Personally I think you leave the tablet hooked up and charging all of the time. Most tablets will allow you to leave them plugged in and only charge when they are not 100%. Only display the monitor when the vehicle is turned on. If you have a blue tooth ODBII connector maybe you could have it monitor that port for activity and disable the screen when there isn't any. Or if you have another way to detect if the car is on or off use that and turn the screen off when not needed.