same problem with the garmin gps>? whenever i plug in the power from either the car charger or the ac adapter… it just charge it without enabling me to continue using it… this sucks because my gamin battery have a very low life and i cannot go anywhere just depending on the battery itself…. anyone have similar issue?