A cellular-ready tablet makes a good car navigation system, particular when it's fitted snugly in a car mount. Why? Because a tablet's big screen is a lot easier to read than a puny smartphone screen.
But location services, such as GPS mapping apps, are proven power hogs, and it's very easy to turn on a navigation app and forget it's running in the background. You should use location services only when necessary, or take the extreme approach and disable them altogether. On Android tablets, go to Settings > Location and turn off GPS services and other location-based tools by unchecking the appropriate box. To disable location services on an iPad, visit Settings > Privacy > Location Services.